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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 allocating 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

December 01, 2020

Thomas Huth

QEMU Advent Calendar 2020 starts

Starting today, on December 1st, the first door of the QEMU Advent Calendar 2020 can now be opened! The advent calendar reveals a new disk image or something similar for download on each of the first 24 days in December 2020, to create a fun experience for the QEMU community, brightening your days in the winter season, and to provide some good images for testing various aspects of QEMU. There is also the possibility to contribute your own ideas for disk images - so if you’re interested in helping, please have a look at the announcement in the QEMU blog for details.

December 01, 2020 06:15 AM

November 30, 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 November 30, 2020 09:27 AM

November 27, 2020

Stefan Hajnoczi

Call for QEMU Advent Calendar 2020 disk images

QEMU Advent Calendar publishes a disk image surprise each day from December 1-24. It's a QEMU community tradition that is back again this year!

If you want to contribute disk images to this year's advent calendar (puzzles, games, niche operating systems, retro computing fun, etc), please check out the call for disk images for details.

by Unknown ( at November 27, 2020 07:19 AM

November 26, 2020

Thomas Huth

Secure Execution for zKVM Introduction and Demo

The recent generation of the mainframe (i.e. the z15) has the possibility to run protected KVM guests, so that the administrator of the host LPAR does not have the possibility anymore to read or alter the memory of a guest. This feature is called “Secure Execution” on the IBM Z.

IBM now published a nice introduction and demo of this new feature, which is in my opinion quite worthwhile to watch if you want to get a basic idea about running protected guests:

The video contains a short introduction into the concepts of the secure execution feature, followed by a demo that shows how to prepare a protected guest VM.

November 26, 2020 01:45 PM

QEMU project

QEMU Advent Calendar 2020 Announcement and Call for Images

QEMU Advent Calendar 2020 is around the corner and we are looking for volunteers to contribute disk images that showcase something cool, bring back retro computing memories, or simply entertain with a puzzle or game.

QEMU Advent Calendar publishes a QEMU disk image each day from December 1-24. Each image is a surprise designed to delight an audience consisting of the QEMU community and beyond. You can see previous years at

You can help us make this year’s calendar awesome by:

Here are the requirements for disk images:

  • Content must be freely redistributable (i.e. no proprietary license that prevents distribution). For GPL based software, you need to provide the source code, too.
  • Provide a name and a short description of the disk image (e.g. with hints on what to try)
  • Provide a ./run shell script that prints out the name and description/hints and launches QEMU
  • Provide a 320x240 screenshot/image/logo for the website
  • Size should be ideally under 100 MB per disk image (but if some few images are bigger, that should be OK, too)

Check out this disk image as an example of how to distribute an image. Links to files over 25MB are appreciated in lieu of email attachments.

PS: QEMU Advent Calendar is a secular calendar (not religious). The idea is to create a fun experience for the QEMU community which can be shared with everyone. You don’t need to celebrate Christmas or another religious festival to participate!

November 26, 2020 09:00 AM

November 24, 2020

Cornelia Huck

s390x changes in QEMU 5.2

 As, once again, a new QEMU release is around the corner, the time has come to list some s390x changes in there.

  • TCG has gained emulation support for some additional instructions that had been introduced with the z14. More enhancements needed to be able to run distributions built for the z14 will likely come in the future.
  • When running under KVM, QEMU now supports the diagnose 0x318 instruction. This can be used to set some diagnostic information (such as the operating system), which may be helpful when servicing the hardware. With this comes support for extended SCCBs; this is needed as the facility indication for diag318 encroaches into the control block used for reporting CPU information. A guest needs support for extended SCCBs to be able to see information for all CPUs if diag318 support is provided.
  • You can now use virtiofs on s390x, thanks to some endianness fixes, and a vhost-user-fs-ccw device has been added.
  • Up to now, both fully emulated PCI functions and PCI functions passed via vfio-pci reported the same values when the guest issued CLP instructions. However, the passed through functions may use different values for things such as the supported DMA range. If the host kernel supplies the respective capabilities for the vfio-pci device, QEMU can now provide the real values in the CLP queries.
  • zPCI is now also able to honour vfio DMA limits, if passed via the vfio-pci device, and can trigger the guest to flush its DMA mappings when needed.
  • The s390-ccw bios now tries harder to find a bootable device, if the first device is not suitable. This brings s390x booting a bit closer to what other architectures do.
  • And the usual fixes and cleanups.

by Cornelia Huck ( at November 24, 2020 04:22 PM

November 17, 2020

KVM on Z

RHEL 8.3 Released

RHEL 8.3 is out, see the official announcement.
The unquestionable highlight from a KVM on Z perspective is certainly the addition of the Secure Execution functionality. Also, ECKD DASD can be transparently passed through to KVM guests, allowing full exploitation of all DASD features, including raw track access and IPL

For further details on the changes, see the release notes, and the respective blog entry in the Linux on Z blog here.

by Stefan Raspl ( at November 17, 2020 09:31 AM

November 11, 2020

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.

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 November 11, 2020 04:12 PM

November 04, 2020

QEMU project

Using virtio-fs on a unikernel

This article provides an overview of virtio-fs, a novel way for sharing the host file system with guests and OSv, a specialized, lightweight operating system (unikernel) for the cloud, as well as how these two fit together.


Virtio-fs is a new host-guest shared filesystem, purpose-built for local file system semantics and performance. To that end, it takes full advantage of the host’s and the guest’s colocation on the same physical machine, unlike network-based efforts, like virtio-9p.

As the name suggests, virtio-fs builds on virtio for providing an efficient transport: it is included in the (currently draft, to become v1.2) virtio specification as a new device. The protocol used by the device is a slightly extended version of FUSE, providing a solid foundation for all file system operations native on Linux. Implementation-wise, on the QEMU side, it takes the approach of splitting between the guest interface (handled by QEMU) and the host file system interface (the device “backend”). The latter is handled by virtiofsd (“virtio-fs daemon”), running as a separate process, utilizing the vhost-user protocol to communicate with QEMU.

One prominent performance feature of virtio-fs is the DAX (Direct Access) window. It’s a shared memory window between the host and the guest, exposed as device memory (a PCI BAR) to the second. Upon request, the host (QEMU) maps file contents to the window for the guest to access directly. This bears performance gains due to taking VMEXITs out of the read/write data path and bypassing the guest page cache on Linux, while not counting against the VM’s memory (since it’s just device memory, managed on the host).

virtio-fs DAX architecture

Virtio-fs is under active development, with its community focussing on a pair of device implementation in QEMU and device driver in Linux. Both components are already available upstream in their initial iterations, while upstreaming continues further e.g. with DAX window support.


OSv is a unikernel (framework). The two defining characteristics of a unikernel are:

  • Application-specialized: a unikernel is an executable machine image, consisting of an application and supporting code (drivers, memory management, runtime etc.) linked together, running in a single address space (typically in guest “kernel mode”).
  • Library OS: each unikernel only contains the functionality mandated by its application in terms of non-application code, i.e. no unused drivers, or even whole subsystems (e.g. networking, if the application doesn’t use the network).

OSv in particular strives for binary compatibility with Linux, using a dynamic linker. This means that applications built for Linux should run as OSv unikernels without requiring modifications or even rebuilding, at least most of the time. Of course, not the whole Linux ABI is supported, with system calls like fork() and relatives missing by design in all unikernels, which lack the notion of a process. Despite this limitation, OSv is quite full featured, with full SMP support, virtual memory, a virtual file system (and many filesystem implementations, including ZFS) as well as a mature networking stack, based on the FreeBSD sources.

At this point, one is sure to wonder “Why bother with unikernels?”. The problem they were originally introduced to solve is the bloated software stack in modern cloud computing. Running general-purpose operating systems as guests, typically for a single application/service, on top of a hypervisor which already takes care of isolation and provides a standard device model means duplication, as well as loss of efficiency. This is were unikernels come in, trying to be just enough to support a single application and as light-weight as possible, based on the assumption that they are executing inside a VM. Below is an illustration of the comparison between general-purpose OS, unikernels and containers (as another approach to the same problem, for completeness).

Unikernels vs GPOS vs containers

OSv, meet virtio-fs

As is apparent e.g. from the container world, it is very common for applications running in isolated environments (such as containers, or unikernels even more so) to require host file system access. Whereas containers sharing the host kernel thus have an obvious, controlled path to the host file system, with unikernels this has been more complex: all solutions were somewhat heavyweight, requiring a network link or indirection through network protocols. Virtio-fs then provided a significantly more attractive route: straight-forward mapping of fs operations (via FUSE), reusing the existing virtio transport and decent performance without high memory overhead.

The OSv community quickly identified the opportunity and came up with a read-only implementation on its side, when executing under QEMU. This emphasized being lightweight complexity-wise, while catering to many of its applications’ requirements (they are stateless, think e.g. serverless). Notably, it includes support for the DAX window (even before that’s merged in upstream QEMU), providing excellent performance, directly rivalling that of its local (non-shared) counterparts such as ZFS and ROFS (an OSv-specific read-only file system).

One central point is OSv’s support for booting from virtio-fs: this enables deploying a modified version or a whole new application without rebuilding the image, just by adjusting its root file system contents on the host. Last, owing to the DAX window practically providing low-overhead access to the host’s page cache, scalability is also expected to excel, with it being a common concern due to the potentially high density of unikernels per host.

For example, to build the cli OSv image, bootable from virtio-fs, using the core OSv build system:

scripts/build fs=virtiofs export=all image=cli

This results in a minimal image (just the initramfs), while the root fs contents are placed in a directory on the host (build/export here, by default).

Running the above image is just a step away (may want to use the virtio-fs development version of QEMU, e.g. for DAX window support):

scripts/ --virtio-fs-tag=myfs --virtio-fs-dir=$(pwd)/build/export

This orchestrates running both virtiofsd and QEMU, using the contents of build/export as the root file system. Any changes to this directory, directly from the host will be visible in the guest without re-running the previous build step.


