KVM: x86/mmu: Optimize not-present/MMIO SPTE check in get_mmio_spte()

Check only the terminal leaf for a "!PRESENT || MMIO" SPTE when looking
for reserved bits on valid, non-MMIO SPTEs.  The get_walk() helpers
terminate their walks if a not-present or MMIO SPTE is encountered, i.e.
the non-terminal SPTEs have already been verified to be regular SPTEs.
This eliminates an extra check-and-branch in a relatively hot loop.
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20201218003139.2167891-5-seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -3511,7 +3511,7 @@ static int get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes, int *root_level
 	return leaf;
 }
 
-/* return true if reserved bit is detected on spte. */
+/* return true if reserved bit(s) are detected on a valid, non-MMIO SPTE. */
 static bool get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
 {
 	u64 sptes[PT64_ROOT_MAX_LEVEL + 1];
@@ -3534,11 +3534,20 @@ static bool get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
 		return reserved;
 	}
 
+	*sptep = sptes[leaf];
+
+	/*
+	 * Skip reserved bits checks on the terminal leaf if it's not a valid
+	 * SPTE.  Note, this also (intentionally) skips MMIO SPTEs, which, by
+	 * design, always have reserved bits set.  The purpose of the checks is
+	 * to detect reserved bits on non-MMIO SPTEs. i.e. buggy SPTEs.
+	 */
+	if (!is_shadow_present_pte(sptes[leaf]))
+		leaf++;
+
 	rsvd_check = &vcpu->arch.mmu->shadow_zero_check;
 
-	for (level = root; level >= leaf; level--) {
-		if (!is_shadow_present_pte(sptes[level]))
-			break;
+	for (level = root; level >= leaf; level--)
 		/*
 		 * Use a bitwise-OR instead of a logical-OR to aggregate the
 		 * reserved bit and EPT's invalid memtype/XWR checks to avoid
@@ -3546,7 +3555,6 @@ static bool get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
 		 */
 		reserved |= __is_bad_mt_xwr(rsvd_check, sptes[level]) |
 			    __is_rsvd_bits_set(rsvd_check, sptes[level], level);
-	}
 
 	if (reserved) {
 		pr_err("%s: detect reserved bits on spte, addr 0x%llx, dump hierarchy:\n",
@@ -3556,8 +3564,6 @@ static bool get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
 			       sptes[level], level);
 	}
 
-	*sptep = sptes[leaf];
-
 	return reserved;
 }