in `lib/vhost/vhost.c`:

```
static inline int
vhost_enable_notify_split(struct virtio_net *dev,
                struct vhost_virtqueue *vq, int enable)
{
        if (vq->used == NULL)
                return -1;

        if (!(dev->features & (1ULL << VIRTIO_RING_F_EVENT_IDX))) {
                if (enable)
                        vq->used->flags &= ~VRING_USED_F_NO_NOTIFY;
                else
                        vq->used->flags |= VRING_USED_F_NO_NOTIFY;
        } else {
                if (enable)
                        vhost_avail_event(vq) = vq->last_avail_idx;
        }
        return 0;
}
```

when enable=0, this function do nothing when VIRTIO_RING_F_EVENT_IDX
negotiated, casued the guest do unnecessary kicks when sending & receiving.

it is a bug or feature?