OSv has gained a prominent new feature, powered by virtio-fs and its QEMU implementation. This allows efficient, lightweight and performant access to the host’s file system, thanks to the native virtio transport, usage of the FUSE protocol and the DAX window architecture. In turn, it enables use cases like rapid unikernel reconfiguration.

by Fotis Xenakis at November 04, 2020 12:00 AM

October 30, 2020

KVM on Z

KVM on IBM Z at Virtual SHARE

Don't miss our session dedicated to Secure Execution at this year's virtual SHARE:
Protecting Data In-use With Secure Execution, presented by Reinhard Bündgen, 3:50 PM - 4:35 PM EST on Tuesday, August 4.

by Stefan Raspl ( at October 30, 2020 02:15 PM

October 29, 2020

KVM on Z

2020 Linux on IBM Z and LinuxONE Client Workshop, November 9-14

Get the latest news about the Linux exploitation and advantages of the IBM Z and LinuxONE platform in this technical 5 day workshop. Focusing also on new solutions and capabilities, such as Hybrid Cloud, Red Hat OpenShift Container Platform, Security, Performance, Networking and Virtualization presented by our development experts. We will start with introduction sessions on the first day, continue with 3 days of technical deep dive topics, and resume with the latest client experiences, and a panel discussion on the last day. You will also get a chance to interact directly with IBM developers and solution experts during the workshop.

Agenda highlights:

  • Introduction day
    • Linux on IBM Z and LinuxONE introduction and client stories
    • Virtualization options with Linux on IBM Z and LinuxONERed Hat OpenShift Container Platform in the Hybrid Cloud Strategy for IBM Z & LinuxONE
    • Cloud Paks for IBM Z overview
  • Deep dive sessions
    • What's new Linux on IBM Z and LinuxONE
    • z/VM Platform update
    • IBM Secure Execution for Linux - Introduction and Overview
    • Hyper Protect Virtual Server onPrem - Differences to Cloud
    • Red Hat OpenShift on IBM Z - Performance Experiences, Hints and Tips & Networking
    • Securing the Workloads on Red Hat OpenShift on IBM Z
    • IBM z15 Hardware Compression
    • Preparing for Multifactor Authentication for z/VM
    • Best Practices of Installing Red Hat OpenShift on IBM Z
    • Competitive application performance on Red Hat OpenShift
    • Cloud Paks for Data on IBM Z and LinuxONE
    • KVM Network Performance - Best Practices and Tuning Recommendations
  • Experiences and open discussion
    • Client experiences and lessons learned
    • Panel discussion for all

Date: November 9 - 13, 2020
Time schedule: 8:30 AM- 12:00 PM EDT / 2:30 - 6:00 PM CEST daily

Registration is open here till November 5.

by Stefan Raspl ( at October 29, 2020 09:41 AM

October 26, 2020

KVM on Z

Ubuntu 20.10 released

Ubuntu Server 20.10 is out!
It ships

See the release notes here, and the blog entry at canonical with Z-specific highlights here.

by Stefan Raspl ( at October 26, 2020 11:13 AM

October 18, 2020

Gerd Hoffmann

Improving microvm support in qemu and seabios.

In version 4.2 the microvm machine type was added to qemu. The initial commit describes it this way:

It's a minimalist machine type without PCI nor ACPI support, designed for short-lived guests. microvm also establishes a baseline for benchmarking and optimizing both QEMU and guest operating systems, since it is optimized for both boot time and footprint.

The initial code uses the minimal qboot firmware to initialize the guest, to load a linux kernel and boot it. For network/storage/etc devices virtio-mmio is used. The configuration is passed to the linux kernel on the command line so the guest is able to find the devices.

That works for direct kernel boot, using qemu -kernel vmlinuz, because qemu can easily patch the kernel command line then. But what if you want - for example - boot the Fedora Cloud image? Using the Fedora kernel stored within the image?

A better plan for device discovery

When not using direct kernel boot patching the kernel command line for device discovery isn't going to fly, so we need something else. There are two established standard ways to do that in modern systems. One is device trees. The other is ACPI. Both have support for virtio-mmio.

A device tree entry for virtio-mmio looks like this:

virtio_block@3000 {
	compatible = "virtio,mmio";
	reg = <0x3000 0x100>;
	interrupts = <41>;

And this is the ACPI DSDT version:

Device (VR06)
    Name (_HID, "LNRO0005")  // _HID: Hardware ID
    Name (_UID, 0x06)  // _UID: Unique ID
    Name (_CCA, One)  // _CCA: Cache Coherency Attribute
    Name (_CRS, ResourceTemplate ()  // _CRS: Current Resource Settings
        Memory32Fixed (ReadWrite,
            0xFEB00C00,         // Address Base
            0x00000200,         // Address Length
        Interrupt (ResourceConsumer, Level, ActiveHigh, Exclusive, ,, )

Both carry essentially the same information: What kind of device that is and which resources (registers & interrupt) it uses.

On the arm platform both are established, with device trees being common for small board computers like the raspberry pi and ACPI being used in the arm server space. On the x86 platform we don't have much of a choice though. There are some niche attempts to establish device trees, the google android goldfish platform for example. But for widespread support there is no way around using ACPI.

The nice thing about arm server using ACPI too is that this paved the way for us. The linux kernel supports both device tree and ACPI for the discovery of virtio-mmio devices:

root@fedora-bios ~/acpi# modinfo virtio-mmio | grep alias
alias:          of:N*T*Cvirtio,mmioC*
alias:          of:N*T*Cvirtio,mmio
alias:          acpi*:LNRO0005:*

So linux kernel support is a solved problem already. Yay!

virtio-mmio support for seabios

If we want load a kernel from the disk image the firmware must be able to find and read the disk. We already have virtio-pci support for blk and scsi in seabios. The differences between virtio-pci and virtio-mmio transports are not that big. Also some infrastructure for different transport modes was already there to deal with legacy vs. modern virtio-pci. So adding virtio-mmio support to the drivers wasn't much of a problem.

But of course seabios also has the problem that it must discover the devices before it can initialize the driver. Various approaches to find virtio-mmio devices using the available information sources where tried. All of them had the one or the other non-working corner case, except using ACPI. So seabios ended up getting a simple DSDT parser for device discovery.

While being at it some other small fixes where added to seabios too to make it work better with microvm. The hard dependency on the RTC CMOS has been removed for example, so latest seabios works fine with qemu -M microvm,rtc=off.

This ships with seabios version 1.14.

While speaking about seabios: When using a serial console I'd strongly recommend to run with qemu -M microvm,graphics=off. That will enable serial console support in seabios. This is one of the tweaks done by the qemu -nographic shortcut. The machine option works with the pc and q35 machine types too.

ACPI cleanups in qemu

Hooking up ACPI support for microvm on the qemu side turned out to be surprisingly difficuilt due to some historical baggage.

Years ago qemu used to have a static ACPI DSDT table. All ISA devices (serial & parallel ports, floppy, ...) are declared there, but they might not be actually present depending on qemu configuration. The LPC/ISA bride has some bits in pci config space saying whenever a device is actually present or not (qemu emulation follows physical hardware behavior here). So the devices have a _STA method looking up those bits and returning the device status. The guest had to run the method using AML interpreter to figure whenever the declared device is actually there.

// this is the qemu q35 ISA bridge at 00:1f.0
Device (ISA)
    Name (_ADR, 0x001F0000)  // _ADR: Address
    // ... snip ...
    OperationRegion (LPCE, PCI_Config, 0x82, 0x02)
    Field (LPCE, AnyAcc, NoLock, Preserve)
        CAEN,   1,  // serial port #1 enable bit
        CBEN,   1,  // serial port #2 enable bit
        LPEN,   1   // parallel port enable bit

// serial port #1
Device (COM1)
    Name (_HID, EisaId ("PNP0501") /* 16550A-compatible COM Serial Port */)
    Name (_UID, One)  // _UID: Unique ID
    Method (_STA, 0, NotSerialized)  // _STA: Status
        Local0 = CAEN /* \_SB_.PCI0.ISA_.CAEN */
        If ((Local0 == Zero))
            // serial port #1 is disabled
            Return (Zero)
            // serial port #1 is enabled
            Return (0x0F)
    // ... snip ...

The microvm machine type simply has no PCI support, so that approach isn't going to fly. Also these days all ACPI tables are dynamically generated anyway, so there is no reason to have the guests AML interpreter go dig into pci config space. Instead we can handle that in qemu when generating the DSDT table. Disabled devices are simply not listed. For enabled devices this is enough:

// serial port #1
Device (COM1)
    Name (_HID, EisaId ("PNP0501") /* 16550A-compatible COM Serial Port */)
    Name (_UID, One)  // _UID: Unique ID
    Name (_STA, 0x0F)  // _STA: Status
    // ... snip ...

So I've ended up reorganizing and simplifying the code which creates the DSDT entries for ISA devices. This landed in qemu version 5.1.

ACPI support for microvm

Now with the roadblocks out of the way it was finally possible to add acpi support to microvm. There is little reason to worry about backward compatibility to historic x86 platforms here, old guests wouldn't be able to handle virtio-mmio anyway. So this takes a rather modern approach and looks more like an arm virt machines than a x86 q35 machine. Like arm it uses the generic event device for power management support.

ACPI support for microvm is switchable, simliar to the other machine types, using the acpi=on|off machine option. The -no-acpi switch works too. By default ACPI support is enabled.

With ACPI enabled qemu uses virtio-mmio enabled seabios as firmware and doesn't bother patching the linux kernel command line for device discovery.

With ACPI disabled qemu continues to use qboot as firmware like older qemu versions do. Likewise it continues to add virtio-mmio devices to the linux kernel command line.

This will be available in qemu version 5.2. It is already merged in the master branch.

ACPI advantages

  • Number one is device discovery obviously, this is why we started all this in the first place. seabios and linux kernel find virtio-mmio devices automatically. You can boot Fedora cloud images in microvm without needing any tricks. Probably other distros too, even though I didn't try that. Compiling the linux kernel with CONFIG_VIRTIO_MMIO=y (or =m & adding the module to initramfs) is pretty much the only requirement for this to work.

  • Number two is device discovery too. ACPI will also tell the kernel which devices are not there. So with acpi=on the kernel simply skips the PS/2 probe in case the DSDT doesn't list a keyboard controller. With acpi=off the kernel assumes legacy hardware, goes into probe-harder mode and needs one second to figure that there really is no keyboard controller:

    [    0.414840] i8042: PNP: No PS/2 controller found.
    [    0.415287] i8042: Probing ports directly.
    [    1.454723] i8042: No controller found

    We have an simliar effect with the real time clock. With acpi=off the kernel goes register an IRQ line for the RTC even in case the device isn't there.

