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17. Debugging

17.1. Server side

If the virtual machine was started using QEMU directly, SPICE server logs will be output to your console stdout.

When using libvirt, logs are located in /var/log/libvirt/qemu/ for the qemu system instance (qemu:///system), and in ~/.cache/libvirt/qemu/log for the qemu session instance (qemu:///session).

It’s possible to get more verbose output by setting the G_MESSAGES_DEBUG environment variable to Spice or all before starting QEMU as described in GLib documentation.

When using libvirt, the environment variable needs to be set from the XML domain definition as described in libvirt documentation.

17.2. Client side

remote-viewer can be started with SPICE_DEBUG=1 in the environment, or with --spice-debug in order to get it to output more logs on stdout. SPICE_DEBUG should work with any application using spice-gtk (virt-manager, gnome-boxes, …).

17.3. Guest side

QXL KMS driver. On recent Linux kernels using the QXL kms driver, booting the kernel with the drm.debug=0xf parameter. journalctl -k/dmesg will then contain debug logs for the QXL kms driver. This can also be changed at runtime by echo’ing this value to /sys/module/drm/parameters/debug.

qxl.guestdebug QEMU parameter. It’s also possible to get some host-side debugging logs from the guest QXL driver. The driver reads a guestdebug parameter from the rom, which can be set when starting the VM. This can be enabled with -global qxl-vga.guestdebug=3, or -global qxl.guestdebug=3 for secondary devices.

The corresponding libvirt XML is:

<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
  ....
  <qemu:commandline>
    <qemu:arg value='-global'/>
    <qemu:arg value='qxl-vga.guestdebug=3'/>
  </qemu:commandline>
</domain>

(don’t forget to add the xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0' attribute to the toplevel <domain> node)

You can go up to 12 (or more, look for DEBUG_PRINT in the driver), you get really a lot of debug information. Interesting values are:

  • 3 - will give you all the highlevel commands (DrvCopyBits, DrvBitBlt, etc.)
  • 6 - will also show QXLGetBitMap
  • 11 - will show caching of images (this is a driver cache, not to be confused with the cache shared between server and client).

qxl.cmdlog QEMU parameter. This will dump all the commands passing through the ringbuffer on the device side. It is driver and hence guest agnostic. This can be enabled with -global qxl-vga.cmdlog=1, or -global qxl.cmdlog=1 for secondary devices. See the section above for the libvirt XML to use.

Agent

On Linux, journalctl -t spice-vdagentd -t spice-vdagent will display the agent log messages. spice-vdagent can also be restarted by hand with the -d argument in order to display more logs.

On Windows, the agent logs can be found in C:\WINDOWS\TEMP\VDAGENT.LOG and C:\WINDOWS\TEMP\VDSERVICE.LOG

17.4. Recording/replaying SPICE server traffic

Since spice-server 0.12.6, it’s possible to record display traffic sent by the SPICE server in order to replay it afterwards for a client without needing to start a VM. This is achieved by setting the environment variable SPICE_WORKER_RECORD_FILENAME to the filename to write the traffic to before starting QEMU.

Once the recording session is done, the spice-server-replay tool can be used to replay the previously recorded SPICE session, for example:

spice-server-replay -p 5900 -c "remote-viewer spice://localhost:5900" recorded-session.spice
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