Re: [PATCH 4/5] vfio/type1: Flush CPU caches on DMA pages in non-coherent domains

Jason Gunthorpe
2024-05-21T16:07:27Z

On Mon, May 20, 2024 at 02:52:43AM +0000, Tian, Kevin wrote:
> +Daniel
>
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Saturday, May 18, 2024 1:11 AM
> >
> > On Thu, May 16, 2024 at 02:31:59PM -0600, Alex Williamson wrote:
> >
> > > Yes, exactly. Zero'ing the page would obviously reestablish the
> > > coherency, but the page could be reallocated without being zero'd and as
> > > you describe the owner of that page could then get inconsistent
> > > results.
> >
> > I think if we care about the performance of this stuff enough to try
> > and remove flushes we'd be better off figuring out how to disable no
> > snoop in PCI config space and trust the device not to use it and avoid
> > these flushes.
> >
> > iommu enforcement is nice, but at least ARM has been assuming that the
> > PCI config space bit is sufficient.
> >
> > Intel/AMD are probably fine here as they will only flush for weird GPU
> > cases, but I expect ARM is going to be unhappy.
> >
>
> My impression was that Intel GPU is not usable w/o non-coherent DMA,
> but I don't remember whether it's unusable being a functional breakage
> or a user experience breakage. e.g. I vaguely recalled that the display
> engine cannot afford high resolution/high refresh rate using the snoop
> way so the IOMMU dedicated for the GPU doesn't implement the force
> snoop capability.
>
> Daniel, can you help explain the behavior of Intel GPU in case nosnoop
> is disabled in the PCI config space?
>
> Overall it sounds that we are talking about different requirements. For
> Intel GPU nosnoop is a must but it is not currently done securely so we
> need add proper flush to fix it, while for ARM looks you don't have a
> case which relies on nosnoop so finding a way to disable it is more
> straightforward?

Intel GPU weirdness should not leak into making other devices
insecure/slow. If necessary Intel GPU only should get some variant
override to keep no snoop working.

It would make alot of good sense if VFIO made the default to disable
no-snoop via the config space.

Jason