[vfio-users] VGA Passthrough fails without NoSnoop patch

[Alex Williamson]

On Sun, 29 Jan 2017 23:03:38 +0100
Javier Celaya <jcelaya at gmail.com> wrote:

> The host kernel is version 4.9.0 from Debian Sid. That command outputs
> "f0105a". What does it mean?

Bit 7 would indicate if the IOMMU supports Snoop Control.  When it
does, the IOMMU page tables can set the SNP bit, which allows
transactions to snoop the processor cache regardless of transaction
attributes such as NoSnoop.  IOW, it basically negates NoSnoop at the
IOMMU level.  Since this bit is clear on your IOMMU, we need to
register non-coherent DMA with KVM.  This affects memory attributes in
a few places and determines whether or not KVM needs to emulate the
WBINVD (write-back-invalidate) instruction.  To some extent, we do need
to trust guest memory attributes to work in this mode, however the
NoSnoop patch, which simply tries to prevent the device from enabling
NoSnoop transactions was a pretty horrible hack.

I'm sure that lots of us run with the same IOMMU capabilities, I have a
system with identical flags that runs without issue with either an
NVIDIA or AMD card.  I wonder if the problem is more to do with the
fact that your CPU has integrated graphics and you're trying to use
x-vga=on without any mention whether you've patched i915 to properly
work with VGA arbitration.  If the cards support UEFI, you should not
need x-vga=on.  Is the host primary graphics Intel?  I don't understand
why the NoSnoop patch would change your results at all.  Thanks,

Alex