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News from QEMU, KVM, libvirt, libguestfs, virt-manager and related tools


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May 31, 2021

KVM on Z

RHEL 8.4 Released

RHEL 8.4 is out! See the official announcement and the release notes.

KVM is supported via Advanced Virtualization, and provides

  • QEMU v5.2, supporting virtio-fs on IBM Z
  • libvirt v7.0

Furthermore, RHEL 8.4 now supports graphical installation for guest installs. Just add the highlighted arguments to your virt-install command line for an RHEL 8.4 install on an RHEL 8.4 KVM host:

    virt-install --input keyboard,bus=virtio --input mouse,bus=virtio \
    --graphics vnc --video virtio
--disk size=8 --memory 2048 --name rhel84 \
    --cdrom /var/lib/libvirt/images/RHEL-8.4.0-20210503.1-s390x-dvd1.iso

And the installation will enter the fancy graphical installer:

Make sure to have package virt-viewer installed on the host, and X forwarding enabled (option -X for ssh).

This new support also allows graphical installs started in cockpit:

by Stefan Raspl ( at May 31, 2021 04:55 PM

May 30, 2021

Gerd Hoffmann

Adding cut+paste support to qemu

The spice project supports cut+paste for ages, now the rest of qemu is playing catch up.

Implementation options

So, what are the choices for implementing cut+paste support? Without guest cooperation the only possible way would be to send text as keystrokes to the guest. Which has a number of drawbacks:

  • It works for text only.
  • It is one-way (host to guest) only.
  • Has keyboard mapping problems even when limiting to us-ascii,
    sending unicode (ä ø Я € © 漢字 ❤ 😎) reliably is impossible.
  • Too slow for larger text blocks.

So, this is not something to consider seriously. Instead we need help from the guest, which is typically implemented with some agent process running inside the guest. The options are:

  1. Write a new cut+paste agent.
  2. Add cut+paste support to the qemu guest agent.
  3. Use the spice agent which already supports cut+paste.

Reusing the spice agent has some major advantages. For starters there is no need to write any new guest code for this. Less work for developers and maintainers. Also the agent is packaged since years for most distributions (typically the package is named spice-vdagent). So it is easily available, making things easier for users, and guest images with the agent installed work out-of-the-box.

Downside is that this is a bit confusing as you need the spice agent in the guest even when not using spice on the host. So I'm writing this article to address that ...

Some background on spice cut+paste

The spice guest agent is not a single process but two: One global daemon running as system service (spice-vdagentd) and one process (spice-vdagent) running in desktop session context.

The desktop process will handle everything which needs access to your display server. That includes cut+paste support. It will also talk to the system service. The system service in turn connects to the host using a virtio-serial port. It will relay data messages between desktop process and host and also process some of the requests (mouse messages for example) directly.

On the host side qemu simply forwards the agent data stream to the spice client and visa versa. So effectively the spice guest agent can communicate directly with the spice client. It's configured this way:

qemu-system-x86_64 [ ... ] \
  -chardev spicevmc,id=ch1,name=vdagent \
  -device virtio-serial-pci \
  -device virtserialport,chardev=ch1,id=ch1,name=com.redhat.spice.0
This is the data channel to the spice client.
The virtio device which manages the ports.
The port for the guest/host connection. It'll show up as /dev/virtio-ports/com.redhat.spice.0 inside the guest.

The qemu clipboard implementation.

The central piece of code is the new qemu clipboard manager (ui/clipboard.c). Initially it supports only plain text. The interfaces are designed for multiple data types though, so adding support for more data types later on is possible.

There are three peers which can talk to the qemu clipboard manager:

The vnc server got clipboard support (ui/vnc-clipboard.c), so vnc clients with cut+paste support can exchange data with the qemu clipboard.
The gtk ui got clipboard support too (ui/gtk-clipboard.c) and connects the qemu clipboard manager with your desktop clipboard.
Qemu got an implementation of the spice agent protocol (ui/vdagent.c), which connects the guest to the qemu clipboard.

This landed in the qemu upstream repo a few days ago and will be shipped with the qemu 6.1 release.

Configure the qemu vdagent

The qemu vdagent is implemented as chardev. It is a drop-in replacement for the spicevmc chardev, and instead of forwarding everything to the spice client it implements the spice agent protocol and parses the messages itself. So only the chardev configuration changes, the virtserialport stays as-is:

qemu-system-x86_64 [ ... ] \
  -chardev qemu-vdagent,id=ch1,name=vdagent,clipboard=on \
  -device virtio-serial-pci \
  -device virtserialport,chardev=ch1,id=ch1,name=com.redhat.spice.0

The vdagent has two options to enable/disable vdagent protocol features:

enable/disable mouse messages. When enabled absolute mouse events can travel this way instead of using an usb or virtio tablet device for that. Default is on.
enable/disable clipboard support. Default is off (for security reasons).

Future work

No immediate plans right now, but I have some ideas what could be done:

Add more peers
Obvious candidates are the other UIs (SDL, cocoa). Possibly also more guest protocols, I think vmware supports cut+paste too (via vmport and agent).
Add more data types
With image support being a hot candidate. Chances are high that this involves more than just passing data. spice uses png as baseline image format, whereas vnc uses bmp. So qemu most likely has to do image format conversions.

Maybe I look into them when I find some time. No promise though. Patches are welcome.

by Gerd Hoffmann at May 30, 2021 10:00 PM

May 20, 2021

Gerd Hoffmann

virtio-gpu and qemu graphics in 2021

Time for an update, a few things did happen since the previous update in November 2019.

virtio-gpu features

Progress is rather slow in qemu due to shifted priorities. That doesn't mean virglrenderer development is completely stalled though. crosvm (aka Chrome OS Virtual Machine Monitor) has virtio-gpu support too and is pushing forward virglrenderer development these days. There is good progress in virglrenderer library (although I don't follow that closely any more these days), crosvm and linux kernel driver.

Lets go through the feature list from 2019 for a quick update first:

shared mappings
Not a separate feature, it's part of blob resources now.
blob resources
Specification is final. Linux kernel got support. crosvm too as far I know. qemu lags behind.
metadata query
Is implemented using execbuffer commands. Which is the communication path between guest driver and virglrenderer, so virtio-gpu doesn't need any changes for this.
host memory
Also part of blob resources.
vulkan support
Not fully sure where we stand here. Blob resources are designed with vulkan memory management needs in mind, so there shouldn't be any blockers left in the virtio-gpu guest/host interface. It should "only" be a matter of coding things up in guest driver and virglrenderer. And add blob resource support to qemu of course.

Another feature which was added to virtio-gpu which is not on the 2019 list is UUID support. This allows to attach a UUID to a virtio-gpu resource, which is specifically useful for dma-buf sharing between virtio drivers inside the guest. A guest driver importing a virtio-gpu resource can send the UUID to the host device for lookup, so the host devices can easily share the resource too.

virtio-gpu state in qemu

Not much progress here up to qemu 6.0. There are a few changes merged or in the pipeline for the next release (6.1) though.

First, the virtio-gpu device is splitted. It will loose the virgl=on|off property. There will be two devices instead: virtio-vga and virtio-vga-gl (same for the other device variants).

This will de-clutter the source code and it will also remove hard virglrenderer dependency from virtio-gpu. With modular qemu builds you'll now have two modules: One for the simple virtio-vga device, without any external dependencies, and one for the virtio-vga-gl device, which supports virgl and thus depends on the virglrenderer library.

Second, blob resource support for the simple virtio-vga device is in progress, and it will bring support shared resource mappings to qemu. This will accelerate the display path due to less or no copying of pixel data.

You may wonder why this is useful for the non-virgl device. The use case is 3D-rendering using a pci-assigned GPU (or vgpu). In that model the GPU handles only the rendering, virtio-gpu handles the display scanout, and framebuffers are shared between drivers using dma-bufs. If you worked with arm socs before this may sound familiar because they often handle rendering and scanout with separate hardware blocks too. So virt graphics will use the same approach, and userspace (xorg + wayland) luckily is already prepared for it.

modular graphics in qemu

When building qemu with configure --enable-modules you'll get a modular qemu build, which means some functionality is build as separate module (aka shared object) which is loaded on demand. This allows distributions to move some functionality to separate, optional sub-packages, which is especially useful for modules which depend on shared libraries. That way you can have a rather lightweight qemu install by simply not installing the sub-packages not needed.

Block backend drivers where modularized first, audio backend drivers next. The easy UI code was modularized early on too: curses, gtk and sdl.

Last year we took a big step forward in modularizing qemu graphics. Two features got added to qemu: First support for building devices as modules, and second support for modules depending on other modules. This allowed building pretty much all qemu graphics code with external shared library dependencies modular:

opengl, egl-headless
depend on mesa libraries and drivers
spice-core, spice-app
depend on libspice-server (plus more indirect deps)
depend on qemu opengl module
qxl device
depends on libspice-server too
depends on qemu spice-core module
virtio-vga-gl device
depends on virglrenderer

You can see the results on Fedora 34. Installing qemu-system-x86-core on a fresh system installs 29 packages, summing up to an 18M download and 74M installed size. Installing qemu-system-x86 (which pulls in all module sub-packages) on top adds 125 more packages with 45M download size and 145M installed size.

linux kernel drm driver updates

As already mentioned above the virtio drm driver got support for blob resources. Also a collection of bugfixes all over the place.

The ttm (drm memory manager) got a bunch of cleanups over the last few upstream kernel releases. A bunch of sanity checks have been added along the way, and they flagged a number of issues in the qxl drm driver (which uses ttm to manage video memory). That in turn caused a bunch of bugfixes and some other improvements in the qxl drm driver. Merged upstream for linux 5.13, most important fixes have been backported to 5.10+ stable branches.

by Gerd Hoffmann at May 20, 2021 10:00 PM

April 30, 2021

QEMU project

QEMU version 6.0.0 released

We’d like to announce the availability of the QEMU 6.0.0 release. This release contains 3300+ commits from 268 authors.

You can grab the tarball from our download page. The full list of changes are available in the Wiki.

Highlights include:

  • 68k: new ‘virt’ machine type based on virtio devices
  • ARM: support for ARMv8.1-M ‘Helium’ architecture and Cortex-M55 CPU
  • ARM: support for ARMv8.4 TTST, SEL2, and DIT extensions
  • ARM: ARMv8.5 MemTag extension now available for both system and usermode emulation
  • ARM: support for new mps3-an524, mps3-an547 board models
  • ARM: additional device emulation support for xlnx-zynqmp, xlnx-versal, sbsa-ref, npcm7xx, and sabrelite board models
  • Hexagon: new emulation support for Qualcomm hexagon DSP units
  • MIPS: new Loongson-3 ‘virt’ machine type
  • PowerPC: external BMC support for powernv machine type
  • PowerPC: pseries machines now report memory unplug failures to management tools, as well as retrying unsuccessful CPU unplug requests
  • RISC-V: Microchip PolarFire board now supports QSPI NOR flash
  • Tricore: support for new TriBoard board model emulating Infineon TC27x SoC
  • x86: AMD SEV-ES support for running guests with secured CPU register state
  • x86: TCG emulation support for protection keys (PKS)

  • ACPI: support for assigning NICs to known names in guest OS independently of PCI slot placement
  • NVMe: new emulation support for v1.4 spec with many new features, experimental support for Zoned Namespaces, multipath I/O, and End-to-End Data Protection.
  • virtiofs: performance improvements with new USE_KILLPRIV_V2 guest feature
  • VNC: virtio-vga support for scaling resolution based on client window size
  • QMP: backup jobs now support multiple asynchronous requests in parallel

  • and lots more…

Thank you to everyone involved!

April 30, 2021 12:39 AM

April 27, 2021

Thomas Huth

How to check your shell scripts for portability

This blog is mainly a reminder for myself for the various possibilities to check my shell scripts for portability, but maybe it’s helpful for some other people, too.

