KVM: x86: hyper-v: Honor HV_POST_MESSAGES privilege bit

Hyper-V partition must possess 'HV_POST_MESSAGES' privilege to issue
HVCALL_POST_MESSAGE hypercalls.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210521095204.2161214-23-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/hyperv.c

@@ -2099,6 +2099,8 @@ static bool hv_check_hypercall_access(struct kvm_vcpu_hv *hv_vcpu, u16 code)
 	case HVCALL_NOTIFY_LONG_SPIN_WAIT:
 		return hv_vcpu->cpuid_cache.enlightenments_ebx &&
 			hv_vcpu->cpuid_cache.enlightenments_ebx != U32_MAX;
+	case HVCALL_POST_MESSAGE:
+		return hv_vcpu->cpuid_cache.features_ebx & HV_POST_MESSAGES;
 	default:
 		break;
 	}