[vfio-users] VGA Passthrough limitations?

[Abdulla Bubshait]

I don't think this is an identical MSRS spam, but it might be caused by the
same underlying principal. I think the bottleneck here is the CPU, since
Virtual CPUs are not exactly like CPUs.

Take the Heroes MSRS spam. Each event is a register read in hardware, but
since these are unique registers they will be emulated (if function works)
as software in KVM. What I am guessing is happening here is the 3D code is
running something that requires KVM to do things, which ends up being much
slower than hardware.

I would suggest trying to do something like CPU pinning to see if you can
bump up performance a bit. But if this thing turns out to be a code path
that isn't using hardware virtualization then you might never get the
performance you are looking for.

Maybe someone with more experience with the inner workings of
virtualization could be more useful here.



On Wed, May 18, 2016 at 10:58 AM Andrei Grigore <andrei.grg at gmail.com>
wrote:

> I have passed the ignore msrs parameter but there is no change. I will
> check with dmesg though. This happens in any game: WoW, SC2, HotS, D3 ...I
> do not have anything besides Blizzard games to test this with, but I am
> pretty sure is not game related.
> On May 18, 2016 4:42 PM, "Quentin Deldycke" <quentindeldycke at gmail.com>
> wrote:
>
>> Easy to see, dmesg and you should have plently of flood about it :)
>>
>> --
>> Deldycke Quentin
>>
>>
>> On 18 May 2016 at 16:39, thibaut noah <thibaut.noah at gmail.com> wrote:
>>
>>> I'm thinking same thing as heroes of the storm, msrs spamming ?
>>>
>>> 2016-05-18 14:27 GMT+02:00 Andrei Grigore <andrei.grg at gmail.com>:
>>>
>>>> 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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>>>> vfio-users at redhat.com
>>>> https://www.redhat.com/mailman/listinfo/vfio-users
>>>>
>>>>
>>>
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