KVM: x86: Ignore CR0.WP toggles in non-paging mode

If paging is disabled, there are no permission bits to emulate.
Micro-optimize this case to avoid unnecessary work.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Link: https://lore.kernel.org/r/20230322013731.102955-4-minipli@grsecurity.net
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/x86.c

@@ -908,14 +908,20 @@ void kvm_post_set_cr0(struct kvm_vcpu *vcpu, unsigned long old_cr0, unsigned lon
 {
 	/*
 	 * CR0.WP is incorporated into the MMU role, but only for non-nested,
-	 * indirect shadow MMUs.  If TDP is enabled, the MMU's metadata needs
-	 * to be updated, e.g. so that emulating guest translations does the
-	 * right thing, but there's no need to unload the root as CR0.WP
-	 * doesn't affect SPTEs.
+	 * indirect shadow MMUs.  If paging is disabled, no updates are needed
+	 * as there are no permission bits to emulate.  If TDP is enabled, the
+	 * MMU's metadata needs to be updated, e.g. so that emulating guest
+	 * translations does the right thing, but there's no need to unload the
+	 * root as CR0.WP doesn't affect SPTEs.
 	 */
-	if (tdp_enabled && (cr0 ^ old_cr0) == X86_CR0_WP) {
-		kvm_init_mmu(vcpu);
-		return;
+	if ((cr0 ^ old_cr0) == X86_CR0_WP) {
+		if (!(cr0 & X86_CR0_PG))
+			return;
+
+		if (tdp_enabled) {
+			kvm_init_mmu(vcpu);
+			return;
+		}
 	}
 
 	if ((cr0 ^ old_cr0) & X86_CR0_PG) {