hello ,when I read virtio source code ,I was confused about when back-end
notify front-end。
In function virtio_should_notify，if vq->signalled_used_valid == false
or vring_need_event(vring_get_used_event(vq), new, old) is true，notiry will
occur。but I can not understand when vq->signalled_used_valid will be false.
in virtqueue_flush,
if (unlikely((int16_t)(new - vq->signalled_used) < (uint16_t)(new - old)))
vq->signalled_used_valid = false;
but according code I think vq->signalled_used always equal
to vq->used_idx,that is vq->signalled_used_valid will always be true.
please help me understand it,thanks!