  • Number three is (basic) power management. ACPI provides a virtual power button, so the guest will honor shutdown requests sent that way. ACPI also provides S5 (aka poweroff) support, so qemu gets a notification from the guest when the shutdown is done and can exit.

  • Number four is better IRQ routing. The linux kernel finds the IO-APIC declared in the APIC table and uses it for IRQ routing. It is possible to use lines 16-23 for the virtio-mmio devices, avoiding IRQ sharing. Also we can refine the configuration using IRQ flags in the DSDT table.

    With acpi=off this does not work not reliable. I've seen the kernel ignore the IO-APIC in the past. Doesn't always happen though. Not clear which factors play a role here, I didn't investigate that in detail. Maybe newer kernel versions are a bit more clever here and find the IO-APIC even without ACPI.

Bottom line: ACPI helps moving the microvm machine type forward towards a world without legacy x86 platform devices.

But isn't ACPI bloated and slow?

Well, on my microvm test guest all ACPI tables combined are less than 1k in size:

root@fedora-bios /sys/firmware/acpi/tables# find -type f | xargs ls -l
-r--------. 1 root root  78 Oct  2 09:36 ./APIC
-r--------. 1 root root 482 Oct  2 09:36 ./DSDT
-r--------. 1 root root 268 Oct  2 09:36 ./FACP

I wouldn't call that bloated. This is a rather small virtual machine, with larger configurations (more CPUs, more devices) the tables will grow a bit of course.

When testing boot times I figured it is pretty hard to find any differences due to ACPI initialization. The noise (differences when doing 2-3 runs with identical configuration) is larger than the acpi=on/off difference. Seems to be at most a handful of milliseconds.

When trying that yourself take care to boot the kernel with 'quiet'. This is a good idea anyway if you want boot as fast as possible. The kernel prints more boot information with acpi=on, so slow console logging can skew your numbers if you let the kernel print out everything.

Runtime differences should be zero. There is only one AML method in the DSDT table. It toggles the power button when a notification comes in from the generic event device. It runs only on generic event device interrupts.

USB support for microvm

qemu just got a sysbus (non-pci) version of the xhci host adapter. It is needed for some arm boards. Nice thing is now that we have ACPI we can just wire that up in microvm too, add it in the DSDT table, then linux will find and use it:

Device (XHCI)
    Name (_HID, EisaId ("PNP0D10") /* XHCI USB Controller */)
    Name (_CRS, ResourceTemplate ()  // _CRS: Current Resource Settings
        Memory32Fixed (ReadWrite,
            0xFE900000,         // Address Base
            0x00004000,         // Address Length
        Interrupt (ResourceConsumer, Level, ActiveHigh, Exclusive, ,, )

USB support will be disabled by default, it can be enabled using the usual machine option: qemu -M microvm,usb=on.

Patches for qemu are in flight, should land in version 5.2
Patches for seabios are merged, will be available in version 1.15

PCIe support for microvm

There is one more arm platform thing we can reuse in microvm: The PCI Express host bridge. Again the same approach: Wire everything up, declare it in the ACPI DSDT. Linux kernel finds and uses it.

Not adding an asl snippet this time. The PCIe host bridge is a complex device so the description is a bit larger. It has GSI subdevices, IRQ routing information for each PCI slot, mmconfig configuration etc. Also shows in the DSDT size (even though that is still less than half the size q35 has):

root@fedora-bios /sys/firmware/acpi/tables# ll DSDT
-r--------. 1 root root 3130 Oct  2 10:20 DSDT

PCIe support will be disabled by default, it can be enabled using the new pcie machine option: qemu -M microvm,pcie=on.

This will be available in qemu version 5.2. It is already merged in the master branch.

Future work

My TODO list for qemu isn't very long:

  • Add second IO-APIC, allowing more IRQ lines for more virtio-mmio devices. Experimental patches exist.

  • IOMMU support, using virtio-iommu. Depends on ACPI spec update for virtio-iommu being finalized and support being merged into qemu and linux kernel. The actual wireup for microvm should be easy once all this is done.

Outside qemu there are a few more items:

  • Investigate microvm PCIe support in seabios. Experimental patches exist. I'm not sure yet whenever seabios should care though.

    The linux kernel can initialize the PCIe bus just fine on its own, whereas proper seabios support has its challenges. When running in real mode the mmconfig space can not be reached. Legacy pci config space access via port 0xcf8 is not available. Which breaks pcibios support. Probably fixable with a temporary switch to 32bit mode, at the cost of triggering a bunch of extra vmexits. Beside that seabios will spend more time at boot, initializing the pci devices.

    So, is it worth the effort? The benefit would be that seabios could support booting from pci devices on microvm then.

  • Maybe add microvm support to edk2/ovmf.

    Looks not that easy on a quick glance. ArmVirtPkg depends on device trees for virtio-mmio detection, so while we can re-use the virtio-mmio drivers we can not re-use device discovery code. Unless we maybe have qemu provide both ACPI tables and a device tree, even if ovmf happens to be the only device tree user.

    It also is not clear what other dragons (dependencies on classic x86 platform devices) are lurking in the ovmf codebase.

  • Support the new microvm features (possibly adding microvm support first) in other projects.

    Candidate number one is of course libvirt because it is the foundation for many other projects. Beside that microvm support is probably mostly useful for cloud/container-style workloads, i.e. kata and kubevirt.

by Gerd Hoffmann at October 18, 2020 10:00 PM

October 09, 2020

Stefan Hajnoczi

Requirements for out-of-process device emulation

Over the past months I have participated in discussions about out-of-process device emulation. This post describes the requirements that have become apparent. I hope this will be a useful guide to understanding the big picture about out-of-process device emulation.

What is out-of-process device emulation?

Device emulation is traditionally implemented in the program that executes guest code. This approach is natural because accesses to device registers are trapped as part of the CPU run loop that sits at the core of an emulator or virtual machine monitor (VMM).

In some use cases it is advantageous to perform device emulation in separate processes. For example, software-defined network switches can minimize data copies by emulating network cards directly in the switch process. Out-of-process device emulation also enables privilege separation and tighter sandboxing for security.

Why are these requirements important?

When emulated devices are implemented in the VMM they use common VMM APIs. Adding new devices is relatively easy because the APIs are already there and the developer can focus on the device specifics. Out-of-process device emulation potentially leaves developers without APIs since the device emulation program is a separate program that literally starts from main(). Developers want to focus on implementing their specific device, not on solving general problems related to out-of-process device emulation infrastructure.

It is not only a lot of work to implement an out-of-process device completely from scratch, but there is also a risk of developing the wrong solution because some subtleties of device emulation are not obvious at first glance.

I hope sharing these requirements will help in the creation of common infrastructure so it's easy to implement high-quality out-of-process devices.

Not all use cases have the full set of requirements. Therefore it's best if requirements are addressed in separate, reusable libraries so that device implementors can pick the ones that are relevant to them.

Device emulation

Device resources

Devices provide resources that drivers interact with such as hardware registers, memory, or interrupts. The fundamental requirement of out-of-process device emulation is exposing device resources.

The following types of device resources are needed:

Synchronous MMIO/PIO accesses

The most basic device emulation operation is the hardware register access. This is a memory-mapped I/O (MMIO) or programmed I/O (PIO) access to the device. A read loads a value from a device register. A write stores a value to a device register. These operations are synchronous because the vCPU is paused until completion.

Asynchronous doorbells

Devices often have doorbell registers, allowing the driver to inform the device that new requests are ready for processing. The vCPU does not need to wait since the access is a posted write.

The kvm.ko ioeventfd mechanism can be used to implement asynchronous doorbells.

Shared device memory

Devices may have memory-like regions that the CPU can access (such as PCI Memory BARs). The device emulation process therefore needs to share a region of its memory space with the VMM so the guest can access it. This mechanism also allows device emulation to busy wait (poll) instead of using synchronous MMIO/PIO accesses or asynchronous doorbells for notifications.

Direct Memory Access (DMA)

Devices often require read and write access to a memory address space belonging to the CPU. This allows network cards to transmit packet payloads that are located in guest RAM, for example.

Early out-of-process device emulation interfaces simply shared guest RAM. The allowed DMA to any guest physical memory address. More advanced IOMMU and address space identifier mechanisms are now becoming ubiquitous. Therefore, new out-of-process device emulation interfaces should incorporate IOMMU functionality.

The key requirement for IOMMU mechanisms is allowing the VMM to grant access to a region of memory so the device emulation process can read from and/or write to it.


Devices notify the CPU using interrupts. An interrupt is simply a message sent by the device emulation process to the VMM. Interrupt configuration is flexible on modern devices, meaning the driver may be able to select the number of interrupts and a mapping (using one interrupt with multiple event sources). This can be implemented using the Linux eventfd mechanism or via in-band device emulation protocol messages, for example.

Extensibility for new bus types

It should be possible to support multiple bus types. vhost-user only supports vhost devices. VFIO is more extensible but currently focussed on PCI devices. It is likely that QEMU SysBus devices will be desirable for implementing ad-hoc out-of-process devices (especially for System-on-Chip target platforms).

Bus-level APIs, not protocol bindings

Developers should not need to learn the out-of-process device emulation protocol (vfio-user, etc). APIs should focus on bus-level concepts such as defining VIRTIO or PCI devices rather than protocol bindings for dealing with protocol messages, file descriptor passing, and shared memory.

In other words, developers should be thinking in terms of the problem domain, not worrying about how out-of-process device emulation is implemented. The protocol should be hidden behind bus-level APIs.

Multi-threading support from the beginning

Threading issues arise often in device emulation because asynchronous requests or multi-queue devices can be implemented using threads. Therefore it is necessary to clearly document what threading models are supported and how device lifecycle operations like reset interact with in-flight requests.

Live migration, live upgrade, and crash recovery

There are several related issues around device state and restarting the device emulation program without disrupting the guest.

Live migration

Live migration transfers the state of a device from one device emulation process to another (typically running on another host). This requires the following functionality:

Quiescing the device

Some devices can be live migrated at any point in time without any preparation, while others must be put into a quiescent state to avoid issues. An example is a storage controller that has a write request in flight. It is not safe to live migration until the write request has completed or been canceled. Failure to wait might result in data corruption if the write takes effect after the destination has resumed execution.

