KVM: Hyper-V: do not do hypercall userspace exits if SynIC is disabled

If SynIC is disabled, there is nothing that userspace can do to
handle these exits; on the other hand, userspace probably will
not know about KVM_EXIT_HYPERV_HCALL and complain about it or
even exit.  Just prevent anything bad from happening by handling
the hypercall in KVM and returning an "invalid hypercall" code.

Fixes: 83326e43f27e9a8a501427a0060f8af519a39bb2
Cc: Andrey Smetanin <irqlevel@gmail.com>
Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/hyperv.c

@@ -1116,6 +1116,11 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 		break;
 	case HVCALL_POST_MESSAGE:
 	case HVCALL_SIGNAL_EVENT:
+		/* don't bother userspace if it has no way to handle it */
+		if (!vcpu_to_synic(vcpu)->active) {
+			res = HV_STATUS_INVALID_HYPERCALL_CODE;
+			break;
+		}
 		vcpu->run->exit_reason = KVM_EXIT_HYPERV;
 		vcpu->run->hyperv.type = KVM_EXIT_HYPERV_HCALL;
 		vcpu->run->hyperv.u.hcall.input = param;