[03/24] KVM: x86/mmu: Bail from fast_page_fault() if SPTE is not shadow-present

Commit Message

Sean Christopherson Feb. 25, 2021, 8:47 p.m. UTC

Bail from fast_page_fault() if the SPTE is not a shadow-present SPTE.
Functionally, this is not strictly necessary as the !is_access_allowed()
check will eventually reject the fast path, but an early check on
shadow-present skips unnecessary checks and will allow a future patch to
tweak the A/D status auditing to warn if KVM attempts to query A/D bits
without first ensuring the SPTE is a shadow-present SPTE.

Note, is_shadow_present_pte() is quite expensive at this time, i.e. this
might be a net negative in the short term.  A future patch will optimize
is_shadow_present_pte() to a single AND operation and remedy the issue.

Signed-off-by: Sean Christopherson <[e-mail]>
---
 arch/x86/kvm/mmu/mmu.c | 3 +++
 1 file changed, 3 insertions(+)

Patch

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index d75524bc8423..93b0285e8b38 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3061,6 +3061,9 @@  static int fast_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 			if (!is_shadow_present_pte(spte))
 				break;
 
+		if (!is_shadow_present_pte(spte))
+			break;
+
 		sp = sptep_to_sp(iterator.sptep);
 		if (!is_last_spte(spte, sp->role.level))
 			break;