Blogging about open source virtualization

Yesterday, virt-bootstrap came to life. This is tool aims at simplifying the creation of root file systems for use with libvirt's LXC container drivers. I started prototyping it a few months ago and Radostin Stoyanov wonderfully took it over during this year's Google Summer of Code.

For most users, this tool will just be used by virt-manager (since version 1.4.2). But it can be used directly from any script or command line.

The nice thing about virt-bootstrap is that will allow you to create a root file system out of existing docker images, tarballs or virt-builder templates. For example, the following command will get and unpack the official openSUSE docker image in /tmp/foo.

$ virt-bootstrap docker://opensuse /tmp/foo

Virt-bootstrap offers options to:

• generate qcow2 image with backing chain instead of plain folder
• apply user / group ID mapping
• set the root password in the container

Enjoy easy containers creation with libvirt ecosystem, and have fun!

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

Highlights include:

• Support for ACPI NUMA distance info and control over CPU NUMA assignments via ‘-numa cpu’ parameters
• Support for LUKS encryption format in qcow2 images
• Monitor/Management interface improvments: additional debug information available through ‘info ramblock/cmma/register/qtree’, support for viewing connected clients via ‘info vnc’, improved parsing support for QMP protocol, and other additional commands
• QXL and virtio-gpu support for controlling default display resolution
• Support for vhost-user-scsi devices
• NVMe emulation support for Write Zeroes command and Controller Memory Buffers
• Guest agent support for querying guest hostname, users, timezone, and OS version/release information
• ARM: KVM support for Raspberry Pi 3
• ARM: emulation support for MPS2/MPS2+ FPGA-based dev boards
• ARM: zynq: SPIPS flash support
• ARM: exynos4210: hardware PRNG device, SDHCI, and system poweroff
• Microblaze: support for CPU versions 9.4, 9.5, 9.6, and 10.0
• MIPS: support for Enhanced Virtual Addressing (EVA)
• MIPS: initrd support for kaslr-enabled kernels
• OpenRISC: support for shadow registers, idle states, and numcores/coreid/EVAR/EPH registers
• PowerPC: Multi-threaded TCG emulation support
• PowerPC: OpenBIOS VGA driver for MacOS guests
• PowerPC: pseries: KVM and emulation support for POWER9 guests
• PowerPC: pseries: support for hash page table resizing
• s390: channel device passthrough support via vfio-ccw
• s390: support for channel-attached 3270 “green screen” devices for use as guest consoles or additional TTYs
• s390: improved support for PCI (AEN, AIS, and zPCI)
• s390: support for z14 CPU models and netboot/TFTP via CCW BIOS
• s390: TCG support for atomic “LOAD AND x” and “COMPARE SWAP” operations, LOAD PROGRAM PARAMETER, extended facilities, CPU type, and many more less-common instructions
• SH: TCG support for host atomic instructions for emulating tas.b and gUSA (user-space atomics), and support for fpchg/fsrra instructions
• SPARC: fixes for booting Solaris 2.6 on sun4m/OpenBIOS machines
• x86: Q35 MCH supports TSEG higher than 8MB
• x86: SSE register access via gdbstub
• Xen: support for multi-page shared rings, and 9pfs/virtfs backend
• Xtensa: sim machine console can be directed to chardev via -serial
• and lots more…

Thank you to everyone involved!

I am happy to announce a new bugfix release of virt-viewer 6.0 (gpg), including experimental Windows installers for Win x86 MSI (gpg) and Win x64 MSI (gpg). The virsh and virt-viewer binaries in the Windows builds should now successfully connect to libvirtd, following fixes to libvirt’s mingw port.

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

All historical releases are available from:

http://virt-manager.org/download/

Changes in this release include:

• Mention use of ssh-agent in man page
• Display connection issue warnings in main window
• Switch to GTask API
• Add support changing CD ISO with oVirt foreign menu
• Update various outdated links in README
• Avoid printing password in debug logs
• Pass hostname to authentication dialog
• Fix example URLs in man page
• Add args to virt-viewer to specify whether to resolve VM based on ID, UUID or name
• Fix misc runtime warnings
• Improve support for extracting listening info from XML
• Enable connecting to SPICE over UNIX socket
• Fix warnings with newer GCCs
• Allow controlling zoom level with keypad
• Don’t close app during seemless migration
• Don’t show toolbar in kiosk mode
• Re-show auth dialog in kiosk mode
• Don’t show error when cancelling auth
• Change default screenshot name to ‘Screenshot.png’
• Report errors when saving screenshot
• Fix build with latest glib-mkenums

Thanks to everyone who contributed towards this release.

