Re: [Xen-devel] GPU passthrough issue when VM is configured with 4G memory

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Mon, Mar 4, 2013 at 8:10 AM, Gonglei (Arei) <arei.gonglei@xxxxxxxxxx> wrote:
> Hi,all
>
> I have tried to passthrough GPU card(Nvidia quadro 4000) on the latest Xen
> unstable version (QEMU is using Qemu-upsteam-unstable, not traditional
> Qemu). This issue as below:
>
>        Windows7 64-bit guest will blue screen when GPU passthrough configure
> 4g memory,blue screen code is 50, and SUSE 11 64-bit guest will always stay
> at the grub screen.  I noticed that it will relocate RAM that overlaps PCI
> space in pci_setup()(tools/hvmloader/pci.c). If VM memory is configured with
> 3G, it won't cause relocate RAM that overlaps PCI space in pci_setup(), and
> GPU pass-through is no problem. So it seems this issue is related to
> "relocate RAM" in pci_setup().

So one issue XenServer found with passing through GPUs is that there
are bugs in some PCI bridges that completely break VT-d.  The issue
was that if the *guest* physical address space overlapped the *host*
physical address of a different device, that the PCI bridges would
send traffic from the passed-through card intended for the guest to
another card instead.  The work-around was to make the hole in the
guest MMIO space the same size as the host MMIO hole.  I'm not sure if
that made it upstream or not -- let me check...

 -George

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