Re: [RFC v6 4/5] virtio_ring: add event idx support in packed ring

Jason Wang
2018-06-07T09:50:53Z

On 2018年06月05日 15:40, Tiwei Bie wrote:
> static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
> + u16 bufs, used_idx, wrap_counter;
>
> START_USE(vq);
>
> /* We optimistically turn back on interrupts, then check if there was
> * more to do. */
> + /* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
> + * either clear the flags bit or point the event index at the next
> + * entry. Always update the event index to keep code simple. */
> +

Maybe for packed ring, it's time to treat event index separately to
avoid a virtio_wmb() for event idx is off.

> + /* TODO: tune this threshold */
> + if (vq->next_avail_idx < vq->last_used_idx)
> + bufs = (vq->vring_packed.num + vq->next_avail_idx -
> + vq->last_used_idx) * 3 / 4;
> + else
> + bufs = (vq->next_avail_idx - vq->last_used_idx) * 3 / 4;

vq->next_avail-idx could be equal to vq->last_usd_idx when the ring is
full. Though virito-net is the only user now and it can guarantee this
won't happen. But consider this is a core API, we should make sure it
can work for any cases.

It looks to me that bufs is just vq->vring_packed.num - vq->num_free?

> +
> + wrap_counter = vq->used_wrap_counter;
> +
> + used_idx = vq->last_used_idx + bufs;
> + if (used_idx >= vq->vring_packed.num) {
> + used_idx -= vq->vring_packed.num;
> + wrap_counter ^= 1;
> + }
> +
> + vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
> + used_idx | (wrap_counter << 15));
>
> if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> - vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
> + /* We need to update event offset and event wrap
> + * counter first before updating event flags. */
> + virtio_wmb(vq->weak_barriers);
> + vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC :
> + VRING_EVENT_F_ENABLE;
> vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> vq->event_flags_shadow);
> - /* We need to enable interrupts first before re-checking
> - * for more used buffers. */
> - virtio_mb(vq->weak_barriers);
> }
>
> + /* We need to update event suppression structure first
> + * before re-checking for more used buffers. */
> + virtio_mb(vq->weak_barriers);
> +
> if (more_used_packed(vq)) {
> END_USE(vq);
> return false;

I think what we need to to make sure the descriptor used_idx is used?
Otherwise we may stop and restart qdisc too frequently?

Thanks

> --