On Mon, Mar 21, 2022, David Matlack wrote: > On Mon, Mar 21, 2022 at 3:00 PM David Matlack <dmatlack@xxxxxxxxxx> wrote: > > > > 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? > > Ahh I see, kvm_mmu_zap_collapsible_sptes() only zaps the leaf SPTEs. > Could you add a blurb about this in the commit message for future > reference? > will do. > > > > > 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. > > Nevermind, support for zapping leaf SPTEs already exists for zapping > collapsible 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/ Thanks for that. > > > > > > > > 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 > > >