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

[Jason Wang <jasowang@redhat.com>]

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

> --