I am happy to announce a new release of libosinfo version 1.1.0 is now available, signed with key DAF3 A6FD B26B 6291 2D0E 8E3F BE86 EBB4 1510 4FDF (4096R). All historical releases are available from the project download page.

Changes in this release include:

• Force UTF-8 locale for new glib-mkenums
• Avoid python warnings in example program
• Misc test suite updates
• Fix typo in error messages
• Remove ISO header string padding
• Disable bogus gcc warning about unsafe loop optimizations
• Remove reference to fedorahosted.org
• Don’t hardcode /usr/bin/perl, use /usr/bin/env
• Support eject-after-install parameter in OsinfoMedia
• Fix misc warnings in docs
• Fix error propagation when loading DB
• Add usb.ids / pci.ids locations for FreeBSD
• Don’t include private headers in gir/vapi generation

Thanks to everyone who contributed towards this release.

QEMU has a lot of interfaces (like command line options or HMP commands) and old features (like certain devices) which are considered as deprecated since other more generic or better interfaces/features have been established instead. While the QEMU developers are generally trying to keep each QEMU release compatible with the previous ones, the old legacy sometimes gets into the way when developing new code and/or causes quite some burden of maintaining it.

Thus we are currently considering to get rid of some of the old interfaces and features in a future release and have started to collect a list of such old items in our QEMU documentation. If you are running QEMU directly, please have a look at this deprecation chapter of the QEMU documentation to see whether you are still using one of these old interfaces or features, so you can adapt your setup to use the new interfaces/features instead. Or if you rather think that one of the items should not be removed from QEMU at all, please speak up on the qemu-devel mailing list to explain why the interface or feature is still required.

The perf(1) tool added support for userspace static probes in Linux 4.8. Userspace static probes are pre-defined trace points in userspace applications. Application developers add them so frequently needed lifecycle events are available for performance analysis, troubleshooting, and development.

Static userspace probes are more convenient than defining your own function probes from scratch. You can save time by using them and not worrying about where to add probes because that has already been done for you.

On my Fedora 26 machine the QEMU, gcc, and nodejs packages ship with static userspace probes. QEMU offers probes for vcpu events, disk I/O activity, device emulation, and more.

Without further ado, here is how to trace static userspace probes with perf(1)!

Scan the binary for static userspace probes

The perf(1) tool needs to scan the application's ELF binaries for static userspace probes and store the information in $HOME/.debug/usr/:

# perf buildid-cache --add /usr/bin/qemu-system-x86_64

List static userspace probes

Once the ELF binaries have been scanned you can list the probes as follows:

# perf list sdt_*:*

List of pre-defined events (to be used in -e):

  sdt_qemu:aio_co_schedule                           [SDT event]
  sdt_qemu:aio_co_schedule_bh_cb                     [SDT event]
  sdt_qemu:alsa_no_frames                            [SDT event]
  ...

Let's trace something!

First add probes for the events you are interested in:

# perf probe sdt_qemu:blk_co_preadv
Added new event:
  sdt_qemu:blk_co_preadv (on %blk_co_preadv in /usr/bin/qemu-system-x86_64)

You can now use it in all perf tools, such as:

 perf record -e sdt_qemu:blk_co_preadv -aR sleep 1

Then capture trace data as follows:

# perf record -a -e sdt_qemu:blk_co_preadv
^C
[ perf record: Woken up 3 times to write data ]
[ perf record: Captured and wrote 2.274 MB perf.data (4714 samples) ]

The trace can be printed using perf-script(1):

# perf script
 qemu-system-x86  3425 [000]  2183.218343: sdt_qemu:blk_co_preadv: (55d230272e4b) arg1=94361280966400 arg2=94361282838528 arg3=0 arg4=512 arg5=0
 qemu-system-x86  3425 [001]  2183.310712: sdt_qemu:blk_co_preadv: (55d230272e4b) arg1=94361280966400 arg2=94361282838528 arg3=0 arg4=512 arg5=0
 qemu-system-x86  3425 [001]  2183.310904: sdt_qemu:blk_co_preadv: (55d230272e4b) arg1=94361280966400 arg2=94361282838528 arg3=512 arg4=512 arg5=0
 ...

If you want to get fancy it's also possible to write trace analysis scripts with perf-script(1). That's a topic for another post but see the --gen-script= option to generate a skeleton script.