Therefore it is necessary to quiesce a device. After this point there is no further device activity and no guest-visible changes will be made by the device.

Saving/loading device state

It must be possible to save and load device state. Device state includes the contents of hardware registers as well as device-internal state necessary for resuming operation.

It is typically necessary to determine whether the device emulation processes on the migration source and destination are compatible before attempting migration. This avoids migration failure when the destination tries to load the device state and discovers it doesn't support it. It may be desirable to support loading device state that was generated by a different implementation of the same device type (for example, two virtio-net implementations).

Dirty memory logging

Pre-copy live migration starts with an iterative phase where dirty memory pages are copied from the migration source to the destination host. Devices need to participate in dirty memory logging so that all written pages are transferred to the destination and no pages are "missed".

Crash recovery

If the device emulation process crashes it should be possible to restart it and resume device emulation without disrupting the guest (aside from a possible pause during reconnection).

Doing this requires maintaining device state (contents of hardware registers, etc) outside the device emulation process. This way the state remains even if the process crashes and it can be resume when a new process starts.

Live upgrade

It must be possible to upgrade the device emulation process and the VMM without disrupting the guest. Upgrading the device emulation process is similar to crash recovery in that the process terminates and a new one resumes with the previous state.

Device versioning

The guest-visible aspects of the device must be versioned. In the simplest case the device emulation program would have a --compat-version=Ncommand-line option that controls which version of the device the guest sees. When guest-visible changes are made to the program the version number must be increased.

By giving control of the guest-visible device behavior it is possible to save/load and live migrate reliably. Otherwise loading device state in a newer device emulation program could affect the running guest. Guest drivers typically are not prepared for the device to change underneath them and doing so could result in guest crashes or data corruption.


The trust model

The VMM must not trust the device emulation program. This is key to implementing privilege separation and the principle of least privilege. If a compromised device emulation program is able to gain control of the VMM then out-of-process device emulation has failed to provide isolation between devices.

The device emulation program must not trust the VMM to the extent that this is possible. For example, it must validate inputs so that the VMM cannot gain control of the device emulation process through memory corruptions or other bugs. This makes it so that even if the VMM has been compromised, access to device resources and associated system calls still requires further compromising the device emulation process.

Unprivileged operation

The device emulation program should run unprivileged to the extent that this is possible. If special permissions are required to access hardware resources then these resources can sometimes be provided via file descriptor passing by a more privileged parent process.


Operating system sandboxing mechanisms can be applied to device emulation processes more effectively than monolithic VMMs. Seccomp can limit the Linux system calls that may be invoked. SELinux can restrict access to system resources.

Sandboxing is a common task that most device emulation programs need. Therefore it is a good candidate for a library or launcher tool that is shared by device emulation programs.


Command-line interface

A common command-line interface should be defined where possible. For example, vhost-user's standard --socket-path=PATH argument makes it easy to launch any vhost-user device backend. Protocol-specific options (e.g. socket path) and device type-specific options (e.g. virtio-net) can be standardized.

Some options are necessarily specific to the device emulation program and therefore cannot be standardized.

The advantage of standard options is that management tools like libvirt can launch the device emulation programs without further user configuration.

RPC interface

It may be necessary to issue commands at runtime. Examples include adjusting throttling limits, enabling/disabling logging, etc. These operations can be performed over an RPC interface.

Various RPC interfaces are used throughout open source virtualization software. Adopting a widely-used RPC protocol and standardizing commands is beneficial because it makes it easy to communicate with the software and management tools can support them relatively easily.


This was largely a brain dump but I hope it is useful food for thought as out-of-process device emulation interfaces are designed and developed. There is a lot more to it than simply implementing a protocol for device register accesses and guest RAM DMA. Developing open source libraries in Rust and C that can be used as needed will ensure that out-of-process devices are high-quality and easy for users to deploy.

by Unknown ( at October 09, 2020 05:03 PM

KVM on Z

IBM Cloud Infrastructure Center adds Support for Red Hat KVM

The latest release of IBM Cloud Infrastructure Center 1.1.2 adds support for Red Hat KVM on IBM Z and LinuxONE.

IBM Cloud Infrastructure Center is an Infrastructure-as-a-Service (IaaS) management solution that provides on-premise cloud deployments of Linux virtual machines on IBM Z and LinuxONE.
As a component of the Z Hybrid Cloud solution stack, Cloud Infrastructure Center delivers simplified IaaS management across compute, network, and storage resources, making virtual infrastructure ready to be consumed by PaaS layer solutions.
Among others, version 1.1.2 provides new capabilities, including cloud deployments for Red Hat Enterprise Linux Kernel-based Virtual Machines (KVM).

See here for the official announcement, and here for a blog entry with further details.

by Stefan Raspl ( at October 09, 2020 04:59 PM

September 27, 2020

Stefan Hajnoczi

On unifying vhost-user and VIRTIO

The recent development of a Linux driver framework called VIRTIO Data Path Acceleration (vDPA) has laid the groundwork for exciting new vhost-user features. The implications of vDPA have not yet rippled through the community so I want to share my thoughts on how the vhost-user protocol can take advantage of new ideas from vDPA.

This post is aimed at developers and assumes familiarity with the vhost-user protocol and VIRTIO. No knowledge of vDPA is required.

vDPA helps with writing drivers for hardware that supports VIRTIO offload. Its goal is to enable hybrid hardware/software VIRTIO devices, but as a nice side-effect it has overcome limitations in the kernel vhost interface. It turns out that applying ideas from vDPA to the vhost-user protocol solves the same issues there. In this article I'll show how extending the vhost-user protocol with vDPA has the following benefits:

  • Allows existing VIRTIO device emulation code to act as a vhost-user device backend.
  • Removes the need for shim devices in the virtual machine monitor (VMM).
  • Replaces undocumented conventions with a well-defined device model.

These things can be done while reusing existing vhost-user and VIRTIO code. In fact, this is especially good news for existing codebases like QEMU because they already have a wealth of vhost-user and VIRTIO code that can now finally be connected together!

Let's look at the advantages of extending vhost-user with vDPA first and then discuss how to do it.

Why extend vhost-user with vDPA?

Reusing VIRTIO emulation code for vhost-user backends

It is a common misconception that a vhost device is a VIRTIO device. VIRTIO devices are defined in the VIRTIO specification and consist of a configuration space, virtqueues, and a device lifecycle that includes feature negotiation. A vhost device is a subset of the corresponding VIRTIO device. The exact subset depends on the device type, and some vhost devices are closer to the full functionality of their corresponding VIRTIO device than others. The most well-known example is that vhost-net devices have rx/tx virtqueues and but lack the virtio-net control virtqueue. Also, the configuration space and device lifecycle are only partially available to vhost devices.

This difference makes it impossible to use a VIRTIO device as a vhost-user device and vice versa. There is an impedance mismatch and missing functionality. That's a shame because existing VIRTIO device emulation code is mature and duplicating it to provide vhost-user backends creates additional work.

If there was a way to reuse existing VIRTIO device emulation code it would be easier to move to a multi-process architecture in QEMU. Want to run --netdev user,id=netdev0 --device virtio-net-pci,netdev=netdev0 in a separate, sandboxed process? Easy, run it as a vhost-user-net device instead of as virtio-net.

Making VMM device shims optional

Today each new vhost device type requires a shim device in the VMM. QEMU has --device vhost-user-blk-pci, --device vhost-user-input-pci, and so on. Why can't there be a single --device vhost-user device?

This limitation is a consequence of the fact that vhost devices are not full VIRTIO devices. In fact, a vhost device does not even have a way to report its device type (net, blk, scsi, etc). Therefore it is impossible for today's VMMs to offer a generic device. Each vhost device type requires a shim device.

In some cases a shim device is desirable because it allows the VMM to handle some aspects of the device instead of passing everything through to the vhost device backend. But requiring shims by default involves lots of tedious boilerplate code and prevents new device types from being used by older VMMs.

Providing a well-defined device model in vhost-user

Although vhost-user works well for users, it is difficult for developers to learn and extend. The protocol does not have a well-defined device model. Each device type has its own undocumented set of protocol messages that are used. For example, the vhost-user-blk device uses the configuration space whereas most other device types do not use the configuration space at all.

Since protocol use is not fully documented in the specification, developers might resort to reading Linux, QEMU, and DPDK code in order to figure out how to make their devices work. They typically have to debug vhost-user protocol messages and adjust their code until it appears to work. Hopefully the critical bugs are caught before the code ships. This is problematic because it's hard to produce high-quality vhost-user implementations.

Although the protocol specification can certainly be cleaned up, the problem is more fundamental. vhost-user badly needs a well-defined device model so that protocol usage is clear and uniform for all device types. The best way to do that is to simply adopt the VIRTIO device model. The VIRTIO specification already defines the device lifecycle and details of the device types. By making vhost-user devices full VIRTIO devices there is no need for additional vhost device specifications. The vhost-user specification just becomes a transport for the established VIRTIO device model. Luckily that is effectively what vDPA has done for kernel vhost ioctls.

How to do this in QEMU

The following QEMU changes are needed to implement vhost-user vDPA support. Below I will focus on vhost-user-net but most of the work is generic and benefits all device types.

Import vDPA ioctls into vhost-user

vDPA extends the Linux vhost ioctl interface. It uses a subset of vhost ioctls and adds new vDPA-specific ioctls that are implemented in the vhost_vdpa.ko kernel module. These new ioctls enable the full VIRTIO device model, including device IDs, the status register, configuration space, and so on.

In theory vhost-user could be fixed without vDPA, but it would involve effectively adding the same set of functionality that vDPA has already added onto kernel vhost. Reusing the vDPA ioctls allows VMMs to support both kernel vDPA and vhost-user with minimal code duplication.

This can be done by adding a VHOST_USER_PROTOCOL_F_VDPA feature bit to the vhost-user protocol. If both the vhost-user frontend and backend support vDPA then all vDPA messages are available. Otherwise they can either fall back on legacy vhost-user behavior or drop the connection.

The vhost-user specification could be split into a legacy section and a modern vDPA-enabled section. The modern protocol will remove vhost-user messages that are not needed by vDPA, simplifying the protocol for new developers while allowing existing implementations to support both with minimal changes.