First, why bother? Well, while bash is the default /bin/sh shell on many rpm-based Linux distributions (so it’s also the default shell on the systems I’m developing with and thus referring to here), it’s often not the case on other Linux distributions like Debian or Alpine, and it’s certainly not the case on non-Linux systems like the various *BSD flavors or illumos based installations.

Test your scripts with other shells

The most obvious suggestion is, of course, to run your script with a different shell than bash to see whether it works as expected.

Using dash

The probably most important thing to check is whether your script works with dash. dash is the default /bin/sh shell on most Debian-based distributions, so if you want to make sure that your script also works on such systems, this is the bare minimum that you should check. The basic idea of dash is to run scripts as far as possible, without adding bloat to the shell. Therefore the shell is restricted to a minimum with regards to the syntax that it understands, and with regards to the user interface, e.g. the interactive shell prompt is way less comfortable compared with shells like bash.

Since dash is also available in Fedora and in RHEL via EPEL, its installation is as easy as typing something like:

 sudo dnf install dash

Thus checking your scripts with dash is almost no additional effort and thus a very good place to start.

Using posh

posh stands for “Policy-compliant Ordinary SHell” – it’s another shell that has been developed within the Debian project to check shell scripts for POSIX compliance. Unlike dash, the syntax that this shell understands is really restricted to the bare minimum set that the POSIX standard suggests for shells, so if your script works with posh, you can be pretty sure that it is portable to most POSIX-compliant shells indeed.

Unfortunately, I haven’t seen a pre-compiled binary of posh for Fedora or RHEL yet, and I haven’t spotted a dedicated website for this shell either, so the installation is a little bit more complicated compared to dash. The best thing you can do on a non-Debian based system is to download the tar.xz source package from and compile it on your own:

 tar -xaf posh_0.14.1.tar.xz
 cd posh-0.14.1/
 autoreconf -i
 ./posh ~/

Using virtual machines

Of course you can also check your scripts on other systems using virtual machines, e.g. on guest installations with FreeBSD, NetBSD, OpenBSD or one of the illumos distributions. But since this is quite some additional effort (e.g. you have to boot a guest and make your script available to it), I normally skip this step – testing with dash and posh catches most of the issues already anyway.

Test your scripts with shell checkers

There are indeed programs that help you to check the syntax of your shell scripts. The two I’ve been using so far are checkbashism and ShellCheck:

Using checkbashism

checkbashism is a Perl script, maintained again by the Debian people to check for portability issues that occur when a shell script has been only written with bash in mind. It is part of the devscripts package in Debian. Fortunately, the script is also available in Fedora by installing the so-called devscripts-checkbashisms package (which can also be used on RHEL by way). checkbashism focuses on the syntax constructs that are typically only available in bash, so this is a good and easy way to check your scripts on distributions where /bin/sh is bash by default.

Using ShellCheck

ShellCheck is another static shell script analyzer tool which is available for most distributions or can ben installed via the instructions provided on its GitHub page. The nice thing about ShellCheck is that they even provide the possibility to check your script via upload on their website – so for small, public scripts, you don’t have to install anything at all to try it out, just copy and paste your script into the text box on the website.

April 27, 2021 09:00 AM

April 22, 2021

Daniel Berrange

ANNOUNCE: virt-viewer release 10.0

I am happy to announce a new bugfix release of virt-viewer 10.0 (gpg), including experimental Windows installers for Win x86 MSI (gpg) and Win x64 MSI (gpg).

Signatures are created with key DAF3 A6FD B26B 6291 2D0E 8E3F BE86 EBB4 1510 4FDF (4096R)

With this release the project replaced the autotools build system with Meson and Ninja and re-designed the user interface to eliminate the menu bar

All historical releases are available from:

Changes in this release include:

  • Switch to use Meson for build system instead of autotools
  • Require libvirt >= 1.2.8
  • Redesign UI to use title bar widget instead of menu bar
  • Request use of dark theme by default, if available
  • Don’t filter out oVirt DATA storage domains for ISO image sharing
  • Add –keymap arg to allow keys to be remapped
  • Display error message if no extension is present for screenshot filename
  • Fix misc memory leaks
  • Use nicer error message if not ISOs are available
  • Use more explicit accelerator hint to distinguish left and right ctrl/alt keys
  • Report detailed file transfer errors
  • Use standard about diaglog
  • Refresh and improve translations
  • Install appstream data file in preferred location
  • Refresh appstream data file contents
  • Display VM title when listing VMs, if available
  • Display VM description as tooltop, if available
  • Sort VM names when listing
  • Enable ASLR and NX for Windows builds
  • Add –shared arg to request a shared session for VNC
  • Disable all accels when not grabbed in kiosk mode
  • Allow num keypad to be used for zoom changes
  • Disable grab sequence in kiosk mode to prevent escape
  • Allow zoom hotkeys to be set on the command line / vv file
  • Display error message if VNC connection fails
  • Fix warnings about atomics with new GLib
  • Remove use of deprecated GTK APIs
  • Document cursor ungrab sequence in man pages
  • Honour Ctrl-C when auth dialog is active
  • Minor UI tweaks to auth dialog
  • Support VM power control actions with VNC
  • Add –cursor arg to control whether a local pointer is rendered with VNC
  • Add –auto-resize arg and menu to control whether to resize the remote framebuffer to math local window size
  • Add support for remote framebuffer resize with VNC
  • Handle case sensitivity when parsing accelerator mappings

by Daniel Berrange at April 22, 2021 05:39 PM

April 15, 2021

Daniel Berrange

ANNOUNCE: gtk-vnc release 1.2.0 available

I’m pleased to announce a new release of GTK-VNC, version 1.2.0. (213K)
sha256sum: 7aaf80040d47134a963742fb6c94e970fcb6bf52dc975d7ae542b2ef5f34b94a

Changes in this release include

  • Add API to request fixed zoom level
  • Add API to request fixed aspect ratio when scaling
  • Add APIs for client initiated desktop resize
  • Implement “Extended Desktop Resize” VNC extension
  • Implement “Desktop Rename” VNC extension
  • Implement “Last Rect” VNC extension
  • Implement “XVP” (power control) VNC extension
  • Implement VeNCrypt “plain” auth mode
  • Implement alpha cursor VNC extension
  • Use GTK preferred width/height helpers for resizing
  • Fix misc docs/introspection annotation bugs
  • Honour meson warninglevel setting for compiler flags
  • Fix JPEG decoding in low colour depth modes
  • Fix minor memory leaks
  • Add header file macros for checking API version
  • Change some meson options from “bool” to “feature”
  • Validate GLib/GTK min/max symbol versions at build time
  • Avoid recreating framebuffer if size/format is unchanged
  • Emit resize signal after WMVi update
  • Various fixes & enhancements to python demo program
  • Ensure Gir files build against local libs
  • Enable stack protector on more platforms
  • Don’t force disable introspection on windows
  • Relax min x11 deps for older platforms
  • Avoid mutex deadlock on FreeBSD in test suite
  • Stop using deprecated GLib thread APIs
  • Stop using deprecated GLib main loop APIs
  • Stop using deprecated GObject class private data APIs
  • Add fixes for building on macOS
  • Fix deps for building example program
  • Update translations

Thanks to all those who reported bugs and provided patches that went into this new release.

by Daniel Berrange at April 15, 2021 10:56 AM

April 06, 2021

KVM on Z

Webinar: Red Hat OpenShift for IBM Z and LinuxONE on RHEL 8.3 KVM

Join us for our webinar on Wednesday, April 21, 11:00 AM - 12:00 PM EST!


Red Hat OpenShift is available on RHEL 8.3 KVM starting with Red Hat OpenShift version 4.7 on IBM Z and LinuxONE. We discuss the deployment of a Red Hat OpenShift Cluster on RHEL KVM from a high-level perspective, including supported configurations and requirements, especially the available network and storage options.
Furthermore, we explain the installation steps of Red Hat OpenShift 4.7 on RHEL KVM in detail, including best practices and a short excursion on cluster debugging.


  • Dr. Wolfgang Voesch, Iteration Manager - OpenShift on IBM Z and LinuxONE
  • Holger Wolf, Product Owner - OpenShift on Linux on IBM Z and LinuxONE


Register here. You can check the system requirements here.
After registering, you will receive a confirmation email containing information about joining the webinar.

Replay & Archive

All sessions are recorded. For the archive as well as a replay and handout of this session and all previous webinars see here.

by Stefan Raspl ( at April 06, 2021 01:22 PM

April 05, 2021

Stefan Hajnoczi

Learning programming languages

You may be productive and comfortable in one programming language but find the idea of learning a new programming language daunting. Or you may know and use multiple programming languages but haven't learnt a new one in a while. Or you might be a programming language geek who is just curious about how others dive into new programming languages and get productive quickly. No matter how easy or difficult it is for you to engage in new programming languages, this article explains how I like to learn new programming languages. Although people learn best in different ways, I hope you'll find my thought process interesting even if you decide to take a different approach.


Language N+1

This article isn't aimed at learning to program. Learning your first programming language is much harder than learning an additional one. The reason is that many abstract concepts are involved in computer programming. When you first encounter programming, most languages require you to understand concepts like iteration, scopes, (im)mutability, arrays, modules, functions, and much more. The good news is that when you learn an additional language you'll already be familiar with common concepts and can therefore take a more streamlined approach in order to get up to speed quickly.

Courses, videos, exercises

There is a lot of educational material online that teaches various programming languages, but I don't find structured courses, videos, or exercises efficient. If you already know common programming concepts and have an idea of what you want to build in the new programming language, then it's more efficient to chart your own course. Materials that let you jump/skip around will let you focus on information that is novel and that you actually need. Working through a series of exercises that someone else designed may be time spent practicing the wrong things since usually you are the one with the best idea of what to practice. Courses, videos, and exercises tend to be an "on the rails" experience where you are exposed to information in a linear fashion whether it's useful at the moment or not.

1. Understanding the computational model

The first question about a new programming language is "what is its computational model?". Sadly, many language manuals and websites do not describe the computational model beyond what programming paradigms are supported (object-oriented, concatenative, functional, logic programming, etc). The actual computational model may only become fully apparent later. Or it might be expressed in too much detail in a language standards document to be of use early on. In any case, it's worthwhile reading the programming language's website for information on the computational model to grasp the big picture.

It's the computational model that you need to understand in order to write programs. Often we think about syntax and language features too much when learning a new language. The computational model informs us how to break down requirements into programs. We approach logic programming differently from object-oriented programming in how we organize data and code. The syntax and to an extent even the language features don't matter.

Understanding the computational model also helps you situate the new programming language relative to others, especially programming languages that you already know. It will give you an idea of how different programming will be and where you'll need to learn new concepts.

2. The language tutorial

After familiarizing yourself with the computational model of the programming language, the next step is to learn the basic syntax and concepts. Most modern programming languages have an official tutorial available online. The tutorial introduces the language elements, usually with short examples, and its table of contents gives an overview of what the language consists of. The tutorial can be completed in a few hours or days. Unlike full courses, official programming language tutorials often lend themselves to non-linear reading, which is helpful when certain aspects of the language are already familiar or will not be relevant to you.

I remember reading the Python tutorial in an afternoon years ago, but watch out: at this point you might be able to write valid syntax but you won't be writing idiomatic code yet. There's that saying "you can write FORTRAN in any language". In order to write programs that are expressed naturally and take advantage of the language effectively, more effort will be necessary.

3. Writing toy programs

After becoming aware of the language elements the next step is to explore how the language works. This can be done by writing small programs. Often these toy programs are familiar tasks you've already solved in other languages. If you want to write games, maybe it's Pong. If you write web applications, it could be a todo list. There are lots of different well-known programs to write.

During the course of writing toy programs you'll encounter syntax errors or issues with the program. Learning to interpret common error messages is important because they will come up in more complicated scenarios later where it can be harder to resolve them if you haven't seen them before.

