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

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 <seanjc@google.com>
Message-Id: <20210225204749.1512652-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index cfa8265..474c2b6 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3063,6 +3063,9 @@ 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;