One detail is that vDPA does not use the memory table mechanism for sharing memory. Instead it relies on the richer IOMMU message family that is optional in vhost-user today. This approach can be adopted in vhost-user too, making the IOMMU code path standard for all implementations and dropping the memory table mechanism.

Add vhost-user vDPA to the vhost code

QEMU's hw/virtio/vhost*.c code supports kernel vhost, vhost-user, and kernel vDPA. A vhost-user vDPA mode must be added to implement the new protocol. It can be implemented as a combination of the vhost-user and kernel vDPA code already in QEMU. Most likely the existing vhost-user code can simply be extended to enable vDPA support if the backend supports it.

Only small changes to hw/net/virtio-net.c and net/vhost-user.c are needed to use vhost-user vDPA with net devices. At that point QEMU can connect to a vhost-user-net device backend and use vDPA extensions.

Add a vhost-user vDPA VIRTIO transport

Next a vhost-user-net device backend can be put together using QEMU's virtio-net emulation. A translation layer is needed between the vhost-user protocol and the VirtIODevice type in QEMU. This can be done by implementing a new VIRTIO transport alongside the existing pci, mmio, and ccw transports. The transport processes vhost-user protocol messages from the UNIX domain socket and invokes VIRTIO device emulation code inside QEMU. It acts as a VIRTIO bus so that virtio-net-device, virtio-blk-device, and other device types can be plugged in.

This piece is the most involved but the vhost-user protocol communication part was already implemented in the virtio-vhost-user prototype that I worked on previously. Most of the communication code can be reused and the remainder is implementing the VirtioBusClass interface.

To summarize, a new transport acts as the vhost-user device backend and invokes QEMU VirtIODevice methods in response to vhost-user protocol messages. The syntax would be something like --device virtio-net-device,id=virtio-net0 --device vhost-user-backend,device=virtio-net0,addr.type=unix,addr.path=/tmp/vhost-user-net.sock.

Where this converges with multi-process QEMU

At this point QEMU can run ad-hoc vhost-user backends using existing VIRTIO device models. It is possible to go further by creating a qemu-dev launcher executable that implements the vhost-user spec's "Backend program conventions". This way a minimal device emulator executable hosts the device instead of a full system emulator.

The requirements for this are similar to the multi-process QEMU effort, which aims to run QEMU devices as separate processes. One of the main open questions is how to design build system and Kconfig support for building minimal device emulator executables.

In the case of vhost-user-net the qemu-dev-vhost-user-net executable would contain virtio-net-device, vhost-user-backend, any netdevs the user wishes to include, a QMP monitor, and a vhost-user backend command-line interface.

Where does this leave us? QEMU's existing VIRTIO device models can be used as vhost-user devices and run in a separate processes from the VMM. It's a great way of reusing code and having the flexibility to deploy it in the way that makes most sense for the intended use case.


The vhost-user protocol can be simplified by adopting the vhost vDPA ioctls that have recently been introduced in Linux. This turns vhost-user into a VIRTIO transport and vhost-user devices become full VIRTIO devices. Existing VIRTIO device emulation code can then be reused in vhost-user device backends.

by Unknown ( at September 27, 2020 09:26 AM

September 26, 2020

Cole Robinson

virt-install --cloud-init support

As part of GSOC 2019, Athina Plaskasoviti implemented --cloud-init support for virt-install. This post provides a bit more info about the feature.

Why cloud-init

For a long while, most mainstream Linux distros have shipped 'cloud images': raw or qcow2 formatted disk images with the distro minimally pre-installed on it. These images typically have cloud-init set to run on VM bootup. cloud-init can do a variety of things, like add users, change passwords, register ssh keys, and generally perform any desired action on the VM OS. This only works when cloud-init is passed the right configuration by whatever platform is starting the VM, like OpenStack or virt-install. cloud-init supports many different datasources for getting configuration outside the VM.

Historically though the problem is that slapping these images into virt-install or virt-manager gives crappy results, because these tools were not providing any datasource. In this case, cloud-init reverts to its distro default configured behavior, which in most cases is unusable. For example on Fedora, the result was: hang waiting for cloud-init data, time out, drop to login prompt with all accounts locked.

Prior to virt-install --cloud-init support, the simplest workaround was to use libguestfs, specifically virt-customize, to set a root password and disable cloud-init:

$ virt-customize -a MY-CLOUD-IMAGE.qcow2 \
    --root-password password:SUPER-SECRET-PASSWORD \
    --uninstall cloud-init

--cloud-init option

The --cloud-init option will tell virt-install to set up a nocloud datasource via a specially formatted .iso file that is generated on the fly, and only used for the first VM bootup.

The default behavior when --cloud-init is specified with no suboptions will do the following:

  • Generate a random root password and print it to the terminal.
  • Default to VM serial console access rather than graphical console access. Makes it easier to paste the password and gives more ssh-like behavior.
  • Sets the root password to expire on first login. So the temporary password is only used once.
  • Disables cloud-init for subsequent VM startups. Otherwise on the next VM boot you'd face locked accounts again.

The --cloud-init also has suboptions to specify your own behavior, like transfer in a host ssh public key, pass in raw cloud-init user-data/meta-data, etc. See the virt-install --cloud-init man page section for the specifics.

Room for improvement

This is all a step in the right direction but there's lots more we can do here:

  • Extend virt-install's --install option to learn how to fetch cloud images for the specified distro. libosinfo and osinfo-db already track cloud image download links for many distros so the info we need is already in place. We could make virt-install --install fedora32,cloud=yes a single way to pull down a cloud image, generate a cloud-init datasource, and create the VM in one shot.
  • Use --cloud-init by default when the user passes us a cloud-init enabled disk image. virt-customize has a lot of disk image detection smarts already, but we aren't using that in virt-install yet.
  • virt-manager UI support. There's an issue tracking this with some more thoughts in it.
  • Similar support for CoreOS Ignition which fulfills a similar purpose as cloud-init for distros like Fedora CoreOS. There's an issue tracking this too.

I personally don't have plans to work on these any time soon, but I'm happy to provide guidance if anyone is interested in helping out!

by Cole Robinson at September 26, 2020 04:00 AM

September 24, 2020

KVM on Z

Webinar: KVM Network Performance - Best Practices and Tuning Recommendations

Join us for this webinar session to get an overview of the different networking configuration choices running KVM guests on the IBM Z platforms and details to setup and capabilities. It also provides tuning recommendations for the KVM host and KVM guest environment to achieve greater network performance.

Dr. Jürgen Dölle, Linux on IBM Z Performance Evaluation

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 webcasts see here.

by Stefan Raspl ( at September 24, 2020 10:34 AM

September 17, 2020

Stefan Hajnoczi

Initial applications for Outreachy December-March internships Sept 20

QEMU is participating in the Outreachy December-March open source internship program. The internships are 12 weeks of full-time paid remote work contributing to QEMU/KVM. For more information about eligibility and what the internships are like, please see the Outreachy website.

The initial application deadline is September 20 and it only takes 5-30 minutes to complete:

If you or someone you know are considering doing an internship with QEMU, Linux kernel, or another participating organization, please remember to submit an initial application by September 20th!

by Unknown ( at September 17, 2020 05:07 PM

September 16, 2020

Cole Robinson

virt-manager 3.0.0 released!

Yesterday I released virt-manager 3.0.0. Despite the major version number bump, things shouldn't look too different from the previous release. For me the major version number bump reflects certain feature removals (like dropping virt-convert), and the large amount of internal code changes that were done, though there's a few long awaited features sprinkled in like virt-install --cloud-init support which I plan to write more about later.

This release includes:

  • virt-install --cloud-init support (Athina Plaskasoviti, Cole Robinson)
  • The virt-convert tool has been removed. Please use virt-v2v instead
  • A handful of UI XML configuration options have been removed, in an effort to reduce maintenance ongoing maintenance burden, and to be more consistent with what types of UI knobs we expose. The XML editor is an alternative in most cases. For a larger discussion see this thread and virt-manager's file.
  • The 'New VM' UI now has a 'Manual Install' option which creates a VM without any required install media
  • In the 'New VM' UI, the network/pxe install option has been removed. If you need network boot, choose 'Manual Install' and set the boot device after initial VM creation
  • 'Clone VM' UI has been reworked and simplified
  • 'Migrate VM' UI now has an XML editor for the destination VM
  • Global and per-vm option to disable graphical console autoconnect. This makes it easier to use virt-manager alongside another client like virt-viewer
  • virt-manager: set guest time after VM restore (Michael Weiser)
  • virt-manager: option to delete storage when removing disk device (Lily Nie)
  • virt-manager: show warnings if snapshot operation is unsafe (Michael Weiser)
  • Unattended install improvements (Fabiano Fidêncio)
  • cli: new --xml XPATH=VAL option for making direct XML changes
  • virt-install: new --reinstall=DOMAIN option
  • virt-install: new --autoconsole text|graphical|none option
  • virt-install: new --os-variant detect=on,require=on suboptions
  • cli: --clock, --keywrap, --blkiotune, --cputune additions (Athina Plaskasoviti)
  • cli: add --features kvm.hint-dedicated.state= (Menno Lageman)
  • cli: add --iommu option (Menno Lageman)
  • cli: Add --graphics websocket= support (Petr Benes)
  • cli: Add --disk type=nvme source.* suboptions
  • cli: Fill in all --filesystem suboptions
  • Translation string improvements (Pino Toscano)
  • Convert from .pod to .rst for man pages
  • Switch to pytest as our test runner
  • Massively improved unittest and uitest code coverage
  • Now using github issues as our bug tracker

by Cole Robinson at September 16, 2020 04:00 AM

September 14, 2020

QEMU project

An Overview of QEMU Storage Features

This article introduces QEMU storage concepts including disk images, emulated storage controllers, block jobs, the qemu-img utility, and qemu-storage-daemon. If you are new to QEMU or want an overview of storage functionality in QEMU then this article explains how things fit together.

Storage technologies

Persistently storing data and retrieving it later is the job of storage devices such as hard disks, solid state drives (SSDs), USB flash drives, network attached storage, and many others. Technologies vary in their storage capacity (disk size), access speed, price, and other factors but most of them follow the same block device model.

