KVM: SVM: Don't change target vCPU state on AP Creation VMGEXIT error

If KVM rejects an AP Creation event, leave the target vCPU state as-is.
Nothing in the GHCB suggests the hypervisor is *allowed* to muck with vCPU
state on failure, let alone required to do so.  Furthermore, kicking only
in the !ON_INIT case leads to divergent behavior, and even the "kick" case
is non-deterministic.

E.g. if an ON_INIT request fails, the guest can successfully retry if the
fixed AP Creation request is made prior to sending INIT.  And if a !ON_INIT
fails, the guest can successfully retry if the fixed AP Creation request is
handled before the target vCPU processes KVM's
KVM_REQ_UPDATE_PROTECTED_GUEST_STATE.

Fixes: e366f92 ("KVM: SEV: Support SEV-SNP AP Creation NAE event")
Cc: stable@vger.kernel.org
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Link: https://lore.kernel.org/r/20250227012541.3234589-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/sev.c

@@ -3963,16 +3963,12 @@ static int sev_snp_ap_creation(struct vcpu_svm *svm)
 
 	/*
 	 * The target vCPU is valid, so the vCPU will be kicked unless the
-	 * request is for CREATE_ON_INIT. For any errors at this stage, the
-	 * kick will place the vCPU in an non-runnable state.
+	 * request is for CREATE_ON_INIT.
 	 */
 	kick = true;
 
 	mutex_lock(&target_svm->sev_es.snp_vmsa_mutex);
 
-	target_svm->sev_es.snp_vmsa_gpa = INVALID_PAGE;
-	target_svm->sev_es.snp_ap_waiting_for_reset = true;
-
 	/* Interrupt injection mode shouldn't change for AP creation */
 	if (request < SVM_VMGEXIT_AP_DESTROY) {
 		u64 sev_features;
@@ -4018,20 +4014,23 @@ static int sev_snp_ap_creation(struct vcpu_svm *svm)
 		target_svm->sev_es.snp_vmsa_gpa = svm->vmcb->control.exit_info_2;
 		break;
 	case SVM_VMGEXIT_AP_DESTROY:
+		target_svm->sev_es.snp_vmsa_gpa = INVALID_PAGE;
 		break;
 	default:
 		vcpu_unimpl(vcpu, "vmgexit: invalid AP creation request [%#x] from guest\n",
 			    request);
 		ret = -EINVAL;
-		break;
+		goto out;
 	}
 
-out:
+	target_svm->sev_es.snp_ap_waiting_for_reset = true;
+
 	if (kick) {
 		kvm_make_request(KVM_REQ_UPDATE_PROTECTED_GUEST_STATE, target_vcpu);
 		kvm_vcpu_kick(target_vcpu);
 	}
 
+out:
 	mutex_unlock(&target_svm->sev_es.snp_vmsa_mutex);
 
 	return ret;