Subject: Re: [PATCH net-next v2 3/5] virtio_ring: add packed ring support

From: Tiwei Bie <[email-masked]>
Date: Thu, 8 Nov 2018 19:51:48 +0800

On Thu, Nov 08, 2018 at 04:18:25PM +0800, Jason Wang wrote:
> 
> On 2018/11/8 上午9:38, Tiwei Bie wrote:
> > > > +
> > > > +	if (vq->vq.num_free < descs_used) {
> > > > +		pr_debug("Can't add buf len %i - avail = %i\n",
> > > > +			 descs_used, vq->vq.num_free);
> > > > +		/* FIXME: for historical reasons, we force a notify here if
> > > > +		 * there are outgoing parts to the buffer.  Presumably the
> > > > +		 * host should service the ring ASAP. */
> > > I don't think we have a reason to do this for packed ring.
> > > No historical baggage there, right?
> > Based on the original commit log, it seems that the notify here
> > is just an "optimization". But I don't quite understand what does
> > the "the heuristics which KVM uses" refer to. If it's safe to drop
> > this in packed ring, I'd like to do it.
> 
> 
> According to the commit log, it seems like a workaround of lguest networking
> backend.

Do you know why removing this notify in Tx will break "the
heuristics which KVM uses"? Or what does "the heuristics
which KVM uses" refer to?

> I agree to drop it, we should not have such burden.
> 
> But we should notice that, with this removed, the compare between packed vs
> split is kind of unfair. Consider the removal of lguest support recently,
> maybe we can drop this for split ring as well?
> 
> Thanks
> 
> 
> > 
> > commit 44653eae1407f79dff6f52fcf594ae84cb165ec4
> > Author: Rusty Russell<[email-masked]>
> > Date:   Fri Jul 25 12:06:04 2008 -0500
> > 
> >      virtio: don't always force a notification when ring is full
> >      We force notification when the ring is full, even if the host has
> >      indicated it doesn't want to know.  This seemed like a good idea at
> >      the time: if we fill the transmit ring, we should tell the host
> >      immediately.
> >      Unfortunately this logic also applies to the receiving ring, which is
> >      refilled constantly.  We should introduce real notification thesholds
> >      to replace this logic.  Meanwhile, removing the logic altogether breaks
> >      the heuristics which KVM uses, so we use a hack: only notify if there are
> >      outgoing parts of the new buffer.
> >      Here are the number of exits with lguest's crappy network implementation:
> >      