linux-kernel - [PATCH 04/10] KVM: SVM: Don't change target vCPU state on AP Creation VMGEXIT error

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Message-ID: <20250219012705.1495231-5-seanjc@google.com>
Date: Tue, 18 Feb 2025 17:26:59 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Naveen N Rao <naveen@...nel.org>, Kim Phillips <kim.phillips@....com>, 
	Tom Lendacky <thomas.lendacky@....com>, Alexey Kardashevskiy <aik@....com>
Subject: [PATCH 04/10] 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: e366f92ea99e ("KVM: SEV: Support SEV-SNP AP Creation NAE event")
Cc: stable@...r.kernel.org
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/svm/sev.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index e14a37dbc6ea..07125b2cf0a6 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3959,16 +3959,12 @@ 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;
@@ -4014,20 +4010,23 @@ 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;
-- 
2.48.1.601.g30ceb7b040-goog