[PATCH v2 3/4] KVM: x86/mmu: explicitly check nx_hugepage in disallowed_hugepage_adjust()

[Mingwei Zhang]

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). Note that
when dirty logging is disabled, KVM calls kvm_mmu_zap_collapsible_sptes()
which only zaps leaf SPTEs. 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 KVM zaps only leafs in MMU.

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 issue in shadow mmu and potentially in TDP mmu as well.

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..d9b2001d8217 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 KVM re-generate large
+		 * mappings after dirty logging disabled.
+		 */
+		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--;
 	}
-- 
2.35.1.1021.g381101b075-goog