KVM: arm64: Advertise KVM UID to guests via SMCCC

We can advertise ourselves to guests as KVM and provide a basic features
bitmap for discoverability of future hypervisor services.

Cc: Marc Zyngier <maz@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201209060932.212364-3-jianyong.wu@arm.com

arch/arm64/kvm/hypercalls.c

@@ -12,13 +12,13 @@
 int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
 {
 	u32 func_id = smccc_get_function(vcpu);
-	long val = SMCCC_RET_NOT_SUPPORTED;
+	u64 val[4] = {SMCCC_RET_NOT_SUPPORTED};
 	u32 feature;
 	gpa_t gpa;
 
 	switch (func_id) {
 	case ARM_SMCCC_VERSION_FUNC_ID:
-		val = ARM_SMCCC_VERSION_1_1;
+		val[0] = ARM_SMCCC_VERSION_1_1;
 		break;
 	case ARM_SMCCC_ARCH_FEATURES_FUNC_ID:
 		feature = smccc_get_arg1(vcpu);
@@ -28,10 +28,10 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
 			case SPECTRE_VULNERABLE:
 				break;
 			case SPECTRE_MITIGATED:
-				val = SMCCC_RET_SUCCESS;
+				val[0] = SMCCC_RET_SUCCESS;
 				break;
 			case SPECTRE_UNAFFECTED:
-				val = SMCCC_ARCH_WORKAROUND_RET_UNAFFECTED;
+				val[0] = SMCCC_ARCH_WORKAROUND_RET_UNAFFECTED;
 				break;
 			}
 			break;
@@ -54,22 +54,31 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
 					break;
 				fallthrough;
 			case SPECTRE_UNAFFECTED:
-				val = SMCCC_RET_NOT_REQUIRED;
+				val[0] = SMCCC_RET_NOT_REQUIRED;
 				break;
 			}
 			break;
 		case ARM_SMCCC_HV_PV_TIME_FEATURES:
-			val = SMCCC_RET_SUCCESS;
+			val[0] = SMCCC_RET_SUCCESS;
 			break;
 		}
 		break;
 	case ARM_SMCCC_HV_PV_TIME_FEATURES:
-		val = kvm_hypercall_pv_features(vcpu);
+		val[0] = kvm_hypercall_pv_features(vcpu);
 		break;
 	case ARM_SMCCC_HV_PV_TIME_ST:
 		gpa = kvm_init_stolen_time(vcpu);
 		if (gpa != GPA_INVALID)
-			val = gpa;
+			val[0] = gpa;
+		break;
+	case ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID:
+		val[0] = ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_0;
+		val[1] = ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_1;
+		val[2] = ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_2;
+		val[3] = ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_3;
+		break;
+	case ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID:
+		val[0] = BIT(ARM_SMCCC_KVM_FUNC_FEATURES);
 		break;
 	case ARM_SMCCC_TRNG_VERSION:
 	case ARM_SMCCC_TRNG_FEATURES:
@@ -81,6 +90,6 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
 		return kvm_psci_call(vcpu);
 	}
 
-	smccc_set_retval(vcpu, val, 0, 0, 0);
+	smccc_set_retval(vcpu, val[0], val[1], val[2], val[3]);
 	return 1;
 }