[vfio-users] VGA Passthrough limitations?

[Andrei Grigore]

Dear all,

I have this very strange issue that I can not understand:

Everything works flawlessly or even better than bare metal. ...thing is, if
I enable stereoscopic 3d vision (nvidia) in game, the performance drops to
unplayable. The reason for that are CPU /RAM intensive elements such as
view distances. Dropping other graphical elements to the lowest possible
value makes no difference. This means that the GPU is not the bottleneck.
Tested this on bare metal and i get around 50 fps with everything maxed
out. With VGA passthrough I barely get 20. Disabling 3d vision boosts
everything to 60 fps.

The entire 3d vision sensor usb bus is passed through as well. I am running
just this VM with 8/8 cores of a fx-8230 cpu with 12 gb of ram. The cpu is
passed with host-passthrough. I am even using your latest patch and had
enabled hyperv extensions with nvidia GPUs.

The VM is installed clean with nothing but the games in it. As said using
the same drivers on a bare metal windows works flawlessly.

I cannot understand where the bottleneck is. CPU usage is not even close to
100%, plenty of ram left and not even 1MB of swap is made.

I don't even know where to seek the problem. What can cause such a massive
fps drop? Is there any bandtwidth limit that caps the 2x rendering needed
for 3d vision?

Many thanks,
Andrei.
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