Re: [PATCH 1/5] iommu: Replace uses of IOMMU_CAP_CACHE_COHERENCY with dev_is_dma_coherent()

Alex Williamson
2022-04-07T22:38:18Z

On Thu, 7 Apr 2022 12:23:31 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Thu, Apr 07, 2022 at 04:17:11PM +0100, Robin Murphy wrote:
>
> > For the specific case of overriding PCIe No Snoop (which is more problematic
> > from an Arm SMMU PoV) when assigning to a VM, would that not be easier
> > solved by just having vfio-pci clear the "Enable No Snoop" control bit in
> > the endpoint's PCIe capability?
>
> Ideally.
>
> That was rediscussed recently, apparently there are non-compliant
> devices and drivers that just ignore the bit.
>
> Presumably this is why x86 had to move to an IOMMU enforced feature..

I considered this option when implementing the current solution, but
ultimately I didn't have confidence in being able to prevent drivers
from using device specific means to effect the change anyway. GPUs
especially have various back channels to config space. Thanks,

Alex