[PATCH 2/8] iommu/arm-smmu-v3: Use S2FWB when available

[Jason Gunthorpe]

On Tue, Aug 20, 2024 at 09:01:02AM -0300, Jason Gunthorpe wrote:

> Also bear in mind VFIO won't run unless ARM_SMMU_FEAT_COHERENCY is set
> so we won't even get a chance to ask for a S2 domain.

And I should also say that without iommufd something like the DMA API
could select a S2 with S2FWB enabled, but all that does is change the
encoding of the memattr bits. Requests for !IOMMU_CACHE will still map
to non-cacheble IO PTEs like before - just with a different encoding.

The only thing at issue is nesting which will end up in the guest as
forced cachable - however since VFIO doesn't support non-DMA-coherent
devices at all this is not a problem.

Jason