Re: gdm on a virtual machine commit (sysv)

On Wed, May 7, 2025 at 6:19 PM Joe Locash
<[email protected]> wrote:
>
> I'm some what confused by the recent commit for this.
>
>      gdm: on a virtual machine, prevent using X11 on sysv
>
>      The file /usr/lib/udev/rules.d/61-gdm.rules disables wayland
>      for virtual GPUs. The problem is that X11 has a permission problem
>      on sysv with user gdm, so wayland is mandatory. Describe
>      what to do in "configuration information". This is the same
>      problem as for nvidia GPUs...
>
> I don't see a permission problem and I'm able to start both wayland and
> xorg session with gdm in qemu?
>
> The books states:
>   "However, on sysv, a permission problem prevents running X11 for user
> gdm , so running on wayland is mandatory. If your system is sysv on a
> virtual machine or you have an NVIDIA GPU and still want to try running
> Wayland sessions anyway, execute the following command as the root user:"
> This just doesn't make sense.  If running on wayland is mandatory on a
> sysv build why would there need to be a change to run on wayland?
>
> Tested in qemu-9.2.2.

Hi Joe,

Most of the actual info/bug testing for this commit was in the
developers IRC. Essentially Pierre experienced a bug where he couldn't
launch GDM on X11 due to a permission error switching to a new VT.
After some diagnosis it was determined that the same problem that
exists on NVIDIA with udev switching with a virtual GPU also affected
qemu.

To explain what is said in the book: There are 2 different things
being said in that passage.
1) Wayland sessions are hidden on BOTH NVIDIA and Virtual GPUs
2) Wayland sessions are mandatory for sysv with a vgpu
Thus, re-enable wayland support with the command:
ln -s /dev/null /etc/udev/rules.d/61-gdm.rules
Let me know if you think it isn't clear enough I can modify the text
to better reflect this.

It's interesting that you can run both wayland and X11 on sysv, what
QEMU GPU driver are you using, there was some suspicions  in IRC
saying that maybe the virgl renderer could work (-device
virtio-gpu-pci,virgl=on), also how new is your build, I believe Pierre
was building from the dev branch? Are you using gnome 48?

-- 
Thanks,
Rahul

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