On Fri, Mar 04, 2022, Mingwei Zhang wrote: > On Thu, Mar 03, 2022, Paolo Bonzini wrote: > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c > > index f3939ce4a115..c71debdbc732 100644 > > --- a/arch/x86/kvm/mmu/tdp_mmu.c > > +++ b/arch/x86/kvm/mmu/tdp_mmu.c > > @@ -834,10 +834,8 @@ bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp) > > } > > > > /* > > - * Tears down the mappings for the range of gfns, [start, end), and frees the > > - * non-root pages mapping GFNs strictly within that range. Returns true if > > - * SPTEs have been cleared and a TLB flush is needed before releasing the > > - * MMU lock. > > + * Zap leafs SPTEs for the range of gfns, [start, end). Returns true if SPTEs > > + * have been cleared and a TLB flush is needed before releasing the MMU lock. > > I think the original code does not _over_ zapping. But the new version > does. No, the new version doesn't overzap. > Will that have some side effects? In particular, if the range is > within a huge page (or HugeTLB page of various sizes), then we choose to > zap it even if it is more than the range. The old version did that too. KVM _must_ zap a hugepage that overlaps the range, otherwise the guest would be able to access memory that has been freed/moved. If the operation has unmapped a subset of a hugepage, KVM needs to zap and rebuild the portions that are still valid using smaller pages. > Regardless of side effect, I think we probably should mention that in > the comments? > > - /* > > - * If this is a non-last-level SPTE that covers a larger range > > - * than should be zapped, continue, and zap the mappings at a > > - * lower level, except when zapping all SPTEs. > > - */ > > - if (!zap_all && > > - (iter.gfn < start || > > - iter.gfn + KVM_PAGES_PER_HPAGE(iter.level) > end) && > > + if (!is_shadow_present_pte(iter.old_spte) || > > !is_last_spte(iter.old_spte, iter.level)) It's hard to see in the diff, but the key is the "!is_last_spte()" check. The check before was skipping non-leaf, a.k.a. shadow pages, if they weren't in the range. The new version _always_ skips shadow pages. Hugepages will always return true for is_last_spte() and will never be skipped.