Block device I/O

Block devices are accessed in storage units called blocks. It is not possible to access individual bytes, instead an entire block must be transferred. Block sizes vary between devices with 512 bytes and 4KB block sizes being the most common.

As an emulator and virtualizer of computer systems, QEMU naturally has to offer block device functionality. QEMU is capable of emulating hard disks, solid state drives (SSDs), USB flash drives, SD cards, and more.

Storage for virtual machines

There is more to storage than just persisting data on behalf of a virtual machine. The lifecycle of a disk image includes several operations that are briefly covered below.

Block device I/O

Virtual machines consist of device configuration (how much RAM, which graphics card, etc) and the contents of their disks. Transferring virtual machines either to migrate them between hosts or to distribute them to users is an important workflow that QEMU and its utilities support.

Much like ISO files are used to distribute operating system installer images, QEMU supports disk image file formats that are more convenient for transferring disk images than the raw contents of a disk. In fact, disk image file formats offer many other features such as the ability to import/export disks from other hypervisors, snapshots, and instantiating new disk images from a backing file.

Finally, managing disk images also involves the ability to take backups and restore them should it be necessary to roll back after the current disk contents have been lost or corrupted.

Emulated storage controllers

The virtual machine accesses block devices through storage controllers. These are the devices that the guest talks to in order to read or write blocks. Some storage controllers facilitate access to multiple block devices, such as a SCSI Host Bus Adapter that provides access to many SCSI disks.

Storage controllers vary in their features, performance, and guest operating system support. They expose a storage interface such as virtio-blk, NVMe, or SCSI. Virtual machines program storage controller registers to transfer data between memory buffers in RAM and block devices. Modern storage controllers support multiple request queues so that I/O can processed in parallel at high rates.

The most common storage controllers in QEMU are virtio-blk, virtio-scsi, AHCI (SATA), IDE for legacy systems, and SD Card controllers on embedded or smaller boards.

Disk image file formats

Disk image file formats handle the layout of blocks within a host file or device. The simplest format is the raw format where each block is located at its Logical Block Address (LBA) in the host file. This simple scheme does not offer much in the way of features.

QEMU’s native disk image format is QCOW2 and it offers a number of features:

  • Compactness - the host file grows as blocks are written so a sparse disk image can be much smaller than the virtual disk size.
  • Backing files - disk images can be based on a parent image so that a master image can be shared by virtual machines.
  • Snapshots - the state of the disk image can be saved and later reverted.
  • Compression - block compression reduces the image size.
  • Encryption - the disk image can be encrypted to protect data at rest.
  • Dirty bitmaps - backup applications can track changed blocks so that efficient incremental backups are possible.

A number of other disk image file formats are available for importing/exporting disk images for use with other software including VMware and Hyper-V.

Block jobs

Block jobs are background operations that manipulate disk images:

  • Commit - merging backing files to shorten a backing file chain.
  • Backup - copying out a point-in-time snapshot of a disk.
  • Mirror - copying an image to a new destination while the virtual machine can still write to it.
  • Stream - populating a disk image from its backing file.
  • Create - creating new disk image files.

These background operations are powerful tools for building storage migration and backup workflows.

Some operations like mirror and stream can take a long time because they copy large amounts of data. Block jobs support throttling to limit the performance impact on virtual machines.

qemu-img and qemu-storage-daemon

The qemu-img utility manipulates disk images. It can create, resize, snapshot, repair, and inspect disk images. It has both human-friendly and JSON output formats, making it suitable for manual use as well as scripting.

qemu-storage-daemon exposes QEMU’s storage functionality in a server process without running a virtual machine. It can export disk images over the Network Block Device (NBD) protocol as well as run block jobs and other storage commands. This makes qemu-storage-daemon useful for applications that want to automate disk image manipulation.


QEMU presents block devices to virtual machines via emulated storage controllers. On the host side the disk image file format, block jobs, and qemu-img/qemu-storage-daemon utilities provide functionality for working with disk images. Future blog posts will dive deeper into some of these areas and describe best practices for configuring storage.

September 14, 2020 07:00 AM

September 08, 2020

KVM on Z

QEMU v5.1 released

QEMU v5.1 is out. Here are the highlights from a KVM on Z perspective:

  • Secure Guest support
  • vfio-ccw: Various enhancements
  • DASD IPL via vfio-ccw is now fully functional (requires Linux kernel 5.8)

For further details, see the Release Notes.

by Stefan Raspl ( at September 08, 2020 01:58 PM

August 29, 2020

Stefan Hajnoczi

Using kcov code coverage with meson

The meson build system has built-in code coverage support, making it easy to identify lines of code that are not exercised by tests. Meson's code coverage support works with the gcov-based tools gcovr and lcov. This post shows how to use kcov with meson instead so that code coverage can be reported when gcov is unavailable.

How do code coverage tools work?

The gcov-based tools rely on compiler instrumentation, which both gcc and llvm support. Special compiler options instruct the compiler to emit instrumentation in every compiled function in order to record which lines of code are reached.

The kcov tool takes a different approach that does not require compiler support. It uses run-time instrumentation (like breakpoints) instead of compile-time instrumentation. This makes it possible to use kcov on existing binaries without recompilation, as long as debug information is available. The tool maps program instructions to lines of source code using the debug information.

There are pros and cons regarding exact features, performance, limitations, etc. For the most part the gcov approach works well when recompilation is possible and the compiler supports gcov. In other cases kcov is needed.

How to run meson tests under kcov

Meson's built-in code coverage support is designed for gcov and therefore works as a post-processing step after meson test was run. The workflow is different with kcov since the test itself must be run under kcov so it can instrument the process.

Run meson test as follows to get per-test coverage results:

$ meson test --wrapper='kcov kcov-output'

The $BUILD_DIR/kcov-output/ directory will contain the coverage results, one set for each test that was run.

Merging coverage results

If your goal is a single coverage percentage for the entire test suite, then the per-test results need to be merged. The follow wrapper script can be used:

$ cat
test_name=$(basename $1)
exec kcov kcov-runs/$test_name "$@"

And it is invoked like this:

$ rm -rf $BUILD_DIR/kcov-runs
$ mkdir $BUILD_DIR/kcov-runs
$ meson test --wrapper "$SOURCE_DIR/"
$ rm -rf $BUILD_DIR/kcov-output
$ kcov --merge $BUILD_DIR/kcov-output $BUILD_DIR/kcov-runs/*

The merged results are located in the $BUILD_DIR/kcov-output/ directory.


Meson already has built-in support for gcov-based code coverage. If you cannot use gcov, then kcov is an alternative that is fairly easy to integrate into a meson project.

by Unknown ( at August 29, 2020 01:04 PM

August 24, 2020

Stefan Hajnoczi

QEMU Internals: Event loops

This post explains event loops in QEMU v5.1.0 and their unique features compared to other event loop implementations. The APIs are not covered in detail here since they are explained in doc comments. Instead, the focus is on the big picture and why things work the way they do.

Event loops are central to many I/O-bound applications like network services and graphical desktop applications. QEMU also has I/O-bound work that fits well into an event loop. Examples include the QMP monitor, disk I/O, and timers.

An event loop monitors event sources for activity and invokes a callback function when an event occurs. This makes it possible to process multiple event sources within a single CPU thread. The application can appear to do multiple things at once without multithreading because it switches between handling different event sources. This architecture is common in Javascript, Python Twisted/asyncio, and many other environments. Sometimes the event loop is hidden underneath coroutines or async/await language features (QEMU has coroutines but often the event loop is still used directly).

The most important event sources in QEMU are:

  • File descriptors such as sockets and character devices.
  • Event notifiers (implemented as eventfds on Linux).
  • Timers for delayed function execution.
  • Bottom-halves (BHs) for invoking a function in another thread or deferring a function call to avoid reentrancy.

Event loops and threads

QEMU has several different types of threads:

  • vCPU threads that execute guest code and perform device emulation synchronously with respect to the vCPU.
  • The main loop that runs the event loops (yes, there is more than one!) used by many QEMU components.
  • IOThreads that run event loops for device emulation concurrently with vCPUs and "out-of-band" QMP monitor commands.

It's worth explaining how device emulation interacts with threads. When guest code accesses a device register the vCPU thread traps the access and dispatches it to an emulated device. The device's read/write function runs in the vCPU thread. The vCPU thread cannot resume guest code execution until the device's read/write function returns. This means long-running operations like emulating a timer chip or disk I/O cannot be performed synchronously in the vCPU thread since they would block the vCPU. Most devices solve this problem using the main loop thread's event loops. They add timer or file descriptor monitoring callbacks to the main loop and return back to guest code execution. When the timer expires or the file descriptor becomes ready the callback function runs in the main loop thread. The final part of emulating a guest timer or disk access therefore runs in the main loop thread and not a vCPU thread.

Some devices perform the guest device register access in the main loop thread or an IOThread thanks to ioeventfd. ioeventfd is a Linux KVM API and also has a userspace fallback implementation for TCG that traps vCPU device accesses and writes to a file descriptor so another thread can handle the access.

The key point is that vCPU threads do not run an event loop. The main loop thread and IOThreads run event loops. vCPU threads can add event sources to the main loop or IOThread event loops. Callbacks run in the main loop thread or IOThreads.

How the main loop and IOThreads differ

The main loop and IOThreads share some code but are fundamentally different. The common code is called AioContext and is QEMU's native event loop API. Commonly-used functions include aio_set_fd_handler(), aio_set_event_handler(), aio_timer_init(), and aio_bh_new().

The main loop actually has a glib GMainContext and two AioContext event loops. QEMU components can use any of these event loop APIs and the main loop combines them all into a single event loop function os_host_main_loop_wait() that calls qemu_poll_ns() to wait for event sources. This makes it possible to combine glib-based code with code using the native QEMU AioContext APIs.

The reason why the main loop has two AioContexts is because one, called iohandler_ctx, is used to implement older qemu_set_fd_handler() APIs whose handlers should not run when the other AioContext, called qemu_aio_context, is run using aio_poll(). The QEMU block layer and newer code uses qemu_aio_context while older code uses iohandler_ctx. Over time it may be possible to unify the two by converting iohandler_ctx handlers to safely execute in qemu_aio_context.

IOThreads have an AioContext and a glib GMainContext. The AioContext is run using the aio_poll() API, which enables the advanced features of the event loop. If a glib event loop is needed then the GMainContext can be run using g_main_loop_run() and the AioContext event sources will be included.

