> From: Jason Gunthorpe <jgg@xxxxxxxxxx> > Sent: Tuesday, August 27, 2024 11:52 PM > > 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. > I'm confused here. It is clear that we need a mechanism via which the VM cannot bypass the cache, before Yan's series comes to relax. But according to above description S2FWB cannot 100% guarantee it due to PCI No Snoop. Does it suggest that we should only allow nesting only for CANWBS, or disable/hide PCI No Snoop cap from the guest in case of S2FWB?