Current limitations

As of July 2017 there are a few limitations to be aware of:

Probe arguments are automatically numbered and do not have human-readable names. You will see arg1, arg2, etc and will need to reference the probe definition in the application source code to learn the meaning of the argument. Some versions of perf(1) may not even print arguments automatically since this feature was added later.

The contents of string arguments are not printed, only the memory address of the string.

Probes called from multiple call-sites in the application result in multiple perf probes. For example, if probe foo is called from 3 places you get sdt_myapp:foo, sdt_myapp:foo_1, and sdt_myapp:foo_2 when you run perf probe --add sdt_myapp:foo.

The SystemTap semaphores feature is not supported and such probes will not fire unless you manually set the semaphore inside your application or from another tool like GDB. This means that the sdt_myapp:foo will not fire if the application uses the MYAPP_FOO_ENABLED() macro like this: if (MYAPP_FOO_ENABLED()) MYAPP_FOO();.

Some history and alternative tools

Static userspace probes were popularized by DTrace's <sys/sdt.h> header. Tracers that came after DTrace implemented the same interface for compatibility.

On Linux the initial tool for static userspace probes was SystemTap. In fact, the <sys/sdt.h> header file on my Fedora 26 system is still part of the systemtap-sdt-devel package.

More recently the GDB debugger gained support for static userspace probes. See the Static Probe Points documentation if you want to use userspace static probes from GDB.

Conclusion

It's very handy to have static userspace probing available alongside all the other perf(1) tracing features. There are a few limitations to keep in mind but if your tracing workflow is based primarily around perf(1) then you can now begin using static userspace probes without relying on additional tools.

This post is a 64-bit companion to an earlier post of mine where I described how to get Debian running on QEMU emulating a 32-bit ARM “virt” board. Thanks to commenter snak3xe for reminding me that I’d said I’d write this up…

Why the “virt” board?

For 64-bit ARM QEMU emulates many fewer boards, so “virt” is almost the only choice, unless you specifically know that you want to emulate one of the 64-bit Xilinx boards. “virt” supports supports PCI, virtio, a recent ARM CPU and large amounts of RAM. The only thing it doesn’t have out of the box is graphics.

Prerequisites and assumptions

I’m going to assume you have a Linux host, and a recent version of QEMU (at least QEMU 2.8). I also use libguestfs to extract files from a QEMU disk image, but you could use a different tool for that step if you prefer.

I’m going to document how to set up a guest which directly boots the kernel. It should also be possible to have QEMU boot a UEFI image which then boots the kernel from a disk image, but that’s not something I’ve looked into doing myself. (There may be tutorials elsewhere on the web.)

Getting the installer files

I suggest creating a subdirectory for these and the other files we’re going to create.

wget -O installer-linux http://http.us.debian.org/debian/dists/stretch/main/installer-arm64/current/images/netboot/debian-installer/arm64/linux
wget -O installer-initrd.gz http://http.us.debian.org/debian/dists/stretch/main/installer-arm64/current/images/netboot/debian-installer/arm64/initrd.gz

Saving them locally as installer-linux and installer-initrd.gz means they won’t be confused with the final kernel and initrd that the installation process produces.

(If we were installing on real hardware we would also need a “device tree” file to tell the kernel the details of the exact hardware it’s running on. QEMU’s “virt” board automatically creates a device tree internally and passes it to the kernel, so we don’t need to provide one.)

Installing

First we need to create an empty disk drive to install onto. I picked a 5GB disk but you can make it larger if you like.

qemu-img create -f qcow hda.qcow2 5G

Now we can run the installer:

qemu-system-aarch64 -M virt -m 1024 -cpu cortex-a53 \
  -kernel installer-linux \
  -initrd installer-initrd.gz \
  -drive if=none,file=hda.qcow2,format=qcow,id=hd \
  -device virtio-blk-pci,drive=hd \
  -netdev user,id=mynet \
  -device virtio-net-pci,netdev=mynet \
  -nographic -no-reboot

The installer will display its messages on the text console (via an emulated serial port). Follow its instructions to install Debian to the virtual disk; it’s straightforward, but if you have any difficulty the Debian installation guide may help.

The actual install process will take a few hours as it downloads packages over the network and writes them to disk. It will occasionally stop to ask you questions.