Code that relies on the AioContext aio_*() APIs will work with both the main loop and IOThreads. Older code using qemu_*() APIs only works with the main loop. glib code works with both the main loop and IOThreads.

The key difference between the main loop and IOThreads is that the main loop uses a traditional event loop that calls qemu_poll_ns() while IOThreads AioContext aio_poll() has advanced features that result in better performance.

AioContext features

AioContext has the following event loop features that traditional event loops do not have:

  • Adaptive polling support for lower latency but slightly higher CPU consumption. AioContext event sources can have a userspace polling function that detects events without performing syscalls (e.g. peeking at a memory location). This allows the event loop to avoid block syscalls that might lead the host kernel scheduler to yield the thread and put the physical CPU into a low power state. Keeping the CPU busy and avoiding entering the kernel minimizes latency.
  • O(1) time complexity with respect to the number of monitored file descriptors. When there are thousands of file descriptors O(n) APIs like poll(2) spend time scanning over all file descriptors, even those that have no activity. This scalability bottleneck can be avoided with Linux io_uring and epoll APIs, both of which are supported by AioContext aio_poll(2).
  • Nanosecond timers. glib's event loop only has millisecond timers, which is not sufficient for emulating hardware timers.

These features are required for performance reasons. Unfortunately glib's event loop does not support them, otherwise QEMU could use GMainContext as its only event loop.


QEMU uses both its native AioContext event loop and glib's GMainContext. The QEMU main loop and IOThreads work differently, with IOThreads offering the best performance thanks to its AioContext aio_poll() event loop. Modern QEMU code should use AioContext APIs for optimal performance and so that the code can be used in both the main loop and IOThreads.

by Unknown ( at August 24, 2020 07:52 AM

August 12, 2020

Stefan Hajnoczi

Why QEMU should move from C to Rust

Welcome Redditors and HackerNews folks! This post is getting attention outside the QEMU community, so I'd like to highlight two things that may not be immediately clear: I am a QEMU maintainer and I'm not advocating to Rewrite It In Rust. Enjoy! :)

My KVM Forum 2018 presentation titled Security in QEMU: How Virtual Machines provide Isolation (pdf) (video) reviewed security bugs in QEMU and found the most common causes were C programming bugs. This includes buffer overflows, use-after-free, uninitialized memory, and more. In this post I will argue for using Rust as a safer language that prevents these classes of bugs.

In 2018 the choice of a safer language was not clear. C++ offered safe abstractions without an effective way to prohibit unsafe language features. Go also offered safety but with concerns about runtime costs. Rust looked promising but few people had deep experience with it. In 2018 I was not able to argue confidently for moving away from C in QEMU.

Now in 2020 the situation is clearer. C programming bugs are still the main cause of CVEs in QEMU. Rust has matured, its ecosystem is growing and healthy, and there are virtualization projects like Crosvm, Firecracker, and cloud-hypervisor that prove Rust is an effective language for writing Virtual Machine Monitors (VMM). In the QEMU community Paolo Bonzini and Sergio Lopez's work on rust-vmm and vhost-user code inspired me to look more closely at moving away from C.

Do we need to change programming language?

Most security bugs in QEMU are C programming bugs. This is easy to verify by looking through the CVE listings. Although I have only reviewed CVEs it seems likely that non-security bugs are also mostly C programming bugs.

Eliminating C programming bugs does not necessarily require switching programming languages. Other approaches to reducing bug rates in software include:

  • Coding style rules that forbid unsafe language features.
  • Building safe abstractions and prohibiting unsafe language features or library APIs.
  • Static checkers that scan source code for bugs.
  • Dynamic sanitizers that run software with instrumentation to identify bugs.
  • Unit testing and fuzzing.

The problem is, the QEMU community has been doing these things for years but new bugs are still introduced despite these efforts. It is certainly possible to spend more energy on these efforts but the evidence shows that bugs continue to slip through.

There are two issues with these approaches to reducing bugs. First, although these approaches help find existing bugs, eliminating classes of bugs so they cannot exist in the first place is a stronger approach. This is hard to do with C since the language is unsafe, placing the burden of safety on the programmer.

Second, much of the ability to write safe C code comes with experience. Custom conventions, APIs, tooling, and processes to reduce bugs is a hurdle for one-time contributors or newcomers. It makes the codebase inaccessible unless we accept lower standards for some contributors. Code quality should depend as little on experience as possible but C is notorious for being a programming language that requires a lot of practice before you can write production-quality code.

Why Rust?

Safe languages eliminate memory safety bugs (and other classes like concurrency bugs). Rust made this a priority in its design:

  • Use-after-free, double-free, memory leaks, and other lifetime bugs are prevented at compile-time by the borrow checker where the compiler checks ownership of data.
  • Buffer overflows and other memory corruptions are prevented by compile-time and runtime bounds-checking.
  • Pointer deference bugs are prevented by the absense of NULL pointers and strict ownership rules.
  • Uninitialized memory is prevented because all variables and fields must be initialized.

Rust programs can still "panic" at runtime when safety cannot be proven at compile time but this does not result in undefined behavior as seen in C programs. The program simply aborts with a backtrace. Bugs that could have resulted in arbitrary code execution in C become at most denial-of-service bugs in Rust. This reduces the severity of bugs.

As a result of this language design most C programming bugs that plague QEMU today are either caught by the compiler or turn into a safe program termination. It is reasonable to expect CVEs to reduce in number and in severity when switching to Rust.

At the same time Rust eliminates the need for many of the measures that the QEMU community added onto C because the Rust programming language and its compiler already enforce safety. This means newcomers and one-time contributors will not need QEMU-specific experience, can write production-quality code more easily, and can get their code merged more quickly. It also means reviewers will have to spend less time pointing out C programming bugs or asking for changes that comply with QEMU's way of doing things.

That said, Rust has a reputation for being a scary language due to the borrow checker. Most programmers have not thought about object lifetimes and ownership as systematically and explicitly as required by Rust. This raises the bar to learning the language, but I look at it this way: learning Rust is humanly possible, writing bug-free C code is not.

How can we change programming language?

When I checked in 2018 QEMU was 1.5 million lines of code. It has grown since then. Moving a large codebase to a new programming language is extremely difficult. If people want to convert QEMU to Rust that would be great, but I personally don't have the appetite to do it because I think the integration will be messy, result in a lot of duplication, and there is too much un(der)maintained code that is hard to convert.

The reason I am writing this post is because device emulation, the main security attack surface for VMMs, can be done in a separate program. That program can be written in any language and this is where Rust comes in. For vhost devices it is possible to write Rust device backends today and I hope this will become the default approach to writing new devices.

For non-vhost devices the vfio-user project is working on an interface out-of-process device emulation. It will be possible to implement devices in Rust there too.

If you are implementing new device emulation code please consider doing it in Rust!


Most security bugs in QEMU today are C programming bugs. Switching to a safer programming language will significantly reduce security bugs in QEMU. Rust is now mature and proven enough to use as the language for device emulation code. Thanks to vhost-user and vfio-user using Rust for device emulation does not require a big conversion of QEMU code, it can simply be done in a separate program. This way attack surfaces can be written in Rust to make them less susceptible to security bugs going forward.

by Unknown ( at August 12, 2020 07:32 PM

August 11, 2020

QEMU project

QEMU version 5.1.0 released

We’d like to announce the availability of the QEMU 5.1.0 release. This release contains 2500+ commits from 235 authors.

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

Highlights include:

  • ARM: support for ARMv8.2 TTS2UXN architecture feature
  • ARM: support for ARMv8.5 MemTag architecture feature
  • ARM: new board support for sonorapass-bmc
  • ARM: virt: support for memory hot-unplug
  • ARM: support for nvdimm hotplug for ACPI guests
  • AVR: new architecture support for AVR CPUs
  • AVR: new board support for Arduino Duemilanove, Arduino Mega 2560, Arduino Mega, and Arduino UNO
  • MIPS: support for Loongson 3A CPUs (R1 and R4)
  • MIPS: performance improvements for FPU and MSA instruction emulation
  • PowerPC: support for guest error recovery via FWNMI
  • RISC-V: support for SiFive E34 and Ibex CPUs
  • RISC-V: new board support for HiFive1 revB and OpenTitan
  • RISC-V: Spike machine now supports more than 1 CPU
  • s390: KVM support for protected virtualization (secure execution mode)
  • x86: improvements to HVF acceleration support on macOS
  • x86: reduced virtualization overhead for non-enlightened Windows guests via Windows ACPI Emulated Device Table
  • block: support for 2MB logical/physical blocksizes for virtual storage devices
  • crypto: support for passing secrets to QEMU via Linux keyring
  • crypto: support for LUKS keyslot management via qemu-img
  • NVMe: support for Persistent Memory Region from NVMe 1.4 spec
  • qemu-img: additional features added for map/convert/measure commands, as well as support for zstd compression
  • qemu-img: support for new ‘bitmap’ command for manipulating persistent bitmaps in qcow2 files
  • virtio: TCG guests can now use vhost-user threads
  • virtio: vhost-user now supports registering more than 8 RAM slots
  • and lots more…

Thank you to everyone involved!

August 11, 2020 11:00 PM

August 08, 2020

Thomas Huth

How to speed up your QEMU development cycles

Here’s another bunch of tips for QEMU developer – if you are not working with the QEMU source code, you can certainly ignore this article.

Compiling QEMU is a tedious action, since it takes quite some time to compile the whole code base. But with some basic tricks, you can shorten your code-change → compile → test→ repeat cycles quite a bit. These tricks might be quite well-known to the “older” QEMU developers already, but if you just started working on QEMU, they might be useful for you:

Compile and run tests in parallel

Well, this is likely the most obvious hint, but in case you didn’t know it yet: depending on the amount of CPU cores of your build machine, you can of course build the QEMU sources in parallel, for example compile with:

 make -j$(nproc)

The nice thing is that this also works for running the test suite:

 make -j$(nproc) check

This runs quite a bit faster on modern machines than simply running make check only.

