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CMO for non-FWB. > > 2) If cacheable PFNMAP: > a) With FWB, use a cacheable mapping > b) Without FWB, fail. > > 3) If VM_ALLOW_ANY_UNCACHED, use Normal Non-Cacheable mapping > > 4) Otherwise, Device-nGnRE > > I understand 2b breaks ABI, but the 'typical' VFIO usages fall into (3) > and (4). +1 (and +1 to Sean's remark about strictly tracking the VMA as well) IMHO not doing 2b is a "security" bug today. Catalin suggested we fix it as a first step to get agreement on this assement and fix. Once fixed there is no way for KVM to create a S2 with cachable semantics different from the VMA, or to have missing CMOs. That simplifies the discussion of adding 2a to strictly track cachable. I also don't see a need for a flag here. > A pedantic but correct live migration / snapshotting implementation > on non-FWB would need to do CMOs in case the VM used a non-WB > mapping for memory. >From a live migration perspecive, we have already built in a lot of things on the VFIO side where a live migration can be attempted and then fail because of late-detected HW incompatibilities. We've sort of punted this for now to the orchestrator and operator. There is an expectation the environment will somehow ensure that live migration machine pools are sufficiently uniform when using VFIO. It is much more restricted than normal no-VFIO VM live migration. Given you can't have VM live migration of MMIO backed Cachable memory without a VFIO live migration driver, I wouldn't worry too much about these fine details from the KVM side. If /proc/cpuinfo shows the FWB that would be approximately similar discoverability to all the other limitations. Jason