Re: [Xen-devel] Fwd: Re: Xen 4.3 / 4.4 - concurrent APIs, VGA Passthru

[Gordan Bobic <gordan@xxxxxxxxxx>]

On 07/28/2014 03:10 PM, Georg Bege wrote:

> Thanks for your response, I can imagine hundred of people asked the same
> questions already -
> but as said it was hard to get an up to date documentation about this.
> However I did experiment more with it, my graphics card is an GeForce
> GTX470.
> I came to the conclusion that passthru works great with XP (both 32bit
> and 64bit) but giving me issue's whenever Im trying on Windows 7 64bit.
Unless something major has changed very recently, an unmodified GTX470 
won't work passed through to a domU using unmodified drivers.
I am using Xen 4.3.0, and am using a pair of 780Ti cards at the moment, 
but this is a relatively recent upgrade from 470/480 cards.
GTX470 can be modified into a Quadro 5000 using a small BIOS change 
(google it, I'm sure you'll find information about it), after which it 
will work just fine passed through to a domU.
Having said that - it has recently been reported that various QEMU 
patches have neutered various ways the Nvidia driver uses to check 
whether it is running in a VM. Including things as lame as looking for 
QEMU's CPU model name/number override. Unsurprisingly, once you prevent 
the driver from detecting it is running in a VM, the card works fine.
One thing you might want to try is an older Nvidia driver. There has 
been a lot of cat-and-mouse game going on with QEMU getting patched to 
neuter checks for whether the stack is running virtualized, and Nvidia 
introducing new checks for it to attempt to continue making the GeForce 
cards not work virtualized.
But if you are not afraid of flipping a few bits on your GTX470 to turn 
it into a Quadro 5000, that is probably the easiest option that 
debugging the issue. IIRC was using my modified GTX470 and 480 cards 
with 320.xx and 331.xx drivers without problems.
> It seems that vga passthru doesnt work for the newer qemu-xen model
> (using Qemu 2.0.0), so Im using
> qemu-xen-traditional along with the old bundled qemu-dm which works fine
> for XP.
I am using qemu traditional on my system. I have two VMs running all the 
time, one running XP64, the other running 64-bit Windows. I have 
observed no issues at all with either guest OS.
> So any idea why this is not working right for Win7?
> Win7 also seems to be a lot slower with qemu-xen-traditional, is that a
> bug in the old model?
It could be something in Xen 4.4. I am using 4.3 with no problems in a 
similar setup. The machine runs 24/7 and the VMs are always running as 
they are different "workstations". Everything works exactly as expected, 
including VM rebooting, and performance is close enough to native to not 
impede gaming (Crysis 3 at 2560x1600, Borderlands 2 at 3840x2400).
Gordan

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