You'll also hit common tasks for which you need to find solutions in the standard library or language reference manual. Whether it's parsing command-line options, regular expression matching, HTTP requests, or error handling, the language probably has a way of doing it. Toy programs present a simple environment in which to explore the basic facilities of a programming language.

4. Gaining a deeper appreciation for the language

Once you have written some toy programs you'll be able to start writing your own programs that solve new problems. At this stage you start being productive but there is still more to learn. In particular, the language's idioms and patterns must be studied in order to write natural code. Once I have experience with the basics of a language I like to read the source code to the standard library, popular libraries, and popular applications. In the beginning this is hard because they use unfamiliar language features or library dependencies, but after following up on unknown parts of one program, you'll find it becomes easier to read other programs because your knowledge of the language has expanded.

At this point it is also worth looking for style guides, manuals on language idioms, and documentation on common gotchas or anti-patterns. These will provide the information about thinking natively in the new programming language. This is what's needed to become fluent in the language and capable of reading and writing real programs confidently.

Although I have presented steps in a linear order, learning complex subjects is often an iterative process. Sometimes I find myself jumping back and forth between steps as my understanding evolves.


Learning a new programming language is time-consuming no matter how you do it. However, it doesn't all need to happen upfront and after a few days of reading the documentation and experimenting with toy programs, it's possible to peform basic tasks. Learning how to use a language effectively by studying popular programs and reading guides is the quickest way I've found to reaching fluency. Finally, it just takes practice!

by Unknown ( at April 05, 2021 09:29 PM

March 26, 2021

KVM on Z

Installing Red Hat OpenShift on KVM on Z

While there is no documentation on how to install Red Hat OCP on Linux on Z with a static IP under KVM today, the instructions here will get you almost there. However, there are a few parts within section Creating Red Hat Enterprise Linux CoreOS (RHCOS) machines that require attention. Here is an updated version that will get you through:
4. You can use an empty QCOW2 image: Using the prepared one will also work, but it will be overwritten anyway.

5. Start the guest with the following modified command-line:
  $ virt-install --noautoconsole
     --boot kernel=/bootkvm/rhcos-4.7.0-s390x-live-kernel-s390x, \
       initrd=/bootkvm/rhcos-4.7.0-s390x-live-initramfs.s390x.img, \
           kernel_args='rd.neednet=1 dfltcc=off coreos.inst.install_dev=/dev/vda \
           /pub/openshift-v4/s390x/dependencies/rhcos/4.7/4.7.0 \
       coreos.inst.ignition_url=http:// \
       /bootstrap.ign ip= \
     --connect qemu:///system
     --name bootstrap-0
     --memory 16384
     --vcpus 8
     --disk /home/libvirt/images/bootstrap-0.qcow2
     --network network=macvtap-mv1
     --qemu-commandline="-drive if=none,id=ignition,format=raw,file=/bootkvm \
           /bootstrap.ign,readonly=on -device virtio-blk, \

Note the following changes:

  • Use the live installer kernel, initrd (you can get them from the redhat mirror) and parmline (this you need to create yourself once for each guest) in the --boot parameter. This is basically like installing on z/VM, and will write the image to your QCOW2 image with the correct static IP configuration. Keep in mind that the ignition file needs to be provided by an http/s server for this method to work
  • dfltcc=off is required for IBM z15 and LinuxONE III

6. To restart the guest later on, you will need to change the guest definition to boot from the QCOW2 image.
When the kernel parms are passed into the installer, the domain xml will look like this once the guest is installed and running:
    <type arch='s390x' machine='s390-ccw-virtio-rhel8.2.0'>hvm</type>
    <cmdline>rd.neednet=1 dfltcc=off coreos.inst.install_dev=/dev/vda \
             s390x/dependencies/rhcos/4.7/4.7.0/rhcos-4.7.0-s390x-live- \
        ip= \
    <boot dev='hd'/>

However, this domain XML still points at the installation media, hence a reboot will not work (it will merely restart the installation).
Remove the <kernel>, <initrd>, <cmdline> elements, so that all that is left is the following:
    <type arch='s390x' machine='s390-ccw-virtio-rhel8.2.0'>hvm</type>
    <boot dev='hd'/>

With this, the guest will start successfully.

 [Content contributed by Alexander Klein]

by Stefan Raspl ( at March 26, 2021 08:59 PM

March 12, 2021

Stefan Hajnoczi

Overcoming fear of public communication in open source

When given the choice between communicating in private or in public, many people opt for private communication. They send questions or unfinished patches to individuals instead of posting on mailing lists, forums, or chat rooms. This post explains why public communication is a faster and more efficient way for developers to communicate in open source communities than private communication. A big factor in the private vs public decision is psychological and I've provided a checklist to give you confidence when communicating in public.

Time and time again I find people initiating discussions through private channels when I know it would be advantageous to have them in public instead. I even keep an email reply template handy asking the sender to reach out to the public mailing list instead of communicating with me in private. Communicating in public is a good default unless you need confidentiality (security bugs, business reasons, etc). So why do people prefer to ask questions or send patches off-list?

Why we fear public communication

I'm interested in understanding why people avoid public communications channels in open source communities. The purpose of mailing lists, chat rooms, and forums is to engage in discussions and share knowledge. Members of these communities are interested in the topic and want to engage in discussion. But there are some common reasons I've found why people don't make use of public communications channels:

  • I don't want to create noise. Mailing lists are home to important discussions between experienced members of the community. Sending a relatively simple question or unfinished patch that no expert would need to ask can make you doubt whether it deserves to be sent at all. This is a fallacy. As long as the question or patches are relevant to the community and you have done your homework (see the checklist below), there is no need to worry about creating noise.
  • I will look dumb or seem like a bad programmer. When you need help it's likely that your understanding is incomplete. We often hold ourselves to artifically high standards when asking for help in public, yet when we observe others interacting in public we don't hold their questions or unfinished code against them. Only if they show a repeated pattern of sloppy work does it damage their reputation. Asking for help in public won't harm your image.
  • Too much traffic. Big open source communities are often so active that no single person can keep track of everything that is going on. Even core members of the community rely on filtering only topics relevant to them. Since filtering becomes necessary at scale anyway, there's no need to worry about sending too many messages.

A note about tone: in the past people were more likely to be put off by the unfriendly tone in chat rooms and mailing lists. Over time the tone seems to have improved in general, probably due to multiple factors like open source becoming more professional, codes of conduct making participants aware of their behavior, etc. I don't want to cover the pros and cons of these factors, but suffice to say that nowadays tone is less of a problem and that's a good thing for everyone.

Why public communication is faster and more efficient

Next let's look at the reasons why public communication is beneficial:

Including the community from the start avoids waste

If you ask for help in private it's possible that the answer you get will not end up being consensus in the community. When you eventually send patches to the community they might disagree with the approach you settled on in private and you'll have to redo your work to get the patches merged. This can be avoided by including the community from the start.

Similarly, you can avoid duplicating work when multiple people are investigating the same problem in private without knowing about each other. If you discuss what you are doing in public then you can collaborate and avoid spending time creating multiple solutions, only one of which can be merged.

Public discussions create a searchable knowledge base

If you find a solution to your problem in private, that doesn't help others who have the same question. Public discussions are typically archived and searchable on the web. When the next person has the same question they will find the answer online and won't need to ask at all!

Additionally, everyone benefits and learns from each other when questions are asked in public. We soak up knowledge by following these discussions. If they are private then we don't have this opportunity.

Get a reply even if the person you asked is unavailable

The person you intended to reach may be away due to timezones, holidays, etc. A private discussion is blocked until they respond to your message. Public discussions, on the other hand, can progress even when the original participants are unavailable. You can get an answer to your question faster by asking in public.

Public activity gives visibility to your work

Private discussions are invisible to your teammates, managers, and other people affected by your progress. When you communicate in public they will be able plan better, help out when needed, and give you credit for the effort you are putting in.

A checklist before you press Send

Hopefully I have encouraged you to go ahead and ask questions or send unfinished patches in public. Here is a checklist that will help you feel confident about communicating in public:

Before asking questions...

  1. Have you searched the web, documentation, and code?
  2. Have you added printfs, GDB breakpoints, or enabled tracing to understand the behavior of the system?

Before sending patches...

  1. Is the code formatted according to the coding standard?
  2. Are the error cases handled, memory allocation/ownership correctly implemented, and thread safety addressed? Most of the time you can figure these out yourself and forgetting to do so may distract from the topics you want help with.
  3. Did you add todo comments pointing out unfinished aspects of the code? Being upfront about what is missing helps readers understand the status of the code and saves them time trying to distinguish requirements you forgot from things that are simply not yet implemented.
  4. Do the commit descriptions explain the purpose of the code changes and does the cover letter give an overview of the patch series?
  5. Are you using git-format-patch --subject-prefix RFC (same for git-publish) to mark your patches as a "request for comment"? This tells people you are seeking input on unfinished code.

Finally, do you know who to CC on emails or mention in comments/chats? Look up the relevant maintainers or active developers using git-log(1), scripts/, etc.


Private communication can be slower, less efficient, and adds less value to an open source community. For many people public communication feels a little scary and they prefer to avoid it. I hope that by understanding the advantages of public communication you will be motivated to use public communication channels more.

by Unknown ( at March 12, 2021 06:15 PM

March 10, 2021

QEMU project

Google Summer of Code 2021 is on!

QEMU has been accepted into Google Summer of Code 2021 and we look forward to mentoring talented students from around the world as they make open source contributions this summer. GSoC is a remote work open source internship program where students work on a project for an open source organization like QEMU.

Check out the project ideas page where there are 10 projects that eligible students can apply for. This year we have C, Rust, and Python projects in various areas related to emulation and virtualization.

If you are a student who is interested in doing an internship this summer, head over to QEMU’s GSoC organization page where you can read about how to apply and learn more about Google Summer of Code in general.

The GSoC 2021 timeline is:

  • Student application period - March 29 - April 13
  • Student projects announced - May 17
  • Community bonding period - May 17 - June 7
  • Coding - June 7 - August 16

We look forward to meeting you and answering questions on the #qemu-gsoc IRC channel on!

by Stefan Hajnoczi at March 10, 2021 07:00 AM

February 24, 2021

Stefan Hajnoczi

Milestone Systems: Software that changes how things are done

Every few years a project comes out with a new approach that becomes influential. Often it involves combining existing concepts in a novel way. People argue about whether the project is actually novel or whether it was just in the right place at the right time and popularized existing technology. Regardless, I find these projects fascinating and try to learn about them because they are milestones that future systems are based on.

Here is a short list of projects that I think fall into this category. I hope you enjoy them (if you haven't already explored them). Send me your picks!


Tor is an onion router. It enables (mostly) anonymous communication by tunneling encrypted connections. The client does not know the IP address of the server (when connecting to so-called hidden services), the server does not know the IP address of the client, and the intermediate hops only know about their immediate predecessor and successor.

The design of Tor is described in a paper.


BitTorrent is a decentralized peer-to-peer file sharing protocol that can be used to reduce load on file hosting servers and improve download times. It's commonly used to share copyrighted material, but is also used by Linux distributions to publish ISO images and by software update systems.

A central aspect to BitTorrent is that peers exchange pieces of the file amongst themselves thanks to a Merkle tree. Pieces received from untrusted peers are checked against the file's Markle tree to ensure that data has not been corrupted or manipulated.

A paper about the economics of BitTorrent described some of the ideas behind it. The actual protocol is described by the protocol specification.


Git is the most popular version control system as of 2020. It replaced the older CVS and Subversion systems that were widely used before it. Other systems like Mercurial, Darcs, Perforce, and BitKeeper had similar use cases and ideas.

Git is a content-addressable object store with a convention for representing trees of files as well as commits and tags. I wrote about how the object store is implemented here if you want to learn about pack files and deltas.


Bitcoin is a decentralized currency, also known as cryptocurrency. A network of mutually untrusted nodes maintains a ledger called the blockchain that records transactions. Bitcoin is famous for mining where nodes compete to solve a computationally-expensive problem in order to extend the ledger.

