Re: vga passthrough // questions about pci passthrough

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 2012-09-28 at 10:12 +0200, Jan Kiszka wrote:
> On 2012-09-27 21:18, Alex Williamson wrote:
> > On Thu, 2012-09-27 at 20:43 +0200, Martin Wolf wrote:
> >> thank you for the information.
> >>
> >> i will try what you mentioned...
> >> do you have some additional information about rebooting a VM with a 
> >> passed through videocard?
> >> (amd / ati 7870)
> > 
> > I don't.  Is the bsod on reboot only or does it also happen on shutdown?
> > There's a slim chance it could be traced by enabling debug in the
> > pci-assign driver and analyzing what the guest driver is trying to do.
> > I'm hoping that q35 chipset support might resolve some issues with vga
> > assignment as it exposes a topology that looks a bit more like one that
> > a driver would expect on physical hardware.  Thanks,
> 
> From our attempts to get more working than what NVIDIA Quadro cards
> support officially, my own experiments with q35 in this context and our
> discussions with NVIDIA, I'm pretty skeptical that this chipset will
> make a difference here. Most problems are due to those non-standard side
> channels to configure the hardware, memory mappings etc. And getting
> this working requires either cooperation of the vendor or *a lot* of
> reverse engineering.

I heard from an nvidia guy that the driver behaves differently depending
on whether it finds an upstream express port, so we're probably causing
ourselves more problems if it's trying to run in AGP mode.  There was
also a lot of FUD in Xen (maybe justified) around how the BIOS
determines the memory ranges and whether it bypasses the PCI BARs and
gets them directly.  That means some cards may require identity mapping
to work.  It seems like the very high-end cards are possibly fixing
this, but they're far more expensive than I can justify.  Thanks,

Alex