On Sun, Mar 20, 2022 at 5:26 PM Mingwei Zhang <mizhang@xxxxxxxxxx> wrote: > > Add extra check to specify the case of nx hugepage and allow KVM to > reconstruct large mapping after dirty logging is disabled. Existing code > works only for nx hugepage but the condition is too general in that does > not consider other usage case (such as dirty logging). KVM calls kvm_mmu_zap_collapsible_sptes() when dirty logging is disabled. Why is that not sufficient? > Moreover, existing > code assumes that a present PMD or PUD indicates that there exist 'smaller > SPTEs' under the paging structure. This assumption may no be true if > consider the zapping leafs only behavior in MMU. Good point. Although, that code just got reverted. Maybe say something like: This assumption may not be true in the future if KVM gains support for zapping only leaf SPTEs. > > Missing the check causes KVM incorrectly regards the faulting page as a NX > huge page and refuse to map it at desired level. And this leads to back > performance in shadow mmu and potentiall TDP mmu. s/potentiall/potentially/ > > Fixes: b8e8c8303ff2 ("kvm: mmu: ITLB_MULTIHIT mitigation") > Cc: stable@xxxxxxxxxxxxxxx > > Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx> > Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx> > --- > arch/x86/kvm/mmu/mmu.c | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > index 5628d0ba637e..4d358c273f6c 100644 > --- a/arch/x86/kvm/mmu/mmu.c > +++ b/arch/x86/kvm/mmu/mmu.c > @@ -2919,6 +2919,16 @@ void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_ > cur_level == fault->goal_level && > is_shadow_present_pte(spte) && > !is_large_pte(spte)) { > + struct kvm_mmu_page *sp; > + u64 page_mask; > + /* > + * When nx hugepage flag is not set, there is no reason to > + * go down to another level. This helps demand paging to > + * generate large mappings. > + */ > + sp = to_shadow_page(spte & PT64_BASE_ADDR_MASK); > + if (!sp->lpage_disallowed) > + return; > /* > * A small SPTE exists for this pfn, but FNAME(fetch) > * and __direct_map would like to create a large PTE > @@ -2926,8 +2936,8 @@ void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_ > * patching back for them into pfn the next 9 bits of > * the address. > */ > - u64 page_mask = KVM_PAGES_PER_HPAGE(cur_level) - > - KVM_PAGES_PER_HPAGE(cur_level - 1); > + page_mask = KVM_PAGES_PER_HPAGE(cur_level) - > + KVM_PAGES_PER_HPAGE(cur_level - 1); > fault->pfn |= fault->gfn & page_mask; > fault->goal_level--; > } > -- > 2.35.1.894.gb6a874cedc-goog >