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

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Date:   Wed, 16 May 2018 21:45:50 +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 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:
> > [...]
> > > >    struct vring_virtqueue {
> > > > @@ -116,6 +117,9 @@ struct vring_virtqueue {
> > > >    			/* Last written value to driver->flags in
> > > >    			 * guest byte order. */
> > > >    			u16 event_flags_shadow;
> > > > +
> > > > +			/* ID allocation. */
> > > > +			struct idr buffer_id;
> > > I'm not sure idr is fit for the performance critical case here. Need to
> > > measure its performance impact, especially if we have few unused slots.
> > I'm also not sure.. But fortunately, it should be quite easy
> > to replace it with something else without changing other code.
> > If it will really hurt the performance, I'll change it.
> 
> We may want to do some benchmarking/profiling to see.

Yeah!

> 
> > 
> > > >    		};
> > > >    	};
> > [...]
> > > > +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.

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?

Best regards,
Tiwei Bie