On Tue, Aug 30, 2022, Sean Christopherson wrote: > Note, this applies on Yosry's stats series (there's a trivial-but-subtle > conflict in the TDP MMU shadow page accounting). > https://lore.kernel.org/all/20220823004639.2387269-1-yosryahmed@xxxxxxxxxx > > Precisely track (via kvm_mmu_page) if a non-huge page is being forced > and use that info to avoid unnecessarily forcing smaller page sizes in > disallowed_hugepage_adjust(). > > KVM incorrectly assumes that the NX huge page mitigation is the only > scenario where KVM will create a non-leaf page instead of a huge page. > As a result, if the original source of huge page incompatibility goes > away, the NX mitigation is enabled, and KVM encounters an present shadow > page when attempting to install a huge page, KVM will force a smaller page > regardless of whether or not a smaller page is actually necessary to > satisfy the NX huge page mitigation. > > Unnecessarily forcing small pages can result in degraded guest performance, > especially on larger VMs. The bug was originally discovered when testing > dirty log performance, as KVM would leave small pages lying around when > zapping collapsible SPTEs. That case was indadvertantly fixed by commit > 5ba7c4c6d1c7 ("KVM: x86/MMU: Zap non-leaf SPTEs when disabling dirty > logging"), but other scenarios are still affected, e.g. KVM will not > rebuild a huge page if the mmu_notifier zaps a range of PTEs because the > primary MMU is creating a huge page. > > v4: > - Collect reviews. [Mingwei] > - Add comment to document possible_nx_huge_pages. [Mingwei] > - Drop extra memory barriers. [Paolo] > - Document ordering providing by TDP SPTE helpers. [Paolo] Hi Paolo and folks, Just a gentle ping. Are we good on this version? It seems the ordering concern in TDP MMU has been addressed. Thanks. -Mingwei
