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

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Message-ID: <20180516143332.GA1957@debian>
Date:   Wed, 16 May 2018 22:33:32 +0800
From:   Tiwei Bie <tiwei.bie@...el.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     mst@...hat.com, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        wexu@...hat.com, jfreimann@...hat.com
Subject: Re: [RFC v4 3/5] virtio_ring: add packed ring support

On Wed, May 16, 2018 at 10:05:44PM +0800, Jason Wang wrote:
> On 2018年05月16日 21:45, Tiwei Bie wrote:
> > On Wed, May 16, 2018 at 08:51:43PM +0800, Jason Wang wrote:
> > > On 2018年05月16日 20:39, Tiwei Bie wrote:
> > > > On Wed, May 16, 2018 at 07:50:16PM +0800, Jason Wang wrote:
> > > > > On 2018年05月16日 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?

> 
> 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