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Bug#983934: linux-image-5.3.0-3-amd64: guest kernel fails inside qemu VM with qxl displays (BUG: unable to handle page fault)

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  Bug#983934: linux-image-5.3.0-3-amd64: guest kernel fails inside qemu VM with qxl displays (BUG: unable to handle page fault) Bernhard Übelacker <bernhardu@mailbox.org> - 2021-03-06 10:50 +0100

#69714 — Bug#983934: linux-image-5.3.0-3-amd64: guest kernel fails inside qemu VM with qxl displays (BUG: unable to handle page fault)

Dear Maintainer,
I investigated a bit further and came across this document [1].
There it is stated that in Linux usually a single qxl device with
multiple outputs [2] is used and in Windows multiple qxl devices with
a single output each [3].

Using with a Linux guest the suggested multiple outputs of a single
qxl device does not produce the kernel oops and works perfect.

This kernel issue is just visible with multiple qxl devices and a Linux guest,
therefore is kind of an unusual setup, nevertheless was not causing
this kernel oops up until 5.2 and fails afterwards.

Kind regards,
Bernhard

[1] https://www.spice-space.org/multiple-monitors.html, paragraph "Spice Protocol"

[2] QEmu parameter: "... -device qxl-vga,max_outputs=4,... ..."
[3] QEmu parameter: "... -device qxl-vga                   -device qxl ..."



As a side note:
In my tests with multiple qxl devices and watching the Linux guest
with remote-viewer the mouse pointer was always stuck at one position.

Debugging remote-viewer/Qemu shows that if multiple qxl devices are used,
the mouse mode gets set to "server", therefore no absolute
but just relative mouse movements get sent to the VM.
My guess is that the guest qxl driver on Linux does
currently not implement that mode.
spice-0.14.3/server/reds.c:640, reds_update_mouse_mode:
SPICE_MOUSE_MODE_CLIENT / SPICE_MOUSE_MODE_SERVER

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