Late in the process, the installer will print the following warning dialog:

   +-----------------| [!] Continue without boot loader |------------------+
   |                                                                       |
   |                       No boot loader installed                        |
   | No boot loader has been installed, either because you chose not to or |
   | because your specific architecture doesn't support a boot loader yet. |
   |                                                                       |
   | You will need to boot manually with the /vmlinuz kernel on partition  |
   | /dev/vda1 and root=/dev/vda2 passed as a kernel argument.             |
   |                                                                       |
   |                              <Continue>                               |
   |                                                                       |
   +-----------------------------------------------------------------------+  

Press continue for now, and we’ll sort this out later.

Eventually the installer will finish by rebooting — this should cause QEMU to exit (since we used the -no-reboot option).

At this point you might like to make a copy of the hard disk image file, to save the tedium of repeating the install later.

Extracting the kernel

The installer warned us that it didn’t know how to arrange to automatically boot the right kernel, so we need to do it manually. For QEMU that means we need to extract the kernel the installer put into the disk image so that we can pass it to QEMU on the command line.

There are various tools you can use for this, but I’m going to recommend libguestfs, because it’s the simplest to use. To check that it works, let’s look at the partitions in our virtual disk image:

$ virt-filesystems -a hda.qcow2 
/dev/sda1
/dev/sda2

If this doesn’t work, then you should sort that out first. A couple of common reasons I’ve seen:

• if you’re on Ubuntu then your kernels in /boot are installed not-world-readable; you can fix this with sudo chmod 644 /boot/vmlinuz*
• if you’re running Virtualbox on the same host it will interfere with libguestfs’s attempt to run KVM; you can fix that by exiting Virtualbox

Looking at what’s in our disk we can see the kernel and initrd in /boot:

$ virt-ls -a hda.qcow2 /boot/
System.map-4.9.0-3-arm64
config-4.9.0-3-arm64
initrd.img
initrd.img-4.9.0-3-arm64
initrd.img.old
lost+found
vmlinuz
vmlinuz-4.9.0-3-arm64
vmlinuz.old

and we can copy them out to the host filesystem:

virt-copy-out -a hda.qcow2 /boot/vmlinuz-4.9.0-3-arm64 /boot/initrd.img-4.9.0-3-arm64 .

(We want the longer filenames, because vmlinuz and initrd.img are just symlinks and virt-copy-out won’t copy them.)

An important warning about libguestfs, or any other tools for accessing disk images from the host system: do not try to use them while QEMU is running, or you will get disk corruption when both the guest OS inside QEMU and libguestfs try to update the same image.

If you subsequently upgrade the kernel inside the guest, you’ll need to repeat this step to extract the new kernel and initrd, and then update your QEMU command line appropriately.

Running

To run the installed system we need a different command line which boots the installed kernel and initrd, and passes the kernel the command line arguments the installer told us we’d need:

qemu-system-aarch64 -M virt -m 1024 -cpu cortex-a53 \
  -kernel vmlinuz-4.9.0-3-arm64 \
  -initrd initrd.img-4.9.0-3-arm64 \
  -append 'root=/dev/vda2' \
  -drive if=none,file=hda.qcow2,format=qcow,id=hd \
  -device virtio-blk-pci,drive=hd \
  -netdev user,id=mynet \
  -device virtio-net-pci,netdev=mynet \
  -nographic

This should boot to a login prompt, where you can log in with the user and password you set up during the install.

The installation has an SSH client, so one easy way to get files in and out is to use “scp” from inside the VM to talk to an SSH server outside it. Or you can use libguestfs to write files directly into the disk image (for instance using virt-copy-in) — but make sure you only use libguestfs when the VM is not running, or you will get disk corruption.

This is a follow-up to Running Hyper-V in a QEMU/KVM Guest published earlier this year. The article provided instructions on setting up Hyper-V in a QEMU/KVM Windows guest as enabled by a particular KVM patchset (on Intel hardware only, as it turned out later). Several issues have been found since then; some already fixed, some in the process of being fixed, and some still not fully understood.

This post aims to be an up-to-date list of issues related to Hyper-V on KVM, showing their current status and, where applicable, upstream commit IDs. The issues are ordered chronologically from the oldest ones to those found recently.

and in kernel 4.13 for >1 vCPU
4aebd0e9ca
ab2f4d73eb
9b61174793
a12713c25b
1a5e185294

For anyone interested in the AF_VSOCK zero-configuration host<->guest communications channel it's important to be able to observe traffic. Packet capture is commonly used to troubleshoot network problems and debug networking applications. Up until now it hasn't been available for AF_VSOCK.

