KVM: x86/mmu: Use '0' as the one and only value for an invalid PAE root

Use '0' to denote an invalid pae_root instead of '0' or INVALID_PAGE.
Unlike root_hpa, the pae_roots hold permission bits and thus are
guaranteed to be non-zero.  Having to deal with both values leads to
bugs, e.g. failing to set back to INVALID_PAGE, warning on the wrong
value, etc...
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210309224207.1218275-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -3197,11 +3197,14 @@ void kvm_mmu_free_roots(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
 		    (mmu->root_level >= PT64_ROOT_4LEVEL || mmu->direct_map)) {
 			mmu_free_root_page(kvm, &mmu->root_hpa, &invalid_list);
 		} else if (mmu->pae_root) {
-			for (i = 0; i < 4; ++i)
-				if (mmu->pae_root[i] != 0)
-					mmu_free_root_page(kvm,
-							   &mmu->pae_root[i],
-							   &invalid_list);
+			for (i = 0; i < 4; ++i) {
+				if (!IS_VALID_PAE_ROOT(mmu->pae_root[i]))
+					continue;
+
+				mmu_free_root_page(kvm, &mmu->pae_root[i],
+						   &invalid_list);
+				mmu->pae_root[i] = INVALID_PAE_ROOT;
+			}
 		}
 		mmu->root_hpa = INVALID_PAGE;
 		mmu->root_pgd = 0;
@@ -3253,8 +3256,7 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
 			return -EIO;
 
 		for (i = 0; i < 4; ++i) {
-			WARN_ON_ONCE(mmu->pae_root[i] &&
-				     VALID_PAGE(mmu->pae_root[i]));
+			WARN_ON_ONCE(IS_VALID_PAE_ROOT(mmu->pae_root[i]));
 
 			root = mmu_alloc_root(vcpu, i << (30 - PAGE_SHIFT),
 					      i << 30, PT32_ROOT_LEVEL, true);
@@ -3328,11 +3330,11 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
 	}
 
 	for (i = 0; i < 4; ++i) {
-		WARN_ON_ONCE(mmu->pae_root[i] && VALID_PAGE(mmu->pae_root[i]));
+		WARN_ON_ONCE(IS_VALID_PAE_ROOT(mmu->pae_root[i]));
 
 		if (mmu->root_level == PT32E_ROOT_LEVEL) {
 			if (!(pdptrs[i] & PT_PRESENT_MASK)) {
-				mmu->pae_root[i] = 0;
+				mmu->pae_root[i] = INVALID_PAE_ROOT;
 				continue;
 			}
 			root_gfn = pdptrs[i] >> PAGE_SHIFT;
@@ -3450,7 +3452,7 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
 	for (i = 0; i < 4; ++i) {
 		hpa_t root = vcpu->arch.mmu->pae_root[i];
 
-		if (root && VALID_PAGE(root)) {
+		if (IS_VALID_PAE_ROOT(root)) {
 			root &= PT64_BASE_ADDR_MASK;
 			sp = to_shadow_page(root);
 			mmu_sync_children(vcpu, sp);
@@ -5307,7 +5309,7 @@ static int __kvm_mmu_create(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu)
 
 	mmu->pae_root = page_address(page);
 	for (i = 0; i < 4; ++i)
-		mmu->pae_root[i] = INVALID_PAGE;
+		mmu->pae_root[i] = INVALID_PAE_ROOT;
 
 	return 0;
 }

arch/x86/kvm/mmu/mmu_audit.c

@@ -70,7 +70,7 @@ static void mmu_spte_walk(struct kvm_vcpu *vcpu, inspect_spte_fn fn)
 	for (i = 0; i < 4; ++i) {
 		hpa_t root = vcpu->arch.mmu->pae_root[i];
 
-		if (root && VALID_PAGE(root)) {
+		if (IS_VALID_PAE_ROOT(root)) {
 			root &= PT64_BASE_ADDR_MASK;
 			sp = to_shadow_page(root);
 			__mmu_spte_walk(vcpu, sp, fn, 2);

arch/x86/kvm/mmu/mmu_internal.h

@@ -20,6 +20,16 @@ extern bool dbg;
 #define MMU_WARN_ON(x) do { } while (0)
 #endif
 
+/*
+ * Unlike regular MMU roots, PAE "roots", a.k.a. PDPTEs/PDPTRs, have a PRESENT
+ * bit, and thus are guaranteed to be non-zero when valid.  And, when a guest
+ * PDPTR is !PRESENT, its corresponding PAE root cannot be set to INVALID_PAGE,
+ * as the CPU would treat that as PRESENT PDPTR with reserved bits set.  Use
+ * '0' instead of INVALID_PAGE to indicate an invalid PAE root.
+ */
+#define INVALID_PAE_ROOT	0
+#define IS_VALID_PAE_ROOT(x)	(!!(x))
+
 struct kvm_mmu_page {
 	struct list_head link;
 	struct hlist_node hash_link;