> From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx> > Sent: Saturday, April 9, 2022 8:51 PM > > On 2022/4/8 16:16, Tian, Kevin wrote: > >> From: Jason Gunthorpe <jgg@xxxxxxxxxx> > >> Sent: Thursday, April 7, 2022 11:24 PM > >> > >> IOMMU_CACHE means "normal DMA to this iommu_domain's IOVA > should > >> be cache > >> coherent" and is used by the DMA API. The definition allows for special > >> non-coherent DMA to exist - ie processing of the no-snoop flag in PCIe > >> TLPs - so long as this behavior is opt-in by the device driver. > >> > >> The flag is mainly used by the DMA API to synchronize the IOMMU setting > >> with the expected cache behavior of the DMA master. eg based on > >> dev_is_dma_coherent() in some case. > >> > >> For Intel IOMMU IOMMU_CACHE was redefined to mean 'force all DMA > to > >> be > >> cache coherent' which has the practical effect of causing the IOMMU to > >> ignore the no-snoop bit in a PCIe TLP. > >> > >> x86 platforms are always IOMMU_CACHE, so Intel should ignore this flag. > >> > >> Instead use the new domain op enforce_cache_coherency() which causes > >> every > >> IOPTE created in the domain to have the no-snoop blocking behavior. > >> > >> Reconfigure VFIO to always use IOMMU_CACHE and call > >> enforce_cache_coherency() to operate the special Intel behavior. > >> > >> Remove the IOMMU_CACHE test from Intel IOMMU. > >> > >> Ultimately VFIO plumbs the result of enforce_cache_coherency() back into > >> the x86 platform code through kvm_arch_register_noncoherent_dma() > >> which > >> controls if the WBINVD instruction is available in the guest. No other > >> arch implements kvm_arch_register_noncoherent_dma(). > >> > >> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx> > > > > Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx> > > > > btw as discussed in last version it is not necessarily to recalculate > > snoop control globally with this new approach. Will follow up to > > clean it up after this series is merged. > > Agreed. But it also requires the enforce_cache_coherency() to be called > only after domain being attached to a device just as VFIO is doing. that actually makes sense, right? w/o device attached it's pointless to call that interface on a domain... > > Anyway, for this change in iommu/vt-d: > > Reviewed-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx> > > Best regards, > baolu
