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@xxxxxxxxxx> Reviewed-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
