On Wed, May 03, 2023, Kai Huang wrote: > > for better or worse, KVM doesn't apply the "zap > > SPTEs" logic to guest PAT changes when the VM has a passthrough device > > with non-coherent DMA. > > Is it a bug? No. KVM's MTRR behavior is using a heuristic to try not to break the VM: if the VM has non-coherent DMA, then honor UC mapping in the MTRRs as such mappings may be coverage the non-coherent DMA. >From vmx_get_mt_mask(): /* We wanted to honor guest CD/MTRR/PAT, but doing so could result in * memory aliases with conflicting memory types and sometimes MCEs. * We have to be careful as to what are honored and when. The PAT is problematic because it is referenced via the guest PTEs, versus the MTRRs being tied to the guest physical address, e.g. different virtual mappings for the same physical address can yield different memtypes via the PAT. My head hurts just thinking about how that might interact with shadow paging :-) Even the MTRRs are somewhat sketchy because they are technically per-CPU, i.e. two vCPUs could have different memtypes for the same physical address. But in practice, sane software/firmware uses consistent MTRRs across all CPUs.
