On 2024/10/10 0:23, Jason Gunthorpe wrote:
From: Nicolin Chen<nicolinc@xxxxxxxxxx> The IORT spec, Issue E.f (April 2024), adds a new CANWBS bit to the Memory Access Flag field in the Memory Access Properties table, mainly for a PCI Root Complex. This CANWBS defines the coherency of memory accesses to be not marked IOWB cacheable/shareable. Its value further implies the coherency impact from a pair of mismatched memory attributes (e.g. in a nested translation case): 0x0: Use of mismatched memory attributes for accesses made by this device may lead to a loss of coherency. 0x1: Coherency of accesses made by this device to locations in Conventional memory are ensured as follows, even if the memory attributes for the accesses presented by the device or provided by the SMMU are different from Inner and Outer Write-back cacheable, Shareable. Note that the loss of coherency on a CANWBS-unsupported HW typically could occur to an SMMU that doesn't implement the S2FWB feature where additional cache flush operations would be required to prevent that from happening. Add a new ACPI_IORT_MF_CANWBS flag and set IOMMU_FWSPEC_PCI_RC_CANWBS upon the presence of this new flag. CANWBS and S2FWB are similar features, in that they both guarantee the VM can not violate coherency, however S2FWB can be bypassed by PCI No Snoop TLPs, while CANWBS cannot. Thus CANWBS meets the requirements to set IOMMU_CAP_ENFORCE_CACHE_COHERENCY. Architecturally ARM has expected that VFIO would disable No Snoop through PCI Config space, if this is done then the two would have the same protections.
Acked-by: Hanjun Guo <guohanjun@xxxxxxxxxx> Thanks Hanjun