Re: [RFC v4 3/5] virtio_ring: add packed ring support

[Jason Wang]

On 2018å05æ16æ 22:33, Tiwei Bie wrote:
> On Wed, May 16, 2018 at 10:05:44PM +0800, Jason Wang wrote:
> > On 2018å05æ16æ 21:45, Tiwei Bie wrote:
> > > On Wed, May 16, 2018 at 08:51:43PM +0800, Jason Wang wrote:
> > > > On 2018å05æ16æ 20:39, Tiwei Bie wrote:
> > > > > On Wed, May 16, 2018 at 07:50:16PM +0800, Jason Wang wrote:
> > > > > > On 2018å05æ16æ 16:37, Tiwei Bie wrote:
> [...]
> > > > > > > +static void detach_buf_packed(struct vring_virtqueue *vq, unsigned int head,
> > > > > > > +			      unsigned int id, void **ctx)
> > > > > > > +{
> > > > > > > +	struct vring_packed_desc *desc;
> > > > > > > +	unsigned int i, j;
> > > > > > > +
> > > > > > > +	/* Clear data ptr. */
> > > > > > > +	vq->desc_state[id].data = NULL;
> > > > > > > +
> > > > > > > +	i = head;
> > > > > > > +
> > > > > > > +	for (j = 0; j < vq->desc_state[id].num; j++) {
> > > > > > > +		desc = &vq->vring_packed.desc[i];
> > > > > > > +		vring_unmap_one_packed(vq, desc);
> > > > > > As mentioned in previous discussion, this probably won't work for the case
> > > > > > of out of order completion since it depends on the information in the
> > > > > > descriptor ring. We probably need to extend ctx to record such information.
> > > > > Above code doesn't depend on the information in the descriptor
> > > > > ring. The vq->desc_state[] is the extended ctx.
> > > > >
> > > > > Best regards,
> > > > > Tiwei Bie
> > > > Yes, but desc is a pointer to descriptor ring I think so
> > > > vring_unmap_one_packed() still depends on the content of descriptor ring?
> > > I got your point now. I think it makes sense to reserve
> > > the bits of the addr field. Driver shouldn't try to get
> > > addrs from the descriptors when cleanup the descriptors
> > > no matter whether we support out-of-order or not.
> > Maybe I was wrong, but I remember spec mentioned something like this.
> You're right. Spec mentioned this. I was just repeating
> the spec to emphasize that it does make sense. :)
>
> > > But combining it with the out-of-order support, it will
> > > mean that the driver still needs to maintain a desc/ctx
> > > list that is very similar to the desc ring in the split
> > > ring. I'm not quite sure whether it's something we want.
> > > If it is true, I'll do it. So do you think we also want
> > > to maintain such a desc/ctx list for packed ring?
> > To make it work for OOO backends I think we need something like this
> > (hardware NIC drivers are usually have something like this).
> Which hardware NIC drivers have this?

It's quite common I think, e.g driver track e.g dma addr and page frag 
somewhere. e.g the ring->rx_info in mlx4 driver.

Thanks

> > Not for the patch, but it looks like having a OUT_OF_ORDER feature bit is
> > much more simpler to be started with.
> +1
>
> Best regards,
> Tiwei Bie