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Re: [Xen-devel] Windows 8 VGA Passthrough

On 10/27/2013 02:27 PM, John Wargo wrote:
On 10/27/2013 04:30 AM, Gordan Bobic wrote:
On 10/26/2013 09:58 PM, John Wargo wrote:

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

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
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

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.

Status update:

The catalyst drivers appear to be working perfectly fine, device manager
shows no errors, and everything works great, except that when I plug my
monitor into the second card, I get "No Signal" on the monitor, then it
shuts off.

I though that secondary passthrough meant that it would pass the virtual
adapter in as primary, but I could use my GPU that I passed through, so
where am I wrong, and how do I do that?

Your understanding is correct - real VGA card will be passed as secondary, and the driver, when it loads, should start it up, and this normally takes over from the primary.

Two things come to mind:
1) I find that sometimes I have to disable the primary emulated VGA adapter.
2) Make sure your desktop settings are correctly set up to use the real VGA card as the primary desktop.


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