What is interesting about Bitcoin is that the blockchain prevents abuse as long as at least half of the nodes are not controlled or colluding. In other words, it is a decentralized consensus - although there can be short-lived splits where not all nodes agree on the current state.

The Bitcoin paper gives an overview of how the system works.


I hope this was a fun post that motivated you to look at a system you haven't studied yet or made you think about systems that you consider milestone systems. Please get in touch if you want to share yours!

by Unknown ( at February 24, 2021 08:29 PM

KVM on Z

QEMU v5.2 released [UPDATE Feb 24, 2021]

QEMU v5.2 is out. A highlight from a KVM on Z perspective:

  • PCI passthrough support now includes any PCI devices other than RoCE Express cards, e.g. including NVME devices. However, ISM devices as needed for SMC-D, require extra support an cannot be used at this point.
  • virtiofs support vi virtio-fs-ccw: Shared Filesystem allowing KVM guests to access host directories.
    Use cases:
    • Container image access in lightweight VMs (e.g. in Kata Containers)
    • CI/CD and development enablement
    • Filesystem as a service, to easily switch backends
    To use, define in the host as follows:
          <access mode='shared'/>
          <filesystem type='mount'
            <driver type='virtiofs'/>
            <source dir='/<hostpath>'/>
            <target dir='mount_tag'/>

    Then mount in guests as follows:
      # mount -t virtiofs mount_tag /mnt/<path>
    Requires Linux kernel 5.4 and libvirt v7.0.

For further details, see the Release Notes.

UPDATE: A previous version had falsely listed ISM devices as supported.

by Stefan Raspl ( at February 24, 2021 05:13 PM

Red Hat OpenShift Cotainer Platform 4.7 Released

Red Hat OCP 4.7 is out!

Among others, it adds support for KVM on Z as provided by RHEL 8.3 as the hypervisor for user-provisioned infrastructure.

See here for the full list of IBM Z-specific changes and improvements.

by Stefan Raspl ( at February 24, 2021 04:54 PM

February 17, 2021

QEMU project

QEMU is applying to Google Summer of Code and Outreachy 2021

QEMU is applying to Google Summer of Code 2021 and is participating in Outreachy May-August 2021. Both of these open source internship programs offer remote work opportunities for new developers wishing to get involved in our community.

Interns work with mentors who support them in their project. The code developed during the project is submitted via the same open source development process that all QEMU code follows. This gives interns experience with contributing to open source software.

QEMU’s mentors are experienced contributors who enjoy working with talented individuals who are getting started in open source. You can find a list of project ideas that mentors are proposing here.


Initial applications are open until February 22nd at 16:00 UTC. Outreachy’s goal is to increase diversity in open source and is open to anyone who faces under-representation, systemic bias, or discrimination in the technology industry of their country.

You can learn more about Outreachy May-August and how to apply at the Outreachy website.

Google Summer of Code

Google Summer of Code (GSOC) is a 10-week internship for students. Applications are open from March 29th to April 13th. You can find the details of how to apply at the Google Summer of Code website.

Google will announced accepted organizations on March 9th. QEMU is applying and we hope to mentors GSoC interns again this year!

Please review the eligibility criteria for GSoC before applying.

by Stefan Hajnoczi at February 17, 2021 07:00 AM

February 16, 2021

Stefan Hajnoczi

Video and slides available for "The Evolution of File Descriptor Monitoring in Linux"

My FOSDEM 2021 talk "The Evolution of File Descriptor Monitoring in Linux: From select(2) to io_uring" is now available:

The talk compares the file descriptor monitoring system calls available in Linux and discusses their design. Benchmark results show how well they scale when there are many file descriptors. I hope this is a useful overview to this important kernel feature that GUI applications, network services, and many other programs rely on.

If you are interested in API design and performance, this talk highlights how different approaches like stateless vs stateful APIs can affect performance and how to minimize the number of API calls through careful design.


by Unknown ( at February 16, 2021 09:24 AM

February 15, 2021

Daniel Berrange

ANNOUNCE: libvirt-glib release 4.0.0

I am pleased to announce that a new release of the libvirt-glib package, version 4.0.0, is now available from

The packages are GPG signed with

Key fingerprint: DAF3 A6FD B26B 6291 2D0E 8E3F BE86 EBB4 1510 4FDF (4096R)

Changes in this release:

  • Replace autotools build system with meson
  • Mandate libvirt >= 1.2.8
  • Mandate libxml2 >= 2.9.1
  • Mandate glib >= 2.48.0
  • Mandate gobject-introspection >= 1.46.0
  • Fix docs incompatibility with gtk-doc >= 1.30
  • Updated translations
  • Misc API docs fixes
  • Add constants related to NVRAM during domain delete
  • Add domain config API for controller ports attribute
  • Fix compat with newer glib by avoid volatile for enum types

Thanks to everyone who contributed to this new release.

by Daniel Berrange at February 15, 2021 12:36 PM

February 03, 2021

Stefan Hajnoczi

Keeping a clean git commit history

Does the commit history of your source repository look like this:

f02af91822 docs: fix incorrect subheadings
dd7bee8b38 cli: add --import option
900ca2936a cli: extract move_topic() helper function

Or like this:

7011cc9868 lunch time
a07c82331d resolve code review comments
331d79a8ff more fixes


The first is a clean git commit history where each commit has a clear purpose and is a single logical change. The second is a messy commit history where the commits have no inherent structure:

  • "more fixes" does not describe clearly what is being fixed and the plural ("fixes") hints it may contain multiple logical changes instead of just one.
  • "resolve code review comments" contains changes requested by code reviewers in relation to another commit, it's not a self-contained logical change.
  • "lunch time" is an unfinished commit that was created because the programmer wanted to save their work.

Commit anti-patterns

The example above illustrates several anti-patterns:

Vague commit messages

If the commit message is vague and does not express a clear purpose, then it is hard to know what a commit does from the commit message. If git-log(1) doesn't provide useful information about commits then one has to resort to searching the code diffs. That is very tedious and sometimes it's almost impossible to come up with a good code search query while a clear commit message would have been easy to search. So clear commit messages are the first step towards clean commit history.

Doing too many things in one commit

Commits that make several logical code changes are hard to review and impede backporting fixes to stable branches. For example, a commit that fixes a bug as well as adding a new feature may need to be rewritten for a stable branch. If instead the code had been split into two commits, then the bug fix commit could have been backported easily. Therefore it is good practice to separate distinct bug fixes, features, and other logical code changes into separate commits.

Addressing code review comments

The code review and testing history is usually not useful information once a commit has been merged. For example, if there was a continuous integration (CI) test failure and a pull request needed to be changed, then the change should be made directly to the buggy commit so that the final commit passes the tests. No one needs to know about the code review or testing history once the code is merged and keeping these artifacts makes the commit history unwieldy by spreading a logical code change across multiple incomplete commits.

Saving work

There are valid reasons to temporarily save your work in a commit, but work-in-progress (WIP) commits should be cleaned up before merging them. For example, sometimes people make arbitrary commits to save work at the end of the day. That is fine in a local branch, but those temporary commits can be restructured into clean commits using git-rebase(1). No one else needs to know about temporary commits.

Broken commits

It can be easy to accidentally include a commit that does not build or fails tests if a later commit happens to resolve the issue. Since the later commit hides the issue it may not be apparent when testing the branch. When reordering commits the risk of introducing broken commits increases because those commits were originally written in a different order. I use the git-rebase(1) exec action to build and run tests after every commit to detect broken commits when doing extensive rebases.

Why clean commit history is important

Not all reasons for maintaining a clean commit history are obvious. Unfortunately all the above anti-patterns make commit history less useful so it's interesting to note that if you value any of the following reasons for keeping a clean commit history, then all anti-patterns need to be avoided.

Code review

Reviewers have an easier time reading clean commits than an unstructured series of commits. For example, if there is a broken commit because a function is used before it is defined in a later commit, then that affects code reviewers who read the commits linearly. They will be puzzled by the non-existent function and unable to decide whether it is being used correctly because it has not been defined yet. Although code reviewers could put in extra effort to reread the commits multiple times and try to remember the misordered changes, it's better to let code reviewers spend time on real issues rather than on untangling poorly structured commits.

Capturing the rationale for code changes

When each commit is a single logical change it becomes possible to write good commit descriptions that give the rationale for the code change. Explanations for why a code change is necessary, as well as links to issue trackers, email discussions, etc can be valuable when revisiting the commit history later. If commits contain multiple logical code changes or are incomplete then it is hard to include a good commit description, so the commit history is less useful when referring back to it later on.

Making cherry-picking easy

Many software projects maintain stable branches that still receive bug fixes for some time. This allows development to introduce new features and less mature code while users can run a mature stable release. However, maintaining stable branches can be time-consuming. Maintainers need to identify commits suitable for stable branches and cherry-pick or backport them. This requires clean commit history so that bug fixes can be applied in isolation without dragging in other code changes that do not fit the criteria for stable branches.

Enabling git-bisect(1)

When a bug is observed it may not be clear which commit introduced it. The git-bisect(1) command systematically searches the commit history and identifies the commit that caused the bug. However, git-bisect(1) only works with clean commit history. If there are broken commits then bisection becomes unreliable because some portions of commit history cannot be tested. Poorly structured commits, such as huge changes that do many different things, also make it difficult to identify which line caused the bug even when git-bisect(1) has determined which commit is to blame.

When clean commit history does not matter

The reason I have found that not everyone practices clean commit history is that they may not need any of this. Especially small projects developed by a single author may involve little code review, backporting changes to stable branches, or git-bisect(1). In that case the effort required to split code changes into clean commits and write good commit messages may seem unjustified. Of course this can change but once the commit history is messy there is not much to be done. So it's worth thinking carefully about whether to take shortcuts.

Another factor is poor tooling. Gerrit and GitHub's code review has historically made it hard to practice clean commit history. They were not designed for reviewing commit series and favored anti-patterns like squashing everything into a single commit or adding additional commits to address code review feedback. These are tool limitations and luckily GitHub code review has become better over the years. Tools that encourage you to review a commit series as a single diff are not conducive to clean commit history.

Finally, clean commit history requires proficiency with git-rebase(1) and that you are comfortable with the idea of rewriting your local branch to clean it up before publishing it. It takes a little practice to become competent at reordering, squashing, and splitting commits. The process can be a little scary, although git-reflog(1) makes it possible to undo even the most serious errors where commits were accidentally lost. On a related note, some people falsely believe that a pull or merge request branch should not be rebased. Although it is good practice to avoid rewriting history of branches that other people track, rewriting history and force-pushing a pull request is different. Most of the time no one else will maintain a local branch based on it and therefore force-pushing will not inconvenience anyone. Even if it is necessary to develop branches based on someone else's not-yet-merged branches, one needs to weigh the trade-offs of having to do more work in the short-term with the drawbacks of having a messy commit history forever.


I hope this is a useful summary of why each commit should have a clear purpose and embody a single logical change. For source repositories that are used by more than one person it is especially important to think about commit best practices. Clean commit history facilitates better code review, bug-finding, and maintaining stable branches. Beyond that it also provides a useful form of communication and sharing knowledge about the codebase that is missing when commit history is disregarded.

by Unknown ( at February 03, 2021 09:25 AM

February 01, 2021

KVM on Z

Webcast: IBM Secure Execution for Linux Introduction and Demo

IBM Secure Execution for Linux allows to build a Trusted Execution Environment for IBM Z and LinuxONE that helps protect data in use.
This webcast gives an overview of the value and the key concepts of the technology, followed by a hands-on demo, outlining the steps needed to secure Linux workloads.

    UPDATE: A recording of the event is now available here.

