On Thu, Jul 28, 2022, Paolo Bonzini wrote: > On 7/23/22 03:23, Sean Christopherson wrote: > > Patch 6 from Mingwei is the end goal of the series. 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. 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(). > > > > v2: Rebase, tweak a changelog accordingly. > > > > v1:https://lore.kernel.org/all/20220409003847.819686-1-seanjc@xxxxxxxxxx > > > > Mingwei Zhang (1): > > KVM: x86/mmu: explicitly check nx_hugepage in > > disallowed_hugepage_adjust() > > > > Sean Christopherson (5): > > KVM: x86/mmu: Tag disallowed NX huge pages even if they're not tracked > > KVM: x86/mmu: Properly account NX huge page workaround for nonpaging > > MMUs > > KVM: x86/mmu: Set disallowed_nx_huge_page in TDP MMU before setting > > SPTE > > KVM: x86/mmu: Track the number of TDP MMU pages, but not the actual > > pages > > KVM: x86/mmu: Add helper to convert SPTE value to its shadow page > > Some of the benefits are cool, such as not having to track the pages for the > TDP MMU, and patch 2 is a borderline bugfix, but there's quite a lot of new > non-obvious complexity here. 100% agree on the complexity. > So the obligatory question is: is it worth a hundred lines of new code? Assuming I understanding the bug Mingwei's patch fixes, yes. Though after re-reading that changelog, it should more explicitly call out the scenario we actually care about. Anyways, the bug we really care about is that by not precisely checking if a huge page is disallowed, KVM would refuse to create huge page after disabling dirty logging, which is a very noticeable performance issue for large VMs if a migration is canceled. That particular bug has since been unintentionally fixed in the TDP MMU by zapping the non-leaf SPTE, but there are other paths that could similarly be affected, e.g. I believe zapping leaf SPTEs in response to a host page migration (mmu_notifier invalidation) to create a huge page would yield a similar result; KVM would see the shadow-present non-leaf SPTE and assume a huge page is disallowed.