In 2016 Gerard Garcia created the vsockmon Linux driver that enables AF_VSOCK packet capture. During the course of his excellent Google Summer of Code work he also wrote patches for libpcap, tcpdump, and Wireshark.

Recently I revisited Gerard's work because Linux 4.12 shipped with the new vsockmon driver, making it possible to finalize the userspace support for AF_VSOCK packet capture. And it's working beautifully:

I have sent the latest patches to the tcpdump and Wireshark communities so AF_VSOCK can be supported out-of-the-box in the future. For now you can also find patches in my personal repositories:

• libpcap
• tcpdump
• Wireshark

The basic flow is as follows:

# ip link add type vsockmon
# ip link set vsockmon0 up
# tcpdump -i vsockmon0
# ip link set vsockmon0 down
# ip link del vsockmon0

It's easiest to wait for distros to package Linux 4.12 and future versions of libpcap, tcpdump, and Wireshark. If you decide to build from source, make sure to build libpcap first and then tcpdump or Wireshark. The libpcap dependency is necessary so that tcpdump/Wireshark can access AF_VSOCK traffic.

Fedora 26 is out of the door, and here are fresh fedora 26 images.

There are raspberry pi images. The aarch64 images requires a model 3, the armv7 image boots on both 2 and 3 models. Unlike the images for the previous fedora releases the new images use the standard fedora kernels instead of a custom kernel. So, the kernel update service for the older images will stop within the next weeks.

There are efi images for qemu. The i386 and x86_64 images use systemd-boot as bootloader. grub2 doesn’t work due to bug 1196114 (unless you create a boot menu entry manually in uefi setup). The arm images use grub2 as bootloader. armv7 isn’t supported by systemd-boot in the first place. The aarch64 versions throws an exception. The efi images can also be booted as container, using "systemd-nspawn --boot --image <file>", but you have to convert them to raw first.

The images don’t have a root password. You have to set one using "virt-customize -a <image> --root-password "password:<secret>", otherwise you can’t login after boot.

The images have been created with imagefish.

Debian 9 (“Stretch”) was released last week and now it’s available in virt-builder, the fast way to build virtual machine disk images:

$ virt-builder -l | grep debian
debian-6                 x86_64     Debian 6 (Squeeze)
debian-7                 sparc64    Debian 7 (Wheezy) (sparc64)
debian-7                 x86_64     Debian 7 (Wheezy)
debian-8                 x86_64     Debian 8 (Jessie)
debian-9                 x86_64     Debian 9 (stretch)

$ virt-builder debian-9 \
    --root-password password:123456
[   0.5] Downloading: http://libguestfs.org/download/builder/debian-9.xz
[   1.2] Planning how to build this image
[   1.2] Uncompressing
[   5.5] Opening the new disk
[  15.4] Setting a random seed
virt-builder: warning: random seed could not be set for this type of guest
[  15.4] Setting passwords
[  16.7] Finishing off
                   Output file: debian-9.img
                   Output size: 6.0G
                 Output format: raw
            Total usable space: 3.9G
                    Free space: 3.1G (78%)

$ qemu-system-x86_64 \
    -machine accel=kvm:tcg -cpu host -m 2048 \
    -drive file=debian-9.img,format=raw,if=virtio \
    -serial stdio

libguestfs is a C library for creating and editing disk images. In the most common (but not the only) configuration, it uses KVM to sandbox access to disk images. The C library talks to a separate daemon running inside a KVM appliance, as in this Unicode-art diagram taken from the fine manual:

 ┌───────────────────┐
 │ main program      │
 │                   │
 │                   │           child process / appliance
 │                   │          ┌──────────────────────────┐
 │                   │          │ qemu                     │
 ├───────────────────┤   RPC    │      ┌─────────────────┐ │
 │ libguestfs  ◀╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍▶ guestfsd        │ │
 │                   │          │      ├─────────────────┤ │
 └───────────────────┘          │      │ Linux kernel    │ │
                                │      └────────┬────────┘ │
                                └───────────────│──────────┘
                                                │
                                                │ virtio-scsi
                                         ┌──────┴──────┐
                                         │  Device or  │
                                         │  disk image │
                                         └─────────────┘

The library has to be written in C because it needs to be linked to any main program. The daemon (guestfsd in the diagram) is also written in C. But there’s not so much a specific reason for that, except that’s what we did historically.