Audience: Clients, Business Partners, IT Architects, Systems Admins

Speaker: Viktor Mihajlovski, Linux on IBM Z Development, Product Owner for KVM on IBM Z

Date: November 18, 11:00 AM - 12:15 PM EST

Registration: Register here, and check system requirements here.

by Stefan Raspl ( at February 01, 2021 08:15 PM

January 30, 2021

Stefan Hajnoczi

Why learning the Vim text editor is worth it

Vim logo, GNU General Public License v2 or Later

Many tools come and go as our software and devices change - or we get bored and want to try something new and shiny. One of the few exceptions for me has been the Vim text editor, which I use for programming, emails, and writing every day. In this post I want to share why Vim is remarkable but more generally why learning a text editor is a great investment.

It may not be obvious why text editors are useful tools. Many programs, like web browsers, email clients, and integrated development environments (IDEs), have built-in text editing functionality. Why use a separate text editor? Text editors go deep. They are much more powerful than text boxes in browsers and email clients while being more general than IDEs. While IDEs often have excellent programming language-specific functionality, they are rarely used for other text editing tasks like writing emails or documents because they are specialized tools. Text editors strike a good balance of powerful editing and support for programming without being boxed into a narrow use-case.

Getting started with a text editor is easy. Vim implements the arcane vi editor user interface but has many videos, cheatsheets, and tutorials that make it fun to try. Really getting familiar with the features and customizing the editor to your own needs takes time though. This is true for any popular text editor because the number of settings, extensions, or plugins available can be huge. However, once you are familiar with a text editor you will have a powerful tool that can be used for most tasks involving writing or manipulating text. The time investment will pay off as you use the editor for todo lists, emails, documentation, programming, configuration files, and more.

This explains why I've found Vim a useful and enduring tool that I use daily. But what makes it a particularly strong text editor compared to the other options? Text editors go in and out of fashion all the time. I remember many that attracted attention for a time but then faded away. Vim has remained popular and I think there are a few reasons for that.

Powerful text editing plus IDE-like functionality. The vi user interface is actually a language of text editing operators. The keys you press aren't just keyboard shortcuts, they are like a bytecode (!) for a text manipulation CPU that is Turing-complete. Years ago I wrote vi macros that solve the stable marriage problem to demonstrate this. For many geeks this alone might be enough to convince you to learn Vim! But on top of this crazy text editing power Vim also has IDE-like functionality including syntax highlighting, completion, code search and navigation, compiler error navigation, and diffing. There is a large collection of plugins and scripts if you want to extend Vim's functionality even further.

Vim is ubiquitous. It runs on all major operating systems but furthermore it is found on devices from tiny Wi-Fi routers to the largest servers. It has a GUI but also a terminal interface if you are connecting to a remote machine over SSH. I always find it strange when I see people use their editor of choice on their laptop but then use another, less-familiar editor when connecting to remote machines. Learn Vim and you can use it everywhere!

Keyboard-friendly. Constantly moving my hand between the mouse and keyboard is tiring and distracting. Vim has excellent keyboard support and many things can be done without leaving the home row on the keyboard, including navigating, inserting, and deleting text. I find there is no need to use the arrow keys, mouse, or anything that is hard to reach.

If you are looking for a powerful text editor that you can use for many years then I recommend Vim. It's also worth looking at Emacs, which has a different angle but is also a good time investment. Looking back on 17 years of using Vim, I'm happy I stopped switching between language-specific IDEs and instead found a text editor capable of handling all tasks.

by Unknown ( at January 30, 2021 11:02 AM

January 22, 2021

Stefano Garzarella

SOCAT now supports AF_VSOCK

SOCAT is a CLI utility which enables the concatenation of two sockets together. It establishes two bidirectional byte streams and transfers data between them.

socat supports several address types (e.g. TCP, UDP, UNIX domain sockets, etc.) to construct the streams. The latest version 1.7.4, released earlier this year [2021-01-04], supports also AF_VSOCK addresses:

  • VSOCK-LISTEN:<port>

    • Listen on port and accepts a VSOCK connection.
  • VSOCK-CONNECT:<cid>:<port>

    • Establishes a VSOCK stream connection to the specified cid and port.


If you are interested on VSOCK, I’ll talk witn Andra Paraschiv (AWS) about it at FOSDEM 2021. The talk is titled Leveraging virtio-vsock in the cloud and containers and it’s scheduled for Saturday, February 6th 2021 at 11:30 AM (CET).

We will show cool VSOCK use cases and some demos about developing, debugging, and measuring the VSOCK performance, including socat demos.


socat could be very useful for concatenating and redirecting sockets. In this section we will see some examples.

All examples below refer to a guest with CID 42 that we created using virt-builder and virt-install .

VM setup

virt-builder is able to download the installer and create the disk image with Fedora 33 or other distros. It is also able to set the root password and inject the ssh public key, simplifying the creation of guest disk image:


host$ virt-builder --root-password=password:mypassword \
        --ssh-inject root:file:/home/user/.ssh/ \
        --output=${VM_IMAGE} \
        --format=qcow2 --size 10G --selinux-relabel \
        --update fedora-33

Once the disk image is ready, we create our VM with virt-install. We can specify the VM settings like the number of vCPUs, the amount of RAM, and the CID assigned to the VM [42]:

host$ virt-install --name vsockguest \
        --ram 2048 --vcpus 2 --os-variant fedora33 \
        --import --disk path=${VM_IMAGE},bus=virtio \
        --graphics none --vsock cid.address=42

After the creation of the VM, we will remain attached to the console and we can detach from it by pressing ctrl-].

We can reattach to the console in this way:

host$ virsh console vsockguest

If the VM is turned off, we can boot it and attach directly to the console in this way:

host$ virsh start --console vsockguest

ncat like

It’s possible to use socat like ncat, transferring stdin and stdout via VSOCK.

Guest listening

In this example we start socat in the guest listening on port 1234:

guest$ socat - VSOCK-LISTEN:1234

Then we connect from the host using the CID 42 assigned to the VM:

host$ socat - VSOCK-CONNECT:42:1234

At this point we can exchange characters between guest and host, since stdin and stdout are linked through the VSOCK socket.

Host listening

In this example we do the opposite, starting socat in the host listening on port 1234:

host$ socat - VSOCK-LISTEN:1234

Then, in the guest, we connect to the host using the well defined CID 2. It’s always used to reach the host:

guest$ socat - VSOCK-CONNECT:2:1234

ssh over VSOCK

The coolest feature of socat is to concatenate sockets of different address families, so in this example we redirect ssh traffic through VSOCK socket exposed by the VM.

This example could be useful if the VM doesn’t have any NIC attached and we want to provide some network connectivity, like the ssh access.

First of all, in the guest we start socat linking the VSOCK socket listening on port 22, to a TCP socket which will connect to the local TCP port 22 where the ssh server is listening:

guest$ socat VSOCK-LISTEN:22,reuseaddr,fork TCP:localhost:22

On the host we link a TCP socket listening on a port of our choice (e.g. 4321) to the guest port 22 just opened using VSOCK:

host$ socat TCP4-LISTEN:4321,reuseaddr,fork VSOCK-CONNECT:42:22

Finally from the host we can connect to the guest using ssh on the local port 4321, where socat is listening:

host$ ssh -p 4321 root@localhost

socat redirects all the traffic between the sockets and allow us to use ssh over VSOCK to reach the guest.

by (Stefano Garzarella) at January 22, 2021 02:16 PM

January 19, 2021

QEMU project

Configuring virtio-blk and virtio-scsi Devices

The previous article in this series introduced QEMU storage concepts. Now we move on to look at the two most popular emulated storage controllers for virtualization: virtio-blk and virtio-scsi.

This post provides recommendations for configuring virtio-blk and virtio-scsi and how to choose between the two devices. The recommendations provide good performance in a wide range of use cases and are suitable as default settings in tools that use QEMU.

Virtio storage devices

Key points

  • Prefer virtio storage devices over other emulated storage controllers.
  • Use the latest virtio drivers.

Virtio devices are recommended over other emulated storage controllers as they are generally the most performant and fully-featured storage controllers in QEMU.

Unlike emulations of hardware storage controllers, virtio-blk and virtio-scsi are specifically designed and optimized for virtualization. The details of how they work are published for driver and device implementors in the VIRTIO specification.

Virtio drivers are available for Linux, Windows, and other operating systems. Installing the latest version is recommended for the latest bug fixes and performance enhancements.

If virtio drivers are not available, the AHCI (SATA) device is widely supported by modern x86 operating systems and can be used as a fallback. On non-x86 guests the default storage controller can be used as a fallback.

Comparing virtio-blk and virtio-scsi

Key points

  • Prefer virtio-blk in performance-critical use cases.
  • Prefer virtio-scsi for attaching more than 28 disks or for full SCSI support.
  • With virtio-scsi, use scsi-block for SCSI passthrough and otherwise use scsi-hd.

Two virtio storage controllers are available: virtio-blk and virtio-scsi.


The virtio-blk device presents a block device to the virtual machine. Each virtio-blk device appears as a disk inside the guest. virtio-blk was available before virtio-scsi and is the most widely deployed virtio storage controller.

The virtio-blk device offers high performance thanks to a thin software stack and is therefore a good choice when performance is a priority. It does not support non-disk devices such as CD-ROM drives.

CD-ROMs and in general any application that sends SCSI commands are better served by the virtio-scsi device, which has full SCSI support. SCSI passthrough was removed from the Linux virtio-blk driver in v5.6 in favor of using virtio-scsi.

Virtual machines that require access to many disks can hit limits based on availability of PCI slots, which are under contention with other devices exposed to the guest, such as NICs. For example a typical i440fx machine type default configuration allows for about 28 disks. It is possible to use multi-function devices to pack multiple virtio-blk devices into a single PCI slot at the cost of losing hotplug support, or additional PCI busses can be defined. Generally though it is simpler to use a single virtio-scsi PCI adapter instead.


The virtio-scsi device presents a SCSI Host Bus Adapter to the virtual machine. SCSI offers a richer command set than virtio-blk and supports more use cases.

Each device supports up to 16,383 LUNs (disks) per target and up to 255 targets. This allows a single virtio-scsi device to handle all disks in a virtual machine, although defining more virtio-scsi devices makes it possible to tune for NUMA topology as we will see in a later blog post.

Emulated LUNs can be exposed as hard disk drives or CD-ROMs. Physical SCSI devices can be passed through into the virtual machine, including CD-ROM drives, tapes, and other devices besides hard disk drives.

Clustering software that uses SCSI Persistent Reservations is supported by virtio-scsi, but not by virtio-blk.

Performance of virtio-scsi may be lower than virtio-blk due to a thicker software stack, but in many use cases, this is not a significant factor. The following graph compares 4KB random read performance at various queue depths:

Comparing virtio-blk and virtio-scsi performance

virtio-scsi configuration

The following SCSI devices are available with virtio-scsi:

Device SCSI Passthrough Performance
scsi-hd No Highest
scsi-block Yes Lower
scsi-generic Yes Lowest

The scsi-hd device is suitable for disk image files and host block devices when SCSI passthrough is not required.

The scsi-block device offers SCSI passthrough and is preferred over scsi-generic due to higher performance.

The following graph compares the sequential I/O performance of these devices using virtio-scsi with an iothread:

Comparing scsi-hd, scsi-block, and scsi-generic performance


The virtio-blk and virtio-scsi offer a choice between a single block device and a full-fledged SCSI Host Bus Adapter. Virtualized guests typically use one or both of them depending on functional and performance requirements. This post compared the two and offered recommendations on how to choose between them.

The next post in this series will discuss the iothreads feature that both virtio-blk and virtio-scsi support for increased performance.

by Stefan Hajnoczi and Sergio Lopez at January 19, 2021 07:00 AM

December 11, 2020

KVM on Z

New Publications


The following videos and publications are now available on the IBM Knowledge Center:

For more updates, check this blog entry.

by Stefan Raspl ( at December 11, 2020 08:04 PM

Stefan Hajnoczi

Building a git forge using git apps

The two previous blog posts about why git forges are von Neumann machines and the Radicle peer-to-peer git forge explored models for git forges. In this final post I want to cover yet another model that draws from the previous ones but has its own unique twist.

