@@ -137,6 +137,7 @@ enum psc_op {
 #define GHCB_HV_FT_SNP			BIT_ULL(0)
 #define GHCB_HV_FT_SNP_AP_CREATION	BIT_ULL(1)
 #define GHCB_HV_FT_SNP_RINJ		(BIT_ULL(2) | GHCB_HV_FT_SNP_AP_CREATION)
+#define GHCB_HV_FT_APIC_ID_LIST		BIT_ULL(4)
 #define GHCB_HV_FT_SNP_MULTI_VMPL	BIT_ULL(5)
 
 /*

@@ -123,6 +123,7 @@
 #define SVM_VMGEXIT_HVDB_QUERY			2
 #define SVM_VMGEXIT_HVDB_CLEAR			3
 #define SVM_VMGEXIT_HV_IPI                      0x80000015ull
+#define SVM_VMGEXIT_GET_APIC_IDS		0x80000017ull
 #define SVM_VMGEXIT_SNP_RUN_VMPL		0x80000018ull
 #define SVM_VMGEXIT_SAVIC			0x8000001aull
 #define SVM_VMGEXIT_SAVIC_REGISTER_GPA		0

@@ -40,9 +40,10 @@
 #define GHCB_VERSION_MAX	2ULL
 #define GHCB_VERSION_MIN	1ULL
 
-#define GHCB_HV_FT_SUPPORTED	(GHCB_HV_FT_SNP |		\
-				 GHCB_HV_FT_SNP_AP_CREATION |	\
-				 GHCB_HV_FT_SNP_RINJ)
+#define GHCB_HV_FT_SUPPORTED	(GHCB_HV_FT_SNP			| \
+				 GHCB_HV_FT_SNP_AP_CREATION	| \
+				 GHCB_HV_FT_SNP_RINJ		| \
+				 GHCB_HV_FT_APIC_ID_LIST)
 
 /*
  * The GHCB spec essentially states that all non-zero error codes other than

@@ -3518,6 +3519,10 @@ static int sev_es_validate_vmgexit(struct vcpu_svm *svm)
 			if (!kvm_ghcb_rax_is_valid(svm))
 				goto vmgexit_err;
 		break;
+	case SVM_VMGEXIT_GET_APIC_IDS:
+		if (!kvm_ghcb_rax_is_valid(svm))
+			goto vmgexit_err;
+		break;
 	case SVM_VMGEXIT_NMI_COMPLETE:
 	case SVM_VMGEXIT_AP_HLT_LOOP:
 	case SVM_VMGEXIT_AP_JUMP_TABLE:

@@ -4439,6 +4444,78 @@ static int sev_snp_hv_ipi(struct vcpu_svm *svm)
 	return 0;
 }
 
+struct sev_apic_id_desc {
+	u32	num_entries;
+	u32	apic_ids[];
+};
+
+static void sev_get_apic_ids(struct vcpu_svm *svm)
+{
+	struct ghcb *ghcb = svm->sev_es.ghcb;
+	struct kvm_vcpu *vcpu = &svm->vcpu, *loop_vcpu;
+	struct kvm *kvm = vcpu->kvm;
+	unsigned int id_desc_size;
+	struct sev_apic_id_desc *desc;
+	struct page *page;
+	gpa_t gpa;
+	u64 pages;
+	unsigned long i;
+	int n;
+
+	pages = vcpu->arch.regs[VCPU_REGS_RAX];
+
+	/* Each APIC ID is 32-bits in size, so make sure there is room */
+	n = atomic_read(&kvm->online_vcpus);
+	/*TODO: is this possible? */
+	if (n < 0)
+		return;
+
+	id_desc_size = sizeof(*desc);
+	id_desc_size += n * sizeof(desc->apic_ids[0]);
+	if (id_desc_size > (pages * PAGE_SIZE)) {
+		vcpu->arch.regs[VCPU_REGS_RAX] = PFN_UP(id_desc_size);
+		return;
+	}
+
+	gpa = svm->vmcb->control.exit_info_1;
+
+	ghcb_set_sw_exit_info_1(ghcb, 2);
+	ghcb_set_sw_exit_info_2(ghcb, 5);
+
+	if (!page_address_valid(vcpu, gpa))
+		return;
+
+	page = gfn_to_page(kvm, gpa_to_gfn(gpa));
+	kvm_release_page_unused(page);
+	if (!page)
+		return;
+
+	if (!pages)
+		return;
+
+	/* Allocate a buffer to hold the APIC IDs */
+	desc = kvzalloc(id_desc_size, GFP_KERNEL_ACCOUNT);
+	if (!desc)
+		return;
+
+	desc->num_entries = n;
+	kvm_for_each_vcpu(i, loop_vcpu, kvm) {
+		/*TODO: is this possible? */
+		if (i >= n)
+			break;
+
+		desc->apic_ids[i] = loop_vcpu->vcpu_id;
+	}
+
+	if (!kvm_write_guest(kvm, gpa, desc, id_desc_size)) {
+		/* IDs were successfully written */
+		ghcb_set_sw_exit_info_1(ghcb, 0);
+		ghcb_set_sw_exit_info_2(ghcb, 0);
+	}
+
+	kvfree(desc);
+}
+
 static int sev_handle_vmgexit_msr_protocol(struct vcpu_svm *svm)
 {
 	struct vmcb_control_area *control = &svm->vmcb->control;

@@ -4730,6 +4807,10 @@ int sev_handle_vmgexit(struct kvm_vcpu *vcpu)
 		}
 		ret = 1;
 		break;
+	case SVM_VMGEXIT_GET_APIC_IDS:
+		sev_get_apic_ids(svm);
+		ret = 1;
+		break;
 	case SVM_VMGEXIT_UNSUPPORTED_EVENT:
 		vcpu_unimpl(vcpu,
 			    "vmgexit: unsupported event - exit_info_1=%#llx, exit_info_2=%#llx\n",