Virtualization - Apr 2022 - [PATCH 1/5] iommu: Replace uses of IOMMU_CAP_CACHE_COHERENCY with dev_is_dma_coherent()

On Thu, 7 Apr 2022 12:23:31 -0300
Jason Gunthorpe <jgg at 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