KVM: SVM: Use guard(mutex) to simplify SNP vCPU state updates

Use guard(mutex) in sev_snp_init_protected_guest_state() and pull in its
lock-protected inner helper.  Without an unlock trampoline (and even with
one), there is no real need for an inner helper.  Eliminating the helper
also avoids having to fixup the open coded "lockdep" WARN_ON().

Opportunistically drop the error message if KVM can't obtain the pfn for
the new target VMSA.  The error message provides zero information that
can't be gleaned from the fact that the vCPU is stuck.

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20250227012541.3234589-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/sev.c

@@ -3842,11 +3842,26 @@ static int snp_begin_psc(struct vcpu_svm *svm, struct psc_buffer *psc)
 	BUG();
 }
 
-static int __sev_snp_update_protected_guest_state(struct kvm_vcpu *vcpu)
+/*
+ * Invoked as part of svm_vcpu_reset() processing of an init event.
+ */
+void sev_snp_init_protected_guest_state(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
+	struct kvm_memory_slot *slot;
+	struct page *page;
+	kvm_pfn_t pfn;
+	gfn_t gfn;
+
+	if (!sev_snp_guest(vcpu->kvm))
+		return;
 
-	WARN_ON(!mutex_is_locked(&svm->sev_es.snp_vmsa_mutex));
+	guard(mutex)(&svm->sev_es.snp_vmsa_mutex);
+
+	if (!svm->sev_es.snp_ap_waiting_for_reset)
+		return;
+
+	svm->sev_es.snp_ap_waiting_for_reset = false;
 
 	/* Mark the vCPU as offline and not runnable */
 	vcpu->arch.pv.pv_unhalted = false;
@@ -3861,79 +3876,48 @@ static int __sev_snp_update_protected_guest_state(struct kvm_vcpu *vcpu)
 	 */
 	vmcb_mark_all_dirty(svm->vmcb);
 
-	if (VALID_PAGE(svm->sev_es.snp_vmsa_gpa)) {
-		gfn_t gfn = gpa_to_gfn(svm->sev_es.snp_vmsa_gpa);
-		struct kvm_memory_slot *slot;
-		struct page *page;
-		kvm_pfn_t pfn;
-
-		slot = gfn_to_memslot(vcpu->kvm, gfn);
-		if (!slot)
-			return -EINVAL;
-
-		/*
-		 * The new VMSA will be private memory guest memory, so
-		 * retrieve the PFN from the gmem backend.
-		 */
-		if (kvm_gmem_get_pfn(vcpu->kvm, slot, gfn, &pfn, &page, NULL))
-			return -EINVAL;
-
-		/*
-		 * From this point forward, the VMSA will always be a
-		 * guest-mapped page rather than the initial one allocated
-		 * by KVM in svm->sev_es.vmsa. In theory, svm->sev_es.vmsa
-		 * could be free'd and cleaned up here, but that involves
-		 * cleanups like wbinvd_on_all_cpus() which would ideally
-		 * be handled during teardown rather than guest boot.
-		 * Deferring that also allows the existing logic for SEV-ES
-		 * VMSAs to be re-used with minimal SNP-specific changes.
-		 */
-		svm->sev_es.snp_has_guest_vmsa = true;
-
-		/* Use the new VMSA */
-		svm->vmcb->control.vmsa_pa = pfn_to_hpa(pfn);
-
-		/* Mark the vCPU as runnable */
-		vcpu->arch.pv.pv_unhalted = false;
-		vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
-
-		svm->sev_es.snp_vmsa_gpa = INVALID_PAGE;
-
-		/*
-		 * gmem pages aren't currently migratable, but if this ever
-		 * changes then care should be taken to ensure
-		 * svm->sev_es.vmsa is pinned through some other means.
-		 */
-		kvm_release_page_clean(page);
-	}
+	if (!VALID_PAGE(svm->sev_es.snp_vmsa_gpa))
+		return;
 
-	return 0;
-}
+	gfn = gpa_to_gfn(svm->sev_es.snp_vmsa_gpa);
 
-/*
- * Invoked as part of svm_vcpu_reset() processing of an init event.
- */
-void sev_snp_init_protected_guest_state(struct kvm_vcpu *vcpu)
-{
-	struct vcpu_svm *svm = to_svm(vcpu);
-	int ret;
+	slot = gfn_to_memslot(vcpu->kvm, gfn);
+	if (!slot)
+		return;
 
-	if (!sev_snp_guest(vcpu->kvm))
+	/*
+	 * The new VMSA will be private memory guest memory, so retrieve the
+	 * PFN from the gmem backend.
+	 */
+	if (kvm_gmem_get_pfn(vcpu->kvm, slot, gfn, &pfn, &page, NULL))
 		return;
 
-	mutex_lock(&svm->sev_es.snp_vmsa_mutex);
+	/*
+	 * From this point forward, the VMSA will always be a guest-mapped page
+	 * rather than the initial one allocated by KVM in svm->sev_es.vmsa. In
+	 * theory, svm->sev_es.vmsa could be free'd and cleaned up here, but
+	 * that involves cleanups like wbinvd_on_all_cpus() which would ideally
+	 * be handled during teardown rather than guest boot.  Deferring that
+	 * also allows the existing logic for SEV-ES VMSAs to be re-used with
+	 * minimal SNP-specific changes.
+	 */
+	svm->sev_es.snp_has_guest_vmsa = true;
 
-	if (!svm->sev_es.snp_ap_waiting_for_reset)
-		goto unlock;
+	/* Use the new VMSA */
+	svm->vmcb->control.vmsa_pa = pfn_to_hpa(pfn);
 
-	svm->sev_es.snp_ap_waiting_for_reset = false;
+	/* Mark the vCPU as runnable */
+	vcpu->arch.pv.pv_unhalted = false;
+	vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
 
-	ret = __sev_snp_update_protected_guest_state(vcpu);
-	if (ret)
-		vcpu_unimpl(vcpu, "snp: AP state update on init failed\n");
+	svm->sev_es.snp_vmsa_gpa = INVALID_PAGE;
 
-unlock:
-	mutex_unlock(&svm->sev_es.snp_vmsa_mutex);
+	/*
+	 * gmem pages aren't currently migratable, but if this ever changes
+	 * then care should be taken to ensure svm->sev_es.vmsa is pinned
+	 * through some other means.
+	 */
+	kvm_release_page_clean(page);
 }
 
 static int sev_snp_ap_creation(struct vcpu_svm *svm)