Re: [PATCH v2 2/8] iommu/arm-smmu-v3: Use S2FWB when available

[Nicolin Chen <nicolinc@nvidia.com>]

On Tue, Aug 27, 2024 at 12:51:32PM -0300, Jason Gunthorpe wrote:
> Force Write Back (FWB) changes how the S2 IOPTE's MemAttr field
> works. When S2FWB is supported and enabled the IOPTE will force cachable
> access to IOMMU_CACHE memory when nesting with a S1 and deny cachable
> access otherwise.
> 
> When using a single stage of translation, a simple S2 domain, it doesn't
> change anything as it is just a different encoding for the exsting mapping
> of the IOMMU protection flags to cachability attributes.
> 
> However, when used with a nested S1, FWB has the effect of preventing the
> guest from choosing a MemAttr in it's S1 that would cause ordinary DMA to
> bypass the cache. Consistent with KVM we wish to deny the guest the
> ability to become incoherent with cached memory the hypervisor believes is
> cachable so we don't have to flush it.
> 
> Turn on S2FWB whenever the SMMU supports it and use it for all S2
> mappings.
> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>