KVM: nVMX: WARN if PML emulation helper is invoked outside of nested guest

WARN if vmx_write_pml_buffer() is called outside of guest mode instead
of silently ignoring the condition.  The only caller is nested EPT's
ept_update_accessed_dirty_bits(), which should only be reachable when
L2 is active.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200622215832.22090-4-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -7485,13 +7485,15 @@ static int vmx_write_pml_buffer(struct kvm_vcpu *vcpu, gpa_t gpa)
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 	gpa_t dst;
 
-	if (is_guest_mode(vcpu)) {
-		WARN_ON_ONCE(vmx->nested.pml_full);
+	if (WARN_ON_ONCE(!is_guest_mode(vcpu)))
+		return 0;
+
+	if (WARN_ON_ONCE(vmx->nested.pml_full))
+		return 1;
 
 	/*
-		 * Check if PML is enabled for the nested guest.
-		 * Whether eptp bit 6 is set is already checked
-		 * as part of A/D emulation.
+	 * Check if PML is enabled for the nested guest. Whether eptp bit 6 is
+	 * set is already checked as part of A/D emulation.
 	 */
 	vmcs12 = get_vmcs12(vcpu);
 	if (!nested_cpu_has_pml(vmcs12))
@@ -7510,7 +7512,6 @@ static int vmx_write_pml_buffer(struct kvm_vcpu *vcpu, gpa_t gpa)
 		return 0;
 
 	vmcs12->guest_pml_index--;
-	}
 
 	return 0;
 }