Re: Graphics pass-through

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-05-09 16:29, Alex Williamson wrote:
> On Mon, 2011-05-09 at 13:14 +0200, Jan Kiszka wrote:
>> On 2011-05-05 17:17, Alex Williamson wrote:
>>>> And what about the host? When does Linux release the legacy range?
>>>> Always or only when a specific (!=vga/vesa) framebuffer driver is loaded?
>>>
>>> Well, that's where it'd be nice if the vga arbiter was actually in more
>>> widespread use.  It currently seems to be nothing more than a shared
>>> mutex, but it would actually be useful if it included backends to do the
>>> chipset vga routing changes.  I think when I was testing this, I was
>>> externally poking PCI bridge chipset to toggle the VGA_EN bit.
>>
>> Right, we had to drop the approach to pass through the secondary card
>> for now, the arbiter was not switching properly. Haven't checked yet if
>> VGA_EN was properly set, though the kernel code looks like it should
>> take care of this.
>>
>> Even with handing out the primary adapter, we had only mixed success so
>> far. The onboard adapter worked well (in VESA mode), but the NVIDIA is
>> not displaying early boot messages at all. Maybe a vgabios issue.
>> Windows was booting nevertheless - until we installed the NVIDIA
>> drivers. Than it ran into a blue screen.
> 
> Interesting, IIRC I could never get VESA modes to work.  I believe I
> only had a basic VGA16 mode running in a Windows guest too.
> 
>> BTW, what ATI adapter did you use precisely, and what did work, what not?
> 
> I have an old X550 (rv380?).  I also have an Nvidia gs8400, but ISTR the
> ATI working better for me.

Is that Nvidia a PCIe adapter? Did it show BIOS / early boot messages
properly?

BTW, we are fighting with a Quadro FX 3800.

> 
>> One thing I was wondering: Most modern adapters should be PCIe these
>> days. Our NVIDIA definitely is. But so far we are claiming to have it
>> attached to a PCI bus. That caps all the extended capabilities e.g.
>> Could this make some relevant difference?
> 
> The BIOS and early boot use shouldn't care too much about that, but I
> could imagine the high performance drivers potentially failing.  Thanks,

Yeah, that was my thinking as well. But we will try to confirm this by
tracing the BIOS activities. There is a telling that some adapters do
not allow reading the true cold-boot ROM content during runtime, thus
booting those adapters inside the guest may fail to some degree.

Anyway, I've hacked on the q35 patches until they allowed me to boot a
Linux guest with an assigned PCIe Atheros WLAN adapter - all caps were
suddenly visible. Those bits are now on their way to our test box. Let's
see if they are able to change the BSOD a bit...

Jan

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