On Wed, Jun 12, 2024 at 10:23:38AM -0700, Sean Christopherson wrote: > On Wed, Jun 12, 2024, Yu Zhao wrote: > > I do think there can be false negatives but we have not been able to > > measure their practical impacts since we disabled the flush on some > > host MMUs long ago (NOT by MGLRU), e.g., on x86 and ppc, > > ptep_clear_flush_young() is just ptep_test_andclear_young(). > > Aha! That's what I was missing, I somehow didn't see x86's ptep_clear_flush_young(). Heh, well the helper name isn't exactly giving any hints... > That begs the question, why does KVM flush TLBs on architectures that don't need > to? And since kvm_mmu_notifier_clear_young() explicitly doesn't flush, are there > even any KVM-supported architectures for which the flush is mandatory? > > Skipping the flush on KVM x86 seems like a complete no-brainer. > > Will, Marc and/or Oliver, what are arm64's requirements in this area? E.g. I see > that arm64's version of __ptep_clear_flush_young() does TLBI but not DSB. Should > KVM be doing something similar? Can KVM safely skip even the TBLI? Short answer, yes, KVM can elide TLBIs when clearing AF. Long answer: Software needs to be extremely careful to ensure that TLBI elision doesn't lead to a failure to uphold break-before-make requirements, if we're only concerned with architecture-specific requirements. IOW, the AF cannot be used as a hint for the presence of TLB entries for a given PTE. There's the obvious failure of skipping TLBIs for old pages when unmapping, but that isn't an architecture-specific issue. So, since KVM/arm64 doesn't play any games with the AF at stage-2, leaving out a TLBI when aging ought to be fine. -- Thanks, Oliver
