KVM: x86/mmu: Move "invalid" check out of kvm_tdp_mmu_get_root()

Move the check for an invalid root out of kvm_tdp_mmu_get_root() and into
the one place it actually matters, tdp_mmu_next_root(), as the other user
already has an implicit validity check.  A future bug fix will need to
get references to invalid roots to honor mmu_notifier requests; there's
no point in forcing what will be a common path to open code getting a
reference to a root.

No functional change intended.

Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211215011557.399940-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -121,9 +121,14 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
 		next_root = list_first_or_null_rcu(&kvm->arch.tdp_mmu_roots,
 						   typeof(*next_root), link);
 
-	while (next_root && !kvm_tdp_mmu_get_root(kvm, next_root))
+	while (next_root) {
+		if (!next_root->role.invalid &&
+		    kvm_tdp_mmu_get_root(kvm, next_root))
+			break;
+
 		next_root = list_next_or_null_rcu(&kvm->arch.tdp_mmu_roots,
 				&next_root->link, typeof(*next_root), link);
+	}
 
 	rcu_read_unlock();
 
@@ -200,7 +205,10 @@ hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu)
 
 	role = page_role_for_level(vcpu, vcpu->arch.mmu->shadow_root_level);
 
-	/* Check for an existing root before allocating a new one. */
+	/*
+	 * Check for an existing root before allocating a new one.  Note, the
+	 * role check prevents consuming an invalid root.
+	 */
 	for_each_tdp_mmu_root(kvm, root, kvm_mmu_role_as_id(role)) {
 		if (root->role.word == role.word &&
 		    kvm_tdp_mmu_get_root(kvm, root))

arch/x86/kvm/mmu/tdp_mmu.h

@@ -10,9 +10,6 @@ hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu);
 __must_check static inline bool kvm_tdp_mmu_get_root(struct kvm *kvm,
 						     struct kvm_mmu_page *root)
 {
-	if (root->role.invalid)
-		return false;
-
 	return refcount_inc_not_zero(&root->tdp_mmu_root_count);
 }