Which display type should I use in a VM with libvirt?

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leyvi · Post by **leyvi** » Wed May 14, 2025 12:43 am

Which display type should I use? I've been using QXL, but that shows up as a VGA device on the guest, and I'm not sure if it's using that or the GPU I provided...

yaslam · Post by **yaslam** » Sun May 18, 2025 8:23 am

leyvi wrote:Which display type should I use? I've been using QXL, but that shows up as a VGA device on the guest, and I'm not sure if it's using that or the GPU I provided...

Need more information, are you using a passed through GPU? Or just QXL?

Post by **NeddySeagoon** » Sun May 18, 2025 11:05 am

leyvi,

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lspci -nnk

from the guest would be good. That will tell about your available GPUs.

If Xorg is running, Xorg.0.log will tell which one is in use for your GUI.
dmesg will show which GPU is used for the console.
Both of those are too big for for a post. They need to be pastebins.

leyvi · Post by **leyvi** » Sun May 18, 2025 1:23 pm

yaslam wrote:
leyvi wrote:Which display type should I use? I've been using QXL, but that shows up as a VGA device on the guest, and I'm not sure if it's using that or the GPU I provided...
Need more information, are you using a passed through GPU? Or just QXL?

I'm using passthrough, with the iGPU on my CPU, since I have a dGPU that I can use for the host. I'm using SPICE to view it.

Post by **NeddySeagoon** » Sun May 18, 2025 1:29 pm

leyvi,

That sounds wrong. Please answer the qustions, so we don't have to guess.

I would expect you to connect a display to the passed through GPU and use it as if it wer e neced to a real hardware system.

leyvi · Post by **leyvi** » Sun May 18, 2025 1:57 pm

NeddySeagoon wrote:leyvi,
Code: Select all
lspci -nnk
from the guest would be good. That will tell about your available GPUs.

If Xorg is running, Xorg.0.log will tell which one is in use for your GUI.
dmesg will show which GPU is used for the console.
Both of those are too big for for a post. They need to be pastebins.

Alright, I'll check as soon as I'm able.

yaslam · Post by **yaslam** » Sun May 18, 2025 3:31 pm

leyvi wrote:Which display type should I use?

When doing GPU passthrough the display type should be set to none, as Neddy said you connect a monitor to the passed through GPU.

leyvi · Post by **leyvi** » Sun May 18, 2025 3:33 pm

Ah... I only have one monitor. Is there any way to accelerate graphics with a GPU, then send the frames over SPICE?

yaslam · Post by **yaslam** » Sun May 18, 2025 3:39 pm

leyvi wrote:Ah... I only have one monitor. Is there any way to accelerate graphics with a GPU, then send the frames over SPICE?

SPICE with VirGL enabled does only OpenGL acceleration using the host GPU, but it only works on Linux guests.

leyvi · Post by **leyvi** » Sun May 18, 2025 3:41 pm

I've done that before, and while it does work, it's a little stutter-y. Is there no other way?

yaslam · Post by **yaslam** » Sun May 18, 2025 3:47 pm

leyvi wrote:Is there no other way?

Not that I know of, no.

Post by **NeddySeagoon** » Sun May 18, 2025 4:32 pm

leyvi,

This is very hacky but it may work.
The problem you face is that the GPU can only render into the GPU pixel buffer. There is normally no way to get the pixel buffer out.
However, VNC has a 'screen scraper' mode,where it does just that. By default, the VNC server uses a chunk of main memory as its pixel buffer.

Step 1 is to get the passed through GPU to work - you will need to use a monitor to check.
Step 2 is to run a VNC Server in the guest and a VNC client on the host and check that out. This VNC server will use a chunk of main guest RAM as its pixel buffer.

Now you have the bits (almost) working individually. Now the tricky bit.
Make Xorg work in the guest when there is no display connected. The passed through GPU will not be able to read the EDID data, so you will need some xorg.conf.d/ fragments to overcome that.
Check that you can connect the display after Xorg has started and its working the way you want.

Reconfigure the VNC Server in the guest to serve 'screen scraper' mode.

Enjoy accelerated graphics from the guest in the VNC client. Well, sort of. Its going to be pushing a lot of data of the host/client network link. The VNC data link normally runs compression and reduced colour depth to get the frame rate up. Many years ago I used VNC over a 56k dial up link. :)
In practice, the bottleneck is going to move to a different place. I don't know if the performance will be any better than Spice.
A 1920x1080@60Hz (FHD) uncompressed video stream needs over 1Gbit/sec

leyvi · Post by **leyvi** » Sun May 18, 2025 4:36 pm

NeddySeagoon:
This looks very cool, and also very time consuming

I will check it out when I can.

Post by **NeddySeagoon** » Sun May 18, 2025 4:44 pm

leyvi,

It need not be time consuming if you make all the bits work separately before you put then together.
Keep your problem space small.

Its possible to run the VNC server and client both on you host for testing. That would let you test 'screen scraper' mode too.
Its has as much practical value as

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ssh localhost

but you get to test the configuration.

Which display type should I use in a VM with libvirt?

Which display type should I use in a VM with libvirt?

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Re: Which display type should I use in a VM with libvirt?

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