RE: [PATCH 1/1] iommu/vt-d: Support enforce_cache_coherency only for empty domains

[Tian, Kevin]

> From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Sent: Tuesday, November 14, 2023 9:11 AM
> 
> The enforce_cache_coherency callback ensures DMA cache coherency for
> devices attached to the domain.
> 
> Intel IOMMU supports enforced DMA cache coherency when the Snoop
> Control bit in the IOMMU's extended capability register is set.
> Supporting it differs between legacy and scalable modes.
> 
> In legacy mode, it's supported page-level by setting the SNP field
> in second-stage page-table entries. In scalable mode, it's supported
> in PASID-table granularity by setting the PGSNP field in PASID-table
> entries.
> 
> In legacy mode, mappings before attaching to a device have SNP
> fields cleared, while mappings after the callback have them set.
> This means partial DMAs are cache coherent while others are not.
> 
> One possible fix is replaying mappings and flipping SNP bits when
> attaching a domain to a device. But this seems to be over-engineered,
> given that all real use cases just attach an empty domain to a device.
> 
> To meet practical needs while reducing mode differences, only support
> enforce_cache_coherency on a domain without mappings if SNP field is
> used.
> 
> Fixes: fc0051cb9590 ("iommu/vt-d: Check domain force_snooping against
> attached devices")
> Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>