Re: Windows 8 VGA Passthrough

[Gordan Bobic <gordan@bobich.net>]

On 10/26/2013 09:58 PM, John Wargo wrote:
> Geniuses,
>
> I've been trying to get VGA pass-through working on Windows 8, however
> I've been running into a few issues.
> Currently I can get Windows 8 running flawlessly without VGA
> pass-through, however I'm planning on using it for gaming, so this wont
> work.
>
> When I start windows 8, I am able to get past the install portion of the
> DVD, however it hangs on "Getting devices ready.. (n%)".
>
> Hardware Information :
> AMD Fx6300
> ASUS M5a99x evo r2.0
> 16GB RAM
> 6670 Radeon HD - Dom0
> 7850 Radeon HD - Win8

I have recently (recently as in over the past 48 hours) re-tried to get 
ATI VGA passthrough working, and the results are... interesting.

HD4850 dom0 + HD7450 domU worked fine - or as fine as ATI passthrough 
ever seems to work (slowdown + white lines when dragging windows after a 
domU reboot). I am using XP64 domU so I can't eject the GPU (it says it 
is being used). But shutting down the domU, doing echo 1 > reset for the 
GPU from dom0 (I don't know how this is implemented and whether it does 
anything) and then starting it up again sometimes recovers it. GPU-Z 
runs and reports the GPU correctly.

HD4850 dom0 + HD7970 domU results in the entire host locking up as soon 
as the driver is loaded in domU. My solution is to swap the dom0 GPU for 
an 8800GT, and the problem now doesn't occur. If I had to guess, BIOS on 
the HD7970 does something bizzare, finds the HD4850 on the bus, tries to 
do something to it, and ends up crashing the host. This could also be 
exacerbated by the fact that I have to run with relaxed checking and no 
PCI ACS due to NF200 bridges on my motherboard. Even so, however, GPU-Z 
crashes the domU and it's GPU _hard_ (but not the host, thankfully). It 
crashes it so hard, that the only cure is to reboot the host. Reseting 
the GPU via /sys/ doesn't seem to do anything to help.

Are you using gfx_passthru=1 in your config? The way most people have 
this kind of thing working is with gfx_passthru=0, i.e. secondary 
passthru. Don't assign the PCI device in the config when installing. 
Once the domU install is complete, shut it down, and add the GPU you 
want to pass through. Then fire up domU in VNC, install the driver, and 
with a bit of luck it will come up with output on the external monitor.

Also make sure that radeon or radeonfb kernel drivers DO NOT load before 
the device is assigned to xen-pciback. The exact way to ensure this will 
depend on what distro and dom0 kernel you are using.

Gordan