KVM: x86/mmu: Drop exec/NX check from "page fault can be fast"

Tweak the "page fault can be fast" logic to explicitly check for !PRESENT
faults in the access tracking case, and drop the exec/NX check that
becomes redundant as a result.  No sane hardware will generate an access
that is both an instruct fetch and a write, i.e. it's a waste of cycles.
If hardware goes off the rails, or KVM runs under a misguided hypervisor,
spuriously running throught fast path is benign (KVM has been uknowingly
being doing exactly that for years).

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220423034752.1161007-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: sean-jc

arch/x86/kvm/mmu/mmu.c

@@ -3044,16 +3044,14 @@ static bool handle_abnormal_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fa
 static bool page_fault_can_be_fast(struct kvm_page_fault *fault)
 {
 	/*
-	 * Do not fix the mmio spte with invalid generation number which
-	 * need to be updated by slow page fault path.
+	 * Page faults with reserved bits set, i.e. faults on MMIO SPTEs, only
+	 * reach the common page fault handler if the SPTE has an invalid MMIO
+	 * generation number.  Refreshing the MMIO generation needs to go down
+	 * the slow path.  Note, EPT Misconfigs do NOT set the PRESENT flag!
 	 */
 	if (fault->rsvd)
 		return false;
 
-	/* See if the page fault is due to an NX violation */
-	if (unlikely(fault->exec && fault->present))
-		return false;
-
 	/*
 	 * #PF can be fast if:
 	 *
@@ -3069,7 +3067,14 @@ static bool page_fault_can_be_fast(struct kvm_page_fault *fault)
 	 *    SPTE is MMU-writable (determined later), the fault can be fixed
 	 *    by setting the Writable bit, which can be done out of mmu_lock.
 	 */
-	return !kvm_ad_enabled() || (fault->write && fault->present);
+	if (!fault->present)
+		return !kvm_ad_enabled();
+
+	/*
+	 * Note, instruction fetches and writes are mutually exclusive, ignore
+	 * the "exec" flag.
+	 */
+	return fault->write;
 }
 
 /*