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Re: [Xen-users] VGA passthru troubleshooting

On 6/23/12 8:28 PM, Casey DeLorme wrote:
> When you pass the card to a DomU, the card is initialized.  However,
> Windows does not reset the card.  So after that DomU is shutdown the
> card remains initialized.
> When a Windows 7 DomU boots if the card is already in an initialized
> state, and driver installation, removal, and 3D applications will fail.

That's interesting. If I understand, the passed gfx device must be reset
before being passed to any domU.

What you described leads me to conclude that one of the configuration
options (that I'm using) described on the Xen.org wiki may be broken.

In my case the dom0 pciback driver is built as a module, so I can't hide
the gfx device from dom0 at boot with xen-pciback.hide.  I'm using the
late binding approach, which means dom0 binds a driver to the device
(e.g. radeon), which I assume has the consequence of initializing the
card. Then I manually unbind the dom0 driver and assign the device to
pciback. Whether the card is reset during unbind is unclear, but I'd
guess not. So, a physical machine reboot currently wouldn't prevent the
gfx device from being initialized before being passed to a domU. (Is
anyone using the late binding approach with a gfx device successfully?)

The alternative I should try is to use modprobe.conf to blacklist the
radeon driver, so only pciback ever binds to the card. I'll give that a
try next. (Is anyone using the modprobe.conf approach with gfx
pass-through successfully?)

Though one thing about your theory is bugging me. My Linux domU was
repeatedly able to drive the gfx device without any physical machine
reboots. It had no 2D or 3D acceleration and - perhaps more importantly
- no driver was obviously bound to the device.

In any case, I'll give the modprobe.conf approach a try. If that fails
I'll rebuild the kernel to include pciback. (I'd be curious if there's
anyone out there with gfx pass-through working with pciback as a module.)


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