KVM: nSVM: fix condition for filtering async PF

Async page faults have to be trapped in the host (L1 in this case),
since the APF reason was passed from L0 to L1 and stored in the L1 APF
data page.  This was completely reversed: the page faults were passed
to the guest, a L2 hypervisor.

Cc: stable@vger.kernel.org
Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/nested.c

@@ -880,8 +880,8 @@ int nested_svm_exit_special(struct vcpu_svm *svm)
 			return NESTED_EXIT_HOST;
 		break;
 	case SVM_EXIT_EXCP_BASE + PF_VECTOR:
-		/* When we're shadowing, trap PFs, but not async PF */
-		if (!npt_enabled && svm->vcpu.arch.apf.host_apf_reason == 0)
+		/* Trap async PF even if not shadowing */
+		if (!npt_enabled || svm->vcpu.arch.apf.host_apf_reason)
 			return NESTED_EXIT_HOST;
 		break;
 	default: