kvm: don't try to register to KVM_FAST_MMIO_BUS for non mmio eventfd

We only want zero length mmio eventfd to be registered on
KVM_FAST_MMIO_BUS. So check this explicitly when arg->len is zero to
make sure this.

Cc: stable@vger.kernel.org
Cc: Gleb Natapov <gleb@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/eventfd.c

@@ -846,7 +846,7 @@ kvm_assign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
 	/* When length is ignored, MMIO is also put on a separate bus, for
 	 * faster lookups.
 	 */
-	if (!args->len && !(args->flags & KVM_IOEVENTFD_FLAG_PIO)) {
+	if (!args->len && bus_idx == KVM_MMIO_BUS) {
 		ret = kvm_io_bus_register_dev(kvm, KVM_FAST_MMIO_BUS,
 					      p->addr, 0, &p->dev);
 		if (ret < 0)
@@ -901,7 +901,7 @@ kvm_deassign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
 			continue;
 
 		kvm_io_bus_unregister_dev(kvm, bus_idx, &p->dev);
-		if (!p->length) {
+		if (!p->length && p->bus_idx == KVM_MMIO_BUS) {
 			kvm_io_bus_unregister_dev(kvm, KVM_FAST_MMIO_BUS,
 						  &p->dev);
 		}