[PATCH] KVM: x86/mmu: Remove KVM MMU write lock when accessing indirect_shadow_pages

[Mingwei Zhang]

Remove KVM MMU write lock when accessing indirect_shadow_pages counter when
page role is direct because this counter value is used as a coarse-grained
heuristics to check if there is nested guest active. Racing with this
heuristics without mmu lock will be harmless because the corresponding
indirect shadow sptes for the GPA will either be zapped by this thread or
some other thread who has previously zapped all indirect shadow pages and
makes the value to 0.

Because of that, remove the KVM MMU write lock pair to potentially reduce
the lock contension and improve the performance of nested VM. In addition
opportunistically change the comment of 'direct mmu' to make the
description consistent with other places.

Reported-by: Jim Mattson <jmattson@xxxxxxxxxx>
Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5ad55ef71433..97cfa5a00ff2 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8585,15 +8585,9 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 
 	kvm_release_pfn_clean(pfn);
 
-	/* The instructions are well-emulated on direct mmu. */
+	/* The instructions are well-emulated on Direct MMUs. */
 	if (vcpu->arch.mmu->root_role.direct) {
-		unsigned int indirect_shadow_pages;
-
-		write_lock(&vcpu->kvm->mmu_lock);
-		indirect_shadow_pages = vcpu->kvm->arch.indirect_shadow_pages;
-		write_unlock(&vcpu->kvm->mmu_lock);
-
-		if (indirect_shadow_pages)
+		if (READ_ONCE(vcpu->kvm->arch.indirect_shadow_pages))
 			kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
 
 		return true;

base-commit: 31b4fc3bc64aadd660c5bfa5178c86a7ba61e0f7
-- 
2.41.0.rc0.172.g3f132b7071-goog