From 76901e56fb517db61939efdf54e9581b117d615d Mon Sep 17 00:00:00 2001 From: Mingwei Zhang Date: Wed, 19 Oct 2022 16:56:17 +0000 Subject: [PATCH] KVM: x86/mmu: explicitly check nx_hugepage in disallowed_hugepage_adjust() Explicitly check if a NX huge page is disallowed when determining if a page fault needs to be forced to use a smaller sized page. KVM currently assumes that the NX huge page mitigation is the only scenario where KVM will force a shadow page instead of a huge page, and so unnecessarily keeps an existing shadow page instead of replacing it with a huge page. Any scenario that causes KVM to zap leaf SPTEs may result in having a SP that can be made huge without violating the NX huge page mitigation. E.g. prior to commit 5ba7c4c6d1c7 ("KVM: x86/MMU: Zap non-leaf SPTEs when disabling dirty logging"), KVM would keep shadow pages after disabling dirty logging due to a live migration being canceled, resulting in degraded performance due to running with 4kb pages instead of huge pages. Although the dirty logging case is "fixed", that fix is coincidental, i.e. is an implementation detail, and there are other scenarios where KVM will zap leaf SPTEs. E.g. 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. Fixes: b8e8c8303ff2 ("kvm: mmu: ITLB_MULTIHIT mitigation") Reviewed-by: Ben Gardon Reviewed-by: David Matlack Signed-off-by: Mingwei Zhang [sean: use spte_to_child_sp(), massage changelog, fold into if-statement] Signed-off-by: Sean Christopherson Reviewed-by: Yan Zhao Message-Id: <20221019165618.927057-8-seanjc@google.com> Signed-off-by: Paolo Bonzini --- arch/x86/kvm/mmu/mmu.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 5f9b57c6e506..efce5e4e24c3 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -3112,7 +3112,8 @@ void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_ if (cur_level > PG_LEVEL_4K && cur_level == fault->goal_level && is_shadow_present_pte(spte) && - !is_large_pte(spte)) { + !is_large_pte(spte) && + spte_to_child_sp(spte)->nx_huge_page_disallowed) { /* * A small SPTE exists for this pfn, but FNAME(fetch) * and __direct_map would like to create a large PTE