Peer-to-peer git apps

I previously showed how applications can be built on centralized git forges using CI/CD functionality for executing code, webhooks for interacting with the outside world, and disjoint branches for storing data.

A more elegant architecture is a peer-to-peer one where instead of many clients and one server there are just peers. Each peer has full access to the data. There is no client/server application code split, instead each peer runs an application for itself.

First, this makes it easier to move the data to new hosting infrastructure or fork a project since all data resides in the git repository. Merge requests, issues, wikis, and even the app settings are all stored in the git repo itself.

Second, this gives more power to the users who can process data however they want without being limited by the server's API. All peers are on equal footing and users don't need permission to alter applications, because they run locally.

Finally, it is easier to develop a local application than a client/server application. Being able to open a file and tweak the code is immediate and less hassle than testing and deploying a server-side application.

Internet peer-to-peer systems typically still require some central point for bootstrapping and this is no exception. A publicly-accessible git repository is still needed so that peers can fetch and push changes. However, in this model the git server does not run application code but "git apps" like merge requests, issue trackers, wikis, etc can still be implemented. Here is how it works...

The anti-application server

The git server is not allowed to run application code in our model, so apps like merge requests won't be processing data on the server side. However, the repository does need some primitives to make peer-to-peer git apps possible. These primitives are access control policies for refs and directories/files.

Peers run applications locally and the git server is "dumb" with the sole job of enforcing access control. You can imagine this like a multi-user UNIX machine where users have access to a shared directory. UNIX file permissions determine how processes can access the data. By choosing permissions carefully, multiple users can collaborate in the shared directory in a safe and controlled manner.

This is an anti-application server because no application code runs on the server side. The server is just a git repository that stores data and enforces access control on git push.

Access control

Repositories that accept push requests need a pre-receive hook (see githooks(5)) that checks incoming requests against the access control policy. If the request complies with the access control policy then the git push is accepted. Otherwise the git push is rejected and changes are not made to the git repository.

The first type of access control is on git refs. Git refs are the namespace where branches and tags are stored in a git repository. If a regular expression matches the ref and the operation type (create, fast-forward, force, delete) then it is allowed. For example, this policy rule allows any user to push to refs/heads/foo but force pushes and deletion are not allowed:

anyone create,fast-forward ^heads/foo$

The operations available on refs include:

create-branchPush a new branch that doesn't exist yet
create-tagPush a new tag that doesn't exist yet
fast-forwardPush a commit that is a descendent of the current commit
forcePush a commit or tag replacing the previous ref
deleteDelete a ref

What's more interesting is that $user_id is expanded to the git push user's identifier so we can write rules to limit access to per-user ref namespaces:

anyone create-branch,fast-forward,force,delete ^heads/$user_id/.*$

This would allow Alice to push her own branches but Alice could not push to Bob's branches.

We have covered how to define access control policies on refs. Access control policies are also needed on branches so that multiple users can modify the same branch in a controlled and safe manner. The syntax is similar but the policy applies to changes made by commits to directories/files (what git calls a tree). The following allows users to create files in a directory but not delete or modify them (somewhat similar to the UNIX restricted deletion or "sticky" bit on world-writable directories):

anyone create-file ^shared-dir/.*$

The operations available on branches include:

create-directoryCreate a new directory
create-fileCreate a new file
create-symlinkCreate a symlink
modifyChange an existing file or symlink
delete-fileDelete a file

$user_id expansion is also available for branch access control. Here the user can create, modify, and delete files in a per-user directory:

anyone create-file,modify,delete-file ^$user_id/.*$

User IDs

You might be wondering how user identifiers work. Git supports GPG-signed push requests with git push --signed. We can use the GPG key ID as the user identifier, eliminating the need for centralized user accounts. Remember that the GPG key ID is based on the public key. Key pairs are randomly generated and it is improbable that the same key will be generated by two different users. That said, GPG key ID uniqueness has been weak in the past when the default size was 32 bits. Git explicitly enables long 64-bit GPG key IDs but I wonder if collisions could be a problem. Maybe an ID with more bits based on the public key should be used instead, but for now let's assume the GPG key ID is unique.

The downside of this approach is that user IDs are not human-friendly. Git apps can allow the user to assign aliases to avoid displaying raw user IDs. Doing this automatically either requires an external ID issuer like confirming email address ownership, which is tedious for new users, or by storing a registry of usernames in the git repo, which means a first-come-first-server policy for username allocation and possible conflicts when merging from two repositories that don't share history. Due to these challenges I think it makes sense to use raw GPG key IDs at the data storage level and make them prettier at the user interface level.

The GPG key ID approach works well for desktop clients but not for web clients. The web application (even if implemently on the client side) would need access to the private key so it can push to the git repository. Users should not trust remotely hosted web applications with their private keys. Maybe there is a standard Web API that can help but I'm not aware one. More thought is needed here.

The pre-receive git hook checks that signature verification passed and has access to the GPG key ID in the GIT_PUSH_CERT_KEY environment variable. Then the access control policy can be checked.

Access control is a git app

Access control is the first and most fundamental git app. The access control policies that were described above are stored as files in the apps/access-control branch in the repository. Pushes to that branch are also subject to access control checks. Here is the branch's initial layout:

branches/ - access control policies for branches
groups/ - group definitions (see below)
refs/ - access control policies for refs

The default branches/owner.conf access control policy is as follows:

owner create-file,create-directory,modify,delete ^.*$

The default refs/owner.conf access control policy is as follows:

owner create-branch,create-tag,fast-foward,force,delete ^.*$

This gives the owner the ability to push refs and modify branches as they wish. The owner can grant other users access by pushing additional access control policy files or changing exsting files on the apps/access-control branch.

Each access control policy file in refs/ or branches/ is processed in turn. If no access control rule matches the operation then the entire git push is rejected.

Groups can be defined to alias one or more user identifiers. This avoids duplicating access control rules when more than one user should have the same access. There are two automatic groups: owner contains just the user who owns the git repository and anyone is the group of all users.

This completes the description of the access control app. Now let's look at how other functionality is built on top of this.

The merge requests app

A merge requests app can be built on top of this model. The refs access control policy is as follows:

# The data branch contains the titles, comments, etc
anyone modify ^apps/merge-reqs/data$

# Each merge request revision is pushed as a tag in a per-user namespace
anyone create-tag ^apps/merge-reqs/$user_id/[0-9]+-v[0-9]+$

The branch access control policy is:

# Merge requests are per-user and numbered
anyone create-directory ^merge-reqs/$user_id/[0-9]+$

# Title string
anyone create-file,modify ^merge-reqs/$user_id/[0-9]+/title$

# Labels (open, needs-review, etc) work like this:
# merge-reqs/<user-id>/<merge-req-num>/labels/
# needs-review -> /labels/needs-review
# ...
# labels/
# needs-review/
# <user-id>/
# <merge-req-num> -> /merge-reqs/<user-id>/<merge-req-num>
# ...
# ...
# ...
# This directory and symlink layout makes it possible to enumerate labels for a
# given merge request and to enumerate merge requests for a given label.
# Both the merge request author and maintainers can add/remove labels to/from a
# merge request.
anyone create-directory ^merge-reqs/[^/]+/[0-9]+/labels$
anyone create-symlink,delete ^merge-reqs/$user_id/[0-9]+/labels/.*$
maintainers create-symlink,delete ^merge-reqs/[^/]+/[0-9]+/labels/.*$
maintainers create-directory ^labels/[^/]+$
anyone create-symlink,delete ^labels/[^/]+/$user_id/[0-9]+$
maintainers create-symlink,delete ^labels/[^/]+/[^/]+/[0-9]+$

# Comments are stored as individual files in per-user directories. Each file
# contains a timestamp and the contents of the comment. The timestamp can be
# used to sort comments chronologically.
anyone create-directory ^merge-reqs/[^/]+/[0-9]+/comments$
anyone create-directory ^merge-reqs/[^/]+/[0-9]+/comments/$user_id$
anyone create-file,modify ^merge-reqs/[^/]+/[0-9]+/comments/$user_id/[0-9]+$

When a user creates a merge request they provide a title, an initial comment, apply labels, and push a v1 tag for review and merging. Other users can comment by adding files into the merge request's per-user comments directory. Labels can be added and removed by changing symlinks in the labels directories.

The user can publish a new revision of the merge request by pushing a v2 tag and adding a comment describing the changes. Once the maintainers are satisfied they merge the final revision tag into the relevant branch (e.g. "main") and relabel the merge request from open/needs-review to closed/merged.

This workflow can be implemented by a tool that performs the necessary git operations so users do not need to understand the git app's internal data layout. Users just need to interact with the tool that displays merge requests, allows commenting, provides searches, etc. A natural way to implement this tool is as a git alias so it integrates alongside git's built-in commands.

One issue with this approach is that it uses the file system as a database. Performance and scalability are likely to be worst than using a database or application-specific file format. However, the reason for this approach is that it allows the access control app to enforce a policy that ensures users cannot modify or delete other user's data without running application-specific code on the server and while keeping everything stored in a git repository.

An example where this approach performs poorly is for full-text search. The application would need to search all title and comment files for a string. There is no index for efficient lookups. However, if applications find that git-grep(1) does not perform well they can maintain their own index and cache files locally.

I hope that this has shown how git apps can be built without application code running on the server.

Continuous integration bots

Now that we have the merge requests app it's time to think how a continuous integration service could interface with it. The goal is to run tests on each revision of a merge request and report failures so the author of the merge request can rectify the situation.

A CI bot watches the repository for changes. In particular, it needs to watch for tags created with the ref name apps/merge-reqs/[^/]+/[0-9]+-v[0-9]+.

When a new tag is found the CI bot checks it out and runs tests. The results of the tests are posted as a comment by creating a file in merge-regs/<user-id>/>merge-req-num>/comments/<ci-bot-user-id>/0 on the apps/merge-reqs/data branch. A ci-pass or ci-fail label can also be applied to the merge request so that the CI status can be easily queried by users and tools.

Going further

There are many loose ends. How can non-git users participate on issue trackers and wikis? It might be possible to implement a full peer as a client-side web application using isomorphic-git, a JavaScript git implementation. As mentioned above, the GPG key ID approach is not very browser-friendly because it requires revealing the private key to the web page and since keys are user identifiers using temporary keys does not work well.

The data model does not allow efficient queries. A full copy of the data is necessary in order to query it. That's acceptable for local applications because they can maintain their own indexes and are expected to keep the data for a long period of time. It works less well for short-lived web page sessions like a casual user filing a new bug on the issue tracker.

The git push --signed technique is not the only option. Git also supports signed commits and signed tags. The difference between signed pushes and signed tags/commits is significant. The signed push approach only validates the access control policy when the repository is changed and leaves no audit log for future reference. The signed commit/tag approach keeps the signatures in the git history. Signed commits/tags can be propagated in a peer-to-peer network and each peer can validate the access control policy itself. While signed commits/tags apply the access control policy to each object in the repository, signed pushes apply the access control policy to each change made to the repository. The difference is that it's easy to rebase and include work from different authors with signed pushes. Signed commits/tags require re-signing for rebasing and each commit is validated against its signature, which may be different from the user who is making the push request.

There are a lot of interesting approaches and trade-offs to explore here. This model we've discussed fits closely with how I've seen developers use git in open source projects. It is designed around a "main" repository/server that contributors push their code to. But each clone of the repository has all the data and can be published as a new "main" repository, if necessary.

Although these ideas are unfinished I decided to write them up with the knowledge that I probably won't implement them myself. QEMU is moving to GitLab with a traditional centralized git forge. I don't think this is the right time to develop this idea and try to convince the QEMU community to use it. For projects that have fewer infrastructure requirements it would give their contributors more power than being confined to a centralized git forge.