Use out-of-tree builds

While in-tree builds might initially work fine, you sooner or later get to a point where you have to compile QEMU with different options. For example, does your code still compile if you run the configure script with --disable-tcg? Or does it still compile with clang instead of gcc? Each time you re-run the configure script to test a different option, you have to re-compile most of the QEMU code base afterwards. Thus it is better to keep different configurations in different build directories, so that you don’t have to recompile almost everything when you switch back and forth between different configurations. For example:

 mkdir build-gcc
 cd build-gcc
 ../configure --cc=gcc
 make -j$(nproc)
 cd ..
 mkdir build-clang
 cd build-clang
 ../configure --cc=clang
 make -j$(nproc)
 cd ..
 cd build-gcc
 make -j$(nproc)         # This should finish quickly now

By the way, in-tree builds are frowned on by many developers and might be disabled in a future release of QEMU, so it’s better to get used to out-of-tree builds anyway.

Only compile the target that you really need

Compiling all QEMU binaries takes a long time, even if you do it in parallel. During development, you often only need one specific binaries, like qemu-system-x86_64. So instead of re-compiling the whole set of binaries each time, simply only recompile the binary that you really need. At a first glance, you might want to use the --target-list option of the configure script to limit the set of targets that get compiled during make. But this has the disadvantage that you still need a second build folder where you can compile and test the other targets before you submit a patch (which you should always do). So there is an easier way to just compile one target binary, where you do not have to use the --target-list configure option at all. Simply run:

 make -j$(nproc) x86_64-softmmu/all

This will only re-compile the x86_64-softmmu/qemu-system-x86_64 binary, in much less time than compiling all target binaries.

Note: In older QEMU versions, this target was called subdir-x86_64-softmmu instead of x86_64-softmmu/all. And in future versions of QEMU (i.e. when the meson build system has been merged), you might need to run make qemu-system-x86_64 instead.

Apart from building the QEMU emulator binaries directly, there are also special targets for the tools (e.g. run make qemu-img) and the documentation (run make html). Have a look at the output of make help for a full list of available targets.

Only test the target that you really need

Before submitting patches, you should of course always run the whole test suite with make -j$(nproc) check to make sure that there are no regressions. But while you are working on a problem, it is often enough to only run single tests that trigger the problem you are working on, or that are at least covering the area you are interested in.

Similar to the possibility to compile only single target binaries, there is also a way to run only a certain group of tests. Run make check-help to get a list of the possible test targets. For example, if you are only working on x86-related code, it might be enough to run only:

 make -j$(nproc) check-qtest-x86_64

This will only execute the x86-related qtests. If that is still taking too long and you are only interested in one single qtest for example, you can also run that test directly like this:

export QTEST_QEMU_BINARY=x86_64-softmmu/qemu-system-x86_64

The QTEST_QEMU_BINARY environment variable is required here to let the test know which QEMU binary it should use.

Once you’re done with developing your patch, you should of course run at least one cycle of the complete make check (or even make check SPEED=slow) test suite to avoid that there are unexpected side effects in other parts of QEMU.

Use the right configure options for your build

You can also speed up your builds a little bit by using the right options when running the configure script – just have a look at the output of configure --help. You are sure that you will never use/touch feature xyz? Then try to compile without it by using configure --disable-xyz. For example, are you only working with KVM (and never with TCG)? Then give --disable-tcg a try. But please note that it’s still a good idea to have a separate build folder with a configuration where you have all features enabled - so that you can double-check that everything is still working fine before submitting your patches. Alternatively, you can (and should) also use one of the CI systems on Gitlab or Github, of course, to test your patches before submitting them to the mailing list.

Apart from the --disable-feature options, there is one option which can help a little bit when you change the HEAD of your git branch quite often: Run configure with --with-pkgversion=somestring. By default, the pkgversion (which will be part of the version string of QEMU) will be created using the current commit ID of the branch you are working on. Thus each time you change your branch or add a new commit to your local branch, this will trigger a rebuild of a bunch of files. Thus if you used --with-pkgversion with a fixed string, you can avoid this recompilation of these files when the HEAD of your current branch changed.

August 08, 2020 12:25 PM

August 07, 2020

ARM Datacenter Project

How to setup NVMe/TCP with NVME-oF using KVM and QEMU

In this post we will explain how to connect two QEMU Guests (using KVM) with NVMe over Fabrics using TCP as the transport.

We will show how to connect an NVMe target to an NVMe initiator, using the NVMe/TCP transport. It is worth mentioning before we get started that we will use the terms of “target” to describe the guest which exports the target, and “initiator” to describe the guest which connects to the target.

The target QEMU guest will export a simulated NVME drive which we will create from an image file. The initiator guest will connect to the target and will be able to access this NVME drive.

Note that this configuration is largely an example to be used for evaluation and/or development with NVME-of. This setup described is not intended to be used for a production environment.

First Step: Create a Guest

Before we can get started, we need to bring up our QEMU guests and get them sharing the same network.

Fortunately, we described in an earlier post how to setup a shared network for two QEMU guests. That’s a good place to start.

We also have other posts for getting an aarch64 VM up and running, including:

Kernel Configuration

Before we get started we will make sure that the guest’s kernel has all the right modules built in.

The guest’s kernel config should have these modules.

$ cat /boot/config-`uname -r` | grep NVME

# NVME Support
# CONFIG_NVME_HWMON is not set
# end of NVME Support


Make sure the nvme-cli is installed on the guests.

sudo apt-get install nvme-cli

Initiator Guest Setup

This is the QEMU command we use to bring up the initiator QEMU guest.

sudo qemu-system-aarch64 -nographic -machine virt,gic-version=max -m 8G -cpu max   \
       -drive file=./ubuntu20-a.img,if=none,id=drive0,cache=writeback              \
       -device virtio-blk,drive=drive0,bootindex=0                                 \
       -drive file=./flash0-a.img,format=raw,if=pflash                             \
       -drive file=./flash1-a.img,format=raw,if=pflash                             \
       -smp 4 -accel kvm -netdev bridge,id=hn1                                     \
       -device virtio-net,netdev=hn1,mac=e6:c8:ff:09:76:99

Target Guest Setup

When you bring up the target’s QEMU guest, be sure to include an NVME disk.

We can create the disk with the below.

qemu-img create -f qcow2 nvme.img 10G

When we bring up QEMU, add this set of options so that the guest sees the NVME disk.

-drive file=./nvme.img,if=none,id=nvme0 -device nvme,drive=nvme0,serial=1234

This is the QEMU command we use to bring up the target QEMU guest.

Note how we added in the options for the NVMe device.

sudo qemu-system-aarch64 -nographic -machine virt,gic-version=max -m 8G -cpu max \
       -drive file=./ubuntu20-b.img,if=none,id=drive0,cache=writeback              \ 
       -device virtio-blk,drive=drive0,bootindex=0                                 \ 
       -drive file=./flash0-b.img,format=raw,if=pflash                             \
       -drive file=./flash1-b.img,format=raw,if=pflash                             \
       -smp 4 -accel kvm -netdev bridge,id=hn1                                     \ 
       -device virtio-net,netdev=hn1,mac=e6:c8:ff:09:76:9c                         \ 
       -drive file=./nvme.img,if=none,id=nvme0 -device nvme,drive=nvme0,serial=1234 

Configure Target

Load the following modules on the target:

sudo modprobe nvmet
sudo modprobe nvmet-tcp

Next, create and configure an NVMe Target subsystem. This includes creating a namespace.

cd /sys/kernel/config/nvmet/subsystems
sudo mkdir nvme-test-target
cd nvme-test-target/
echo 1 | sudo tee -a attr_allow_any_host > /dev/null
sudo mkdir namespaces/1
cd namespaces/1

Before we can attach our NVMe device to this target, we need to find the name.

sudo nvme list

Node             SN     Model            Namespace Usage                      Format           FW Rev          
---------------- ------ ---------------- --------- -------------------------- ---------------- --------
/dev/nvme0n1     1234   QEMU NVMe Ctrl   1          10.74  GB /  10.74  GB    512   B +  0 B   1.0      

The next step attaches our NVMe device /dev/nvme0n1 to this target and enables it.

echo -n /dev/nvme0n1 |sudo tee -a device_path > /dev/null
echo 1|sudo tee -a enable > /dev/null

Next we will create an NVMe target port, and configure the IP address and other parameters.

sudo mkdir /sys/kernel/config/nvmet/ports/1
cd /sys/kernel/config/nvmet/ports/1

echo |sudo tee -a addr_traddr > /dev/null

echo tcp|sudo tee -a addr_trtype > /dev/null
echo 4420|sudo tee -a addr_trsvcid > /dev/null
echo ipv4|sudo tee -a addr_adrfam > /dev/null

The final step creates a link to the subsystem from the port.

sudo ln -s /sys/kernel/config/nvmet/subsystems/nvme-test-target/ /sys/kernel/config/nvmet/ports/1/subsystems/nvme-test-target

At this point we should see a message in the dmesg log

dmesg |grep "nvmet_tcp"

[81528.143604] nvmet_tcp: enabling port 1 (

Mount Target on Initiator

Load the following modules on the initiator:

sudo modprobe nvme
sudo modprobe nvme-tcp

Next, check that we currently do not see any NVMe devices. The output of the following command should be blank.

sudo nvme list

Next, we will attempt to discover the remote target.

When we initially tried the “discover” command we got an error that our hostnqn was needed. In our example below you will notice that we are providing a hostnqn.

sudo nvme discover -t tcp -a -s 4420

Discovery Log Number of Records 1, Generation counter 2
=====Discovery Log Entry 0======
trtype:  tcp
adrfam:  ipv4
subtype: nvme subsystem
treq:    not specified, sq flow control disable supported
portid:  1
trsvcid: 4420
subnqn:  nvme-test-target
sectype: none

Using the subnqn as the -n argument, we will connect to the discovered target.

sudo nvme connect -t tcp -n nvme-test-target -a -s 4420

Success. We can immediately check the nvme list for the attached device.

sudo nvme list
Node             SN               Model  Namespace Usage                      Format           FW Rev  
---------------- ---------------- ------ --------- -------------------------- ---------------- --------
/dev/nvme0n1     84cfc88e9ba4a8f4 Linux  1          10.74  GB /  10.74  GB    512   B +  0 B   5.8.0-rc

To detach the target, run the following command on the initiator.

sudo nvme disconnect /dev/nvme0n1 -n nvme-test-target


by Rob Foley at August 07, 2020 11:52 AM

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