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

From: Tiwei Bie <[email-masked]>
Date: Fri, 8 Jun 2018 16:32:13 +0800

On Thu, Jun 07, 2018 at 05:50:32PM +0800, Jason Wang wrote:
> 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.

Okay. I'll do it.

> 
> > +	/* 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?

I'll try it! Thanks for the suggestion! :)

> 
> > +
> > +	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?

Good catch! Yeah. It's not the best choice to check the
descriptor at last_used_idx. I'll try to fix this!

Best regards,
Tiwei Bie

> Thanks
> > --