I hope this was an interesting read for anyone thinking about git forges and building git apps.

by Unknown ( at December 11, 2020 02:17 PM

December 08, 2020

QEMU project

QEMU version 5.2.0 released

We’d like to announce the availability of the QEMU 5.2.0 release. This release contains 3200+ commits from 216 authors.

You can grab the tarball from our download page. The full list of changes are available in the Wiki.

Note that QEMU has switched build systems so you will need to install ninja to compile it. See the “Build Information” section of the Changelog for more information about this change.

Highlights include:

  • block: support for using qemu-storage-daemon as a vhost-user-blk device backend, and new ‘block-export-add’ QMP command which replaces now-deprecated ‘nbd-server-add’ with support for qemu-storage-daemon
  • block: qcow2 support for subcluster-based allocation (via extended_l2=on qemu-img option), improvements to NBD client network stalls, and qemu-nbd support for exposing multiple dirty bitmaps at once
  • migration: higher-bandwidth encrypted migration via TLS+multifd, new ‘block-bitmap-mapping’ option for finer-grained control over which bitmaps to migrate, and support for migration over a ‘vsock’ device (for nested environments and certain hardware classes)
  • qemu-ga: support for guest-get-devices, guest-get-disks, and guest-ssh-{get,add-remove}-authorized-keys commands.
  • virtiofs: virtiofsd support for new options to control how xattr names are seen by the guest, specify sandboxing alternative to pivot_root, and allowing different host mounts to be seen as separate submounts in the guest to avoid inode clashes
  • ARM: new board support for mp2-an386 (Cortex-M4 based), mp2-an500 (Cortex-M7 based), raspi3ap (Raspberry Pi 3 model A+), raspi0 (Raspberry Pi Zero), raspi1ap (Raspberry Pi A+), and npcm750-evb/quanta-gsj (Nuvoton iBMC)
  • ARM: ARMv8.2 FEAT_FP16 (half-precision flaoting point) support for AArch32 (already supported for AArch64)
  • ARM: virt: support for kvm-steal-time accounting
  • HPPA: support for booting NetBSD and older Linux distros like debian-0.5 and debian-0.6.1
  • PowerPC: pseries: improved support for user-specified NUMA distance topologies
  • RISC-V: live migration support
  • RISC-V: experimental hypervisor support updated to v0.6.1 and other improvements
  • RISC-V: support for NUMA sockets on virt/Spike machine types
  • s390: KVM support for diagnose 0x318 instruction, TCG support for additional z14 instructions
  • s390: vfio-pci devices now report real hardware features for functions instead of just emulated values
  • Xtensa: DFPU co-processor with single/double-precision FP opcodes is now supported
  • x86: improved support for asynchronous page faults via new kvm-async-pf-int -cpu option
  • and lots more…

Thank you to everyone involved!

December 08, 2020 09:00 PM

December 06, 2020

Stefan Hajnoczi

Git Forge Apps: Why git forges are serverless computing providers

In this post I want to explore an idea for a new type of application made possible by the power of git forges like GitLab and GitHub. I don't have a proof-of-concept and in fact we'll discuss hurdles that could make the idea infeasible. But it's an interesting way of thinking that is worth sharing as it offers a new way of looking at and using git forges.

Git forges offer git repository hosting at their core and then add on related features like pull requests, wikis, issue trackers, static website hosting, continuous integration, and more. They are now a long way away from the initial idea of a hosted git repository where you can publish code. They do so much more. They have a von Neumann architecture and are Turing complete. It's possible to do interesting things with that.

A git forge app (GFA) is an application that runs on a git forge. A hosted git repository is not just a place to publish and back up the source code. It's where the app runs. You can view the git forge as a Platform-as-a-Service (PaaS) or serverless computing provider. The app doesn't need to be deployed elsewhere, because the git forge itself is the execution environment.

A GFA must be able to:

  1. Process data.
  2. Store data.
  3. Interact with the outside world.

This is basically what computer applications do. Git forges have become powerful enough to do this.

Imagine the following: you fork a repository and this instantiates the GFA. The GFA is now running under your git forge account. The web interface is available at https://<user><app>/ and HTTP POST requests can also be used to interact with the application. The GFA could be a todo list, an RSS reader, a blog, etc. To the website visitor it appears like any other web application but everything happens within the git forge and no other hosting provider is necessary.

Let's look at how it's possible to use a git forge as the execution environment for an app.

Processing Data

The first order of business is executing code so that the application can process data. A configuration file is placed into a git repository to define runnable jobs. The job execution systems offered by git forges are GitLab CI and GitHub Actions, respectively. When the job is triggered it executes in a temporary environment, for example a Linux container image. It allows code hosted in the git repository to run on a server somewhere for a little while. There is a free allowance that grants a certain number of hours of unpaid execution time. This is how GFAs process data.

For example, say we are building an RSS reader. Our repo looks like this:

.gitlab-ci.yml - the job configuration file - download RSS feeds and check for new posts

The .gitlab-ci.yml file will contain a scheduled pipeline that runs every 15 minutes.

Storing Data

Applications need to store data persistently. The primary way of doing that in a git forge is by storing data in the git repository itself. Mutable data can be kept on a separate branch called data and can be force-pushed to avoid forever increasing the repository size when we don't need to store previous revisions of the data.

This works because each git branch has an independent commit history. The file namespace it stores is completely separate from other branches. This means it's possible to have the GFA's code on the main branch and to store data on the data branch without upsetting the commit history or files on the main branch.

Data storage is available to jobs because they are allowed to manipulate their own git repository thanks to an authentication token. On GitHub GITHUB_TOKEN allows jobs to push to their git repository. This gives jobs a way of storing data.

There are other forms of data storage besides the git repository. Artifactsare files produced by a job that can be downloaded via a URL. Artifacts are subject to expiry time and file size limitations. Caching is available to temporarily store files between job runs, although this data can be lost at any time.

At this point you may be thinking that this is nice but there is no way to store private data if the git repository is public. Git forges offer a secrets mechanism where variables can be stored privately and only made available during job execution. This can be used to stash an encryption key so that the data is stored in public but is encrypted. Anyone can download the encrypted data but they will not have the key needed to decrypt it. Some applications can also take advantage of client-side encryption and homomorphic encryption.

Interacting with the Outside World

Git forges offer a wealth of ways to interact with the outside world, called triggers. Triggers can run a job when an HTTP POST request is made. POST requests typically need a trigger token for authentication, but that token can be published if the triggered job is safe to be invoked by anyone. Both browser clients and Webhooks can invoke triggers through HTTP POST requests.

For example, imagine our RSS reader app needs an API to mark feeds as read. We define an HTTP POST trigger that runs a script that updates the feeds stored in the data branch. Since only the git repository owner should be able to invoke this trigger we do not publish the trigger token. Instead it is stored encrypted in the repository and the user provides a passphrase for decrypting this secret.

At this point it is also useful to offer a web interface. This is possible thanks to the static pages hosting feature called GitLab Pages and GitHub Pages. Pushing HTML, JavaScript, CSS, and images to a special branch publishes the static website at https://<user><app>/.

For the most part GFAs are asynchronous, they cannot handle HTTP requests synchronously like a web application that outputs an HTTP response. Incoming HTTP requests simply schedule GFA jobs that run sometime later. There are a few ways around this. The browser can poll for results using XMLHttpRequest or similar techniques. Alternatively, a GFA can fire up a job that runs for several minutes although I haven't investigated if there is a practical way to communicate with a web application running in a job (I guess incoming HTTPS is tricky to achieve).

Triggers offer a lot more fun than what I've already covered. They can respond to the creation and modification of wiki pages, issues, and pull request comments. This means it's possible to use those entities for interacting with the outside world. The GFA could act as a chat bot on its issue tracker, for example.


Git forges weren't designed for GFAs. But then they weren't initially designed to be Turing complete and offering ways to interact with the outside world either. That was added incrementally as demand for that functionality grew. Maybe git forges will evolve into full-blown serverless hosting competitors to today's cloud hosting providers.

GFAs are a hack that uses features like static pages, CI jobs, and webhooks in a creative way to run applications on a git forge. Building GFAs that are actually useful is likely to hit some challenges:

  • No synchronous requests - it is hard to implement search queries and other dynamic behavior in GFAs because they are primarily asynchronous (and slow!). This limitation matters for certain classes of applications. A lot can be done client-side instead to make up for this deficiency. But maybe someone can figure out how to do synchronous requests in GFAs.
  • Security - I have outlined how to make data publicly available and also how to encrypt it so only the git repository owner can view the plaintext. This is enough for personal web applications, but it's not sufficient for multi-user applications. Maybe git forges offer a form of authentication that works with GFAs so multiple users can store private data in a single GFA instance (the git repository owner could view all users' data but other users could not).
  • Free usage tiers - job execution time, storage capacity, and request throttling will limit how resource-intensive a GFA can be before it outgrows the free tier and eventually even the paid tier.

The first generation of GFAs could be written from scratch with each job carefully designed. Then a second generation of GFAs could be built on top of frameworks that abstract the tedious git forge integration with standard APIs and data models. For example, a Vue.js frontend could use a key/value store API and the whole thing can be hosted as a GFA.

Even if GFAs don't become a thing because git forges decide there is not enough demand to make them work really well, changing your perspective to think of git forges in this way is valuable. For example, I have a git repo for building a container image that I deploy on my server and that pushes output files to a web server host. All of that can be replaced with a git repository that runs a job and publishes to GitLab Pages. This simplifies things and frees me from maintaining infrastructure.


Git forges offer jobs for processing data, git repositories and artifacts for storing data, and triggers for interacting with the outside world. It is possible to build applications that exist solely as a git repository on a git forge. There is no longer a need to deploy code because the git forge itself is powerful enough to run non-trivial applications. I look forward to how this evolves and whether git forges eventually become full-blown cloud service providers.

by Unknown ( at December 06, 2020 10:02 AM

Understanding Peer-to-Peer Git Forges with Radicle

Git is a distributed version control system and does not require a central server. Although repositories are usually published at a well-known location for convenient cloning and fetching of the latest changes, this is actually not necessary. Each clone can have the full commit history and evolve independently. Furthermore, code changes can be exchanged via email or other means. Finally, even the clone itself does not need to be made from a well-known domain that hosts a git repository (see git-bundle(1)).

Given that git itself is already fully decentralized one would think there is no further work to do. I came across the Radicle project and its somewhat psychedelic website. Besides having a website with a wild color scheme, the project aims to offer a social coding experiment or git forge functionality using a peer-to-peer network architecture. According to the documentation the motivation seems to be that git's built-in functionality works but is not user-friendly enough to make it accessible. In particular, it lacks social coding features.

The goal is to add git forge features like project and developer discovery, issue trackers, wikis, etc. Additional, distinctly decentralized functionality, is also touched on involving Ethereum as a way to anchor project metadata, pay contributors, etc. Radicle is still in early development so these features are not yet implemented. Here is my take on the How it Works documentation, which is a little confusing due to its early stage and some incomplete sentences or typos. I don't know whether my understanding actually corresponds to the Radicle implementation that exists today or its eventual vision, because I haven't studied the code or tried running the software. However, the ideas that the documentation has brought up are interesting and fruitful in their own right, so I wanted to share them and explain them in my own words in case you also find them worth exploring.

The git data model

Let's quickly review the git data model because it is important for understanding peer-to-peer git forges. A git repository contains a refs/ subdirectory that provides a namespace for local branch heads (refs/heads/), local and remotely fetched tags (refs/tags/), and remotely fetched branches (refs/remotes/<remote>/). Actually this namespace layout is just a convention for everyday git usage and it's possible to use the refs/ namespace differently as we will see. The git client fetches refs from a remote according to a refspec rule that maps remote refs to local refs. This gives the client the power to fetch only certain refs from the server. The client can also put them in a different location in its local refs/ directory than the server. For details, see the git-fetch(1) man page.

Refs files contain the commit hash of an object stored in the repository's object database. An object can be a commit, tree (directory), tag, or a blob (file). Branch refs point to the latest commit object. A commit object refers to a tree object that may refer to further tree objects for sub-directories and finally the blob objects that make up the files being stored. Note that a git repository supports disjoint branches that share no history. Perhaps the most well-known example of disjoint branches are the GitHub Pages and GitLab Pages features where these git forges publish static websites from the HTML/CSS/JavaScript/image files on a specific branch in the repository. That branch shares no version history with other branches and the directories/files typically have no similarity to the repository's main branch.

Now we have covered enough git specifics to talk about peer-to-peer git forges. If you want to learn more about how git objects are actually stored, check out my article on the repository layout and pack files.

Identity and authority

Normally a git repository has one or more owners who are allowed to push refs. No one else has permission to modify the refs namespace. What if we tried to share a single refs namespace with the whole world and everyone could push? There would be chaos due to naming conflicts and malicious users would delete or change other users' refs. So it seems like an unworkable idea unless there is some way to enforce structure on the global refs namespace.

Peer-to-peer systems have solutions to these problems. First, a unique identity can be created by picking a random number with a sufficient number of bits so that the chance of collision is improbable. That unique identity can be used as a prefix in the global ref namespace to avoid accidental collisions. Second, there needs to be a way to prevent unauthorized users from modifying the part of the global namespace that is owned by other users.

Public-key cryptography provides the primitive for achieving both these things. A public key or its hash can serve as the unique identifier that provides identity and prevents accidental collisions. Ownership can be enforced by verifying that changes to the global namespace are signed with the private key corresponding to the unique identity.

For example, we fetch the following refs from a peer:


This is a simplified example based on the Radicle documentation. Here identity is the unique identity based on a public key. Remember no one else in the world has the same identity because the chance of generating the same public key is improbable. The heads/ refs are normal git refs to commit objects - these are published branches. The signed_refs ref points to an git object that contains a list of commit hashes and a signature generated using the public key. The signature can be verified using the public key.

Next we need to verify these changes to check that they were created with the private key that is only known to the identity's owner. First, we check the signature on the object pointed to by the signed_refs ref. If the signature is not valid we reject these changes and do not store them in our local repository. Next, we look up each ref in heads/ against the list in signed_refs. If a ref is missing from the list then we reject these refs and do not allow them into our local repository.

This scheme lends itself to peer-to-peer systems because the refs can be propagated (copied) between peers and verified at each step. The identity owner does not need to be present at each copy step since their cryptographic signature is all we need to be certain that they authorized these refs. So I can receive refs originally created by identity A from peer B and still be sure that peer B did not modify them since identity A's signature is intact.

Now we have a global refs namespace that is partitioned so that each identity is able to publish refs and peers can verify that these changes are authorized.


It may not be clear yet that it's not necessary to clone the entire global namespace. In fact, it's possible that no single peer will ever have a full copy of the entire global namespace! That's because this is a distributed system. Peers only fetch refs that they care about from their peers. Peers fetch from each other and this forms a network. The network does not need to be fully connected and it's possible to have multiple clusters of peers running without full global connectivity.

To bootstrap the global namespace there are seed repositories. Seeds are a common concept in peer-to-peer systems. They provide an entry point for new peers to learn about and start participating with other peers. In BitTorrent this is called a "tracker" rather than a "seed".

According to the Radicle documentation it is possible to directly fetch from peers. This probably means a git-daemon(1) or git-http-backend(1) needs to be accessible on the public internet. Many peers will not have sufficient network connectivity due to NAT limitations. I guess Radicle does not expect every user to participate as a repository.

Interestingly, there is a gossip system for propagating refs through the network. Let's revisit the refs for an identity in the global namespace:


We can publish identities that we track in remotes/. It's a recursive refs layout. This is how someone tracking our refs can find out about related identities and their refs.

Thanks to git's data model the commit, tree, and blob objects can be shared even though we duplicate refs published by another identity. Since git is a content-addressable object database the data is stored once even though multiple refs point to it.

Now we not only have a global namespace where anyone can publish git refs, but also ways to build a peer-to-peer network and propagate data throughout the network. It's important to note that data is only propagated if peers are interested in fetching it. Peers are not forced to store data that they are not interested in.

How data is stored locally

Let's bring the pieces together and show how the system stores data. The peer creates a local git repository called the monorepo for the purpose of storing portions of the global namespace. It fetches refs from seeds or direct peers to get started. Thanks to the remotes/ refs it also learns about other refs on the network that it did not request directly.

This git repository is just a data store, it is not usable for normal git workflows. The conventional git branch and git tag commands would not work well with the global namespace layout and verification requirements. Instead we can clone a local file:/// repository from the monorepo that fetches a subset of the refs into the conventional git refs layout. The files can be shared because git-clone(1) supports hard links to local repositories. Thanks to githooks(5) and/or extensible git-push(1) remote helper support it's possible to generate the necessary global namespace metadata (e.g. signatures) when we push from the local clone to the local monorepo. The monorepo can then publish the final refs to other peers.

Building a peer-to-peer git forge

There are neat ideas in Radicle and it remains to be seen how well it will grow to support git forge functionality. A number of challenges need to be addressed:

  • Usability - Radicle is a middle-ground between centralized git forges and email-based decentralized development. The goal is to be easy to use like git forges. Peer-to-peer systems often have challenges providing a human-friendly interface on top of public key identities (having usernames without centralized user accounts). Users will probably prefer to think in terms of repositories, merge requests, issues, and wikis instead of peers, gossip, identities, etc.
  • Security - The global namespace and peer-to-peer model is a target that malicious users will attack by trying to impersonate or steal identities, flood the system with garbage, game reputation systems with sockpuppets, etc.
  • Scalability - Peers only care about certain repositories and don't want to be slowed down by all the other refs in the global namespace. The recursive refs layout seems like it could cause performance problems and maybe users will configure clients to limit the depth to a low number like 3. At first glance Radicle should be able to scale well since peers only need to fetch refs they are interested in, but it's a novel way of using git refs, so we can expect scalability problems as the system grows.
  • Data model - How will this data model grow to handle wikis, issue trackers, etc? Issue tracker comments are an example of a data structure that requires conflict resolution in a distributed system. If two users post comments on an issue, how will this be resolved without a conflict? Luckily there is quite a lot of research on distributed data structures such as Conflict-free Replicated Data Types (CRDTs) and it may be possible to avoid most conflicts by eliminating concepts like linear comment numbering.
  • CI/CD - As mentioned in my blog post about why git forges are von Neumann machines, git forges are more than just data stores. They also have a computing model, initially used for Continuous Integration and Continuous Delivery, but really a general serverless computing platform. This is hard to do securely and without unwanted resource usage in a peer-to-peer system. Maybe Radicle will use Ethereum for compute credits?


Radicle is a cool idea and I look forward to seeing where it goes. It is still at an early stage but already shows interesting approaches with the global refs namespace and monorepo data store.

by Unknown ( at December 06, 2020 10:02 AM

December 03, 2020

Alberto Garcia

Subcluster allocation for qcow2 images

In previous blog posts I talked about QEMU’s qcow2 file format and how to make it faster. This post gives an overview of how the data is structured inside the image and how that affects performance, and this presentation at KVM Forum 2017 goes further into the topic.

This time I will talk about a new extension to the qcow2 format that seeks to improve its performance and reduce its memory requirements.

Let’s start by describing the problem.

Limitations of qcow2

One of the most important parameters when creating a new qcow2 image is the cluster size. Much like a filesystem’s block size, the qcow2 cluster size indicates the minimum unit of allocation. One difference however is that while filesystems tend to use small blocks (4 KB is a common size in ext4, ntfs or hfs+) the standard qcow2 cluster size is 64 KB. This adds some overhead because QEMU always needs to write complete clusters so it often ends up doing copy-on-write and writing to the qcow2 image more data than what the virtual machine requested. This gets worse if the image has a backing file because then QEMU needs to copy data from there, so a write request not only becomes larger but it also involves additional read requests from the backing file(s).

Because of that qcow2 images with larger cluster sizes tend to:

  • grow faster, wasting more disk space and duplicating data.
  • increase the amount of necessary I/O during cluster allocation,
    reducing the allocation performance.

Unfortunately, reducing the cluster size is in general not an option because it also has an impact on the amount of metadata used internally by qcow2 (reference counts, guest-to-host cluster mapping). Decreasing the cluster size increases the number of clusters and the amount of necessary metadata. This has direct negative impact on I/O performance, which can be mitigated by caching it in RAM, therefore increasing the memory requirements (the aforementioned post covers this in more detail).

Subcluster allocation

The problems described in the previous section are well-known consequences of the design of the qcow2 format and they have been discussed over the years.

I have been working on a way to improve the situation and the work is now finished and available in QEMU 5.2 as a new extension to the qcow2 format called extended L2 entries.

The so-called L2 tables are used to map guest addresses to data clusters. With extended L2 entries we can store more information about the status of each data cluster, and this allows us to have allocation at the subcluster level.

The basic idea is that data clusters are now divided into 32 subclusters of the same size, and each one of them can be allocated separately. This allows combining the benefits of larger cluster sizes (less metadata and RAM requirements) with the benefits of smaller units of allocation (less copy-on-write, smaller images). If the subcluster size matches the block size of the filesystem used inside the virtual machine then we can eliminate the need for copy-on-write entirely.

So with subcluster allocation we get:

  • Sixteen times less metadata per unit of allocation, greatly reducing the amount of necessary L2 cache.
  • Much faster I/O during allocation when the image has a backing file, up to 10-15 times more I/O operations per second for the same cluster size in my tests (see chart below).
  • Smaller images and less duplication of data.

This figure shows the average number of I/O operations per second that I get with 4KB random write requests to an empty 40GB image with a fully populated backing file.

I/O performance comparison between traditional and extended qcow2 images

Things to take into account:

  • The performance improvements described earlier happen during allocation. Writing to already allocated (sub)clusters won’t be any faster.
  • If the image does not have a backing file chances are that the allocation performance is equally fast, with or without extended L2 entries. This depends on the filesystem, so it should be tested before enabling this feature (but note that the other benefits mentioned above still apply).
  • Images with extended L2 entries are sparse, that is, they have holes and because of that their apparent size will be larger than the actual disk usage.
  • It is not recommended to enable this feature in compressed images, as compressed clusters cannot take advantage of any of the benefits.
  • Images with extended L2 entries cannot be read with older versions of QEMU.

How to use this?

Extended L2 entries are available starting from QEMU 5.2. Due to the nature of the changes it is unlikely that this feature will be backported to an earlier version of QEMU.

In order to test this you simply need to create an image with extended_l2=on, and you also probably want to use a larger cluster size (the default is 64 KB, remember that every cluster has 32 subclusters). Here is an example:

$ qemu-img create -f qcow2 -o extended_l2=on,cluster_size=128k img.qcow2 1T

And that’s all you need to do. Once the image is created all allocations will happen at the subcluster level.

More information

This work was presented at the 2020 edition of the KVM Forum. Here is the video recording of the presentation, where I cover all this in more detail:

You can also find the slides here.


This work has been possible thanks to Outscale, who have been sponsoring Igalia and my work in QEMU.

Igalia and Outscale

And thanks of course to the rest of the QEMU development team for their feedback and help with this!

by berto at December 03, 2020 06:15 PM

December 02, 2020

Stefan Hajnoczi

Software Freedom Conservancy announces donation matching

Software Freedom Conservancy, the non-profit charity home of QEMU, Git, Inkscape, and many other free and open source software projects is running its annual fundraiser. They have announced a generous donation matching pledge so donations made until January 15th 2021 will be doubled! Full details are here.

What makes Software Freedom Conservancy important, besides being a home for numerous high-profile free and open source software projects, is that it is backed by individuals and works for the public interest. It is not a trade association funded by companies to represent their interests. With the success of free and open source software it's important we don't lose these freedoms or use them just to benefit businesses. That's why I support Software Freedom Conservancy.

by Unknown ( at December 02, 2020 12:47 PM

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