The daemon is essentially a big pile of functions, most corresponding to a libguestfs API. Writing the daemon in C is painful to say the least. Because it’s a long-running process running in a memory-constrained environment, we have to be very careful about memory management, religiously checking every return from malloc, strdup etc., making even the simplest task non-trivial and full of untested code paths.

So last week I modified libguestfs so you can now write APIs in OCaml if you want to. OCaml is a high level language that compiles down to object files, and it’s entirely possible to link the daemon from a mix of C object files and OCaml object files. Another advantage of OCaml is that you can call from C ↔ OCaml with relatively little glue code (although a disadvantage is that you still need to write that glue mostly by hand). Most simple calls turn into direct CALL instructions with just a simple bitshift required to convert between ints and bools on the C and OCaml sides. More complex calls passing strings and structures are not too difficult either.

OCaml also turns memory errors into a single exception, which unwinds the stack cleanly, so we don’t litter the code with memory handling. We can still run the mixed C/OCaml binary under valgrind.

Code gets quite a bit shorter. For example the case_sensitive_path API — all string handling and directory lookups — goes from 183 lines of C code to 56 lines of OCaml code (and much easier to understand too).

I’m reimplementing a few APIs in OCaml, but the plan is definitely not to convert them all. I think we’ll have C and OCaml APIs in the daemon for a very long time to come.

There were a couple of QEMU / virtualization related talks at the DevConf 2017 conference that took place at the end of January already, but so far we missed to gather the links to the recordings of these talks. So here is now the list:

• How to write a legacy storage device emulator by John Snow

• GPU Passthrough using GNOME Boxes by Felipe Borges

• Self-virtualizing Linux on x86 by Radim Krčmář

• New ways to remote desktops with GStreamer integration by Victor Toso de Carvalho and Pavel Grunt

• Can VMs networking benefit from DPDK by Maxime Coquelin and Victor Kaplansky

• Windows VMs on KVM: The Good, the Bad and the Ugly by Ladi Prosek

• Pet VMs in Kubernetes? WTH by Fabian Deutsch

• Virtualization development improved with Lago by Rafael Martins

• An ARMful of guests: virtualization on 64-bit ARM by Andrea Bolognani

• Fully virtual (i.e., fully emulated). There is no "real hardware" virtio-ccw device.
• A control unit type of 0x3832.
• One virtual channel path, type 0x32.

• Fully virtual (emulated). While you could passthrough 3270 devices while running under z/VM, this depends on the non-yet-merged vfio-ccw infrastructure (see below) and does not really make much sense.
• A control unit type of 0x3270.
• One virtual channel path, type 0x1a.

• A device that corresponds one-to-one to a device on the host, although it might have a different device number (depending on how it was configured).
• A control unit and device type corresponding to ECKD DASD.  A control unit type of 0x3990 and a device type of 0x3390 are the most likely.
• One to eight channel paths, corresponding to real channel paths.

Seems the world of build systems is going to change. Traditional approach is make. Pimped up with autoconf and automake. But some newcomers provide new approaches to building your projects.

First, there is ninja-build. It’s a workhorse, roughly comparable to make. It isn’t really designed to be used standalone though. Typically the lowlevel ninja build files are generated by some highlevel build tool, similar to how Makefiles are generated by autotools.

Second, there is meson, a build tool which (on unix) by default uses ninja as backend. meson appears to become pretty popular.

So, lets have a closer look at it. I’m working on drminfo right now, a tool to dump information about drm devices, which also comes with a simple test tool, rendering a test image to the display. It is pretty small, doesn’t even use autotools, perfect for trying out something new. Also nice for this post as the build files are pretty small.

So, here is the Makefile:

CC      ?= gcc
CFLAGS  ?= -Os -g -std=c99
CFLAGS  += -Wall

TARGETS := drminfo drmtest gtktest

drminfo : CFLAGS += $(shell pkg-config --cflags libdrm cairo pixman-1)
drminfo : LDLIBS += $(shell pkg-config --libs libdrm cairo pixman-1)

drmtest : CFLAGS += $(shell pkg-config --cflags libdrm gbm epoxy cairo cairo-gl pixman-1)
drmtest : LDLIBS += $(shell pkg-config --libs libdrm gbm epoxy cairo cairo-gl pixman-1)
drmtest : LDLIBS += -ljpeg

gtktest : CFLAGS += $(shell pkg-config --cflags gtk+-3.0 cairo pixman-1)
gtktest : LDLIBS += $(shell pkg-config --libs gtk+-3.0 cairo pixman-1)
gtktest : LDLIBS += -ljpeg

all: $(TARGETS)

clean:
        rm -f $(TARGETS)
        rm -f *~ *.o

drminfo: drminfo.o drmtools.o
drmtest: drmtest.o drmtools.o render.o image.o
gtktest: gtktest.o render.o image.o

Thanks to pkg-config there is no need to use autotools just to figure the cflags and libraries needed, and the Makefile is short and easy to read. The only thing here you might not be familiar with are target specific variables.

Now, compare with the meson.build file:

project('drminfo', 'c')

# pkg-config deps
libdrm_dep    = dependency('libdrm')
gbm_dep       = dependency('gbm')
epoxy_dep     = dependency('epoxy')
cairo_dep     = dependency('cairo')
cairo_gl_dep  = dependency('cairo-gl')
pixman_dep    = dependency('pixman-1')
gtk3_dep      = dependency('gtk+-3.0')

# libjpeg dep
jpeg_dep      = declare_dependency(link_args : '-ljpeg')

drminfo_srcs  = [ 'drminfo.c', 'drmtools.c' ]
drmtest_srcs  = [ 'drmtest.c', 'drmtools.c', 'render.c', 'image.c' ]
gtktest_srcs  = [ 'gtktest.c', 'render.c', 'image.c' ]

drminfo_deps  = [ libdrm_dep, cairo_dep, pixman_dep ]
drmtest_deps  = [ libdrm_dep, gbm_dep, epoxy_dep,
                  cairo_dep, cairo_gl_dep, pixman_dep, jpeg_dep ]
gtktest_deps  = [ gtk3_dep,
                  cairo_dep, cairo_gl_dep, pixman_dep, jpeg_dep ]

executable('drminfo',
           sources      : drminfo_srcs,
           dependencies : drminfo_deps)
executable('drmtest',
           sources      : drmtest_srcs,
           dependencies : drmtest_deps)
executable('gtktest',
           sources      : gtktest_srcs,
           dependencies : gtktest_deps,
           install      : false)

Pretty straight forward translation. So, what are the differences?

First, meson and ninja have built-in support a bunch of features. No need to put anything into your build files to use them, they are just there:

• Automatic header dependencies. When a header file changes all source files which include it get rebuilt.
• Automatic build system dependencies: When meson.build changes the ninja build files are updated.
• Rebuilds on command changes: When the build command line for a target changes the target is rebuilt.

Sure, you can do all that with make too, the linux kernel build system does it for example. But then your Makefiles will be a order of magnitude larger than the one shown above, because all the clever stuff is in the build files instead of the build tool.

Second meson keeps the object files strictly separated by target. The project has some source files shared by multiple executables. drmtools.c for example is used by both drminfo and drmtest. With the Makefile above it get build once. meson builds it separately for each target, with the cflags for the specific target.

Another nice feature is that ninja automatically does parallel builds. It figures the number of processors available and runs (by default) that many jobs.

Overall I’m pretty pleased, I’ll probably use meson more frequently in the future. If you want try it out too I’d suggest to start with the tutorial.

When you are trying to start a KVM guest via libvirt on an s390x Linux installation that is running on an older version of z/VM, you might run into the problem that QEMU refuses to start with this error message:

cannot set up guest memory 's390.ram': Permission denied.

This happens because older versions of z/VM (before version 6.3) do not support the so-called “enhanced suppression on protection facility” (ESOP) yet, so QEMU has to allocate the memory for the guest with a “hack”, and this hack uses mmap(… PROT_EXEC …) for the allocation.

Now this mmap() call is not allowed by the default SELinux rules (at least not on RHEL-based systems), so QEMU fails to allocate the memory for the guest here. Turning off SELinux completely just to run a KVM guest is of course a bad idea, but fortunately there is already a SELinux boolean value called virt_use_execmem which can be used to tune the behavior here:

setsebool virt_use_execmem 1

This configuration switch has originally been introduced for running TCG guests (i.e. running QEMU without KVM), but in this case it also fixes the problem with KVM guests. Anyway, since setting this SELinux variable to 1 is also a slight decrease in security (but still better than disabling SELinux completely), you should better upgrade your z/VM to version 6.3 (or newer) or use a real LPAR for the KVM host installation instead, if that is feasible.

Device   Subchan.  DevType CU Type Use  PIM PAM POM  CHPIDs
----------------------------------------------------------------------
0.0.0000 0.0.0000  0000/00 3832/01 yes  80  80  ff   00000000 00000000
0.0.0042 0.0.0001  0000/00 3832/02 yes  80  80  ff   00000000 00000000

• They take a subchannel identifier as parameter.
• They are privileged: I.e., on a Linux system, they can only be issued by the kernel and not from user space.

• START SUBCHANNEL (SSCH) - start a channel program
• TEST SUBCHANNEL (TSCH) - retrieve subchannel status

• The command. This falls into the categories of read (read data from the device), write (write data to the device) or control (for example, rewinding a tape). An 8-bit value.
• The flags, which control error handling or program flow. I'll ignore them for simplicity here.
• The data address. This is an address in memory where data is written to (read) or read from (write).

[root@localhost ~]# lscss
Device   Subchan.  DevType CU Type Use  PIM PAM POM  CHPIDs
----------------------------------------------------------------------
0.0.0000 0.0.0000  0000/00 3832/01 yes  80  80  ff   00000000 00000000
0.0.0042 0.0.0001  0000/00 3832/02 yes  80  80  ff   00000000 00000000

• The channel subsystem id (cssid) xx (here: 0, as on all current Linux systems)
• The subchannel set id (ssid) y (here: 0, can be any value from 0-3 on current Linux systems)
• The device number (devno) zzzz (here: 0000 respectively 0042, can be any value from 0-0xffff)

• 0.0.0000 (the virtio-net device) has been autogenerated.
• 0.0.0042 (the virtio-blk device) has been specified on the command line.

• The cssid xx (same as for the device)
• The ssid y (again, the same as for the device)
• The subchannel number zzzz (here: 0000 respectively 0001, generally not the same as the devno, although it can be any value from 0-0xffff as well)

• The type aaaa (a value from 0-0xffff; 0x3832 denotes a virtio-ccw control unit)
• The model bb (a value from 0-0xff; for virtio devices, this is the device id as specified by the virtio standard)

• PIM (path installed mask), PAM (path available mask) and POM (path operational mask) are all 8 bit values corresponding bit-by-bit to one of eight channel paths. If the corresponding bit is set in all of the three masks, the channel path can be used for I/O.
• CHPIDs are channel path identifiers: Each channel path has an id from 0-0xff, which is unique combined with the relevant cssid. For virtio devices, there's only one valid channel path with the id 0².

[root@localhost ~]# lschp
CHPID  Vary  Cfg.  Type  Cmg  Shared  PCHID
============================================
0.00   1     -     32    -    -       -   

• CHPID is the channel-path identifier of the form xx.nn, where xx is the cssid and nn the chpid. This is always 0.00 on virtio-only guests.
• Vary means that the channel path is online to the guest. You don't want to change this for the only path.
• Type is the channel-path type. 0x32 is a reserved type for the virtio virtual channel path.

Here is a short list of articles and blog posts about QEMU and KVM, that were posted last month.

• QEMU and the qcow2 metadata checks by Alberto Garcia

• Running Hyper-V in a QEMU/KVM Guest by Ladislav Prošek

• Setting up a nested KVM guest for developing & testing PCI device assignment with NUMA by Daniel P. Berrangé

• Testing SMM with QEMU, KVM and libvirt by László Érsek

• The surprisingly complicated world of disk image sizes by Daniel P. Berrangé

More virtualization blog posts can be found on the virt tools planet.

In other news, QEMU is now in hard freeze for release 2.9.0. The preliminary list of features is on the wiki.

virt-inspector is a very convenient tool to examine a disk image and find out if it contains an operating system, what applications are installed and so on.

If you have an xz-compressed disk image, you can run virt-inspector on it without uncompressing it, using the magic of captive nbdkit. Here’s how:

nbdkit xz file=win7.img.xz \
    -U - \
    --run 'virt-inspector --format=raw -a nbd://?socket=$unixsocket'

What’s happening here is we run nbdkit with the xz plugin, and tell it to serve NBD over a randomly named Unix domain socket (-U -).

We then run virt-inspector as a sub-process. This is called “captive nbdkit”. (Nbdkit is “captive” here, because it will exit as soon as virt-inspector exits, so there’s no need to clean anything up.)

The $unixsocket variable expands to the name of the randomly generated Unix domain socket, forming a libguestfs NBD URL which allows virt-inspector to examine the raw uncompressed data exported by nbdkit.

The nbdkit xz plugin only uncompresses those blocks of the data which are actually accessed, so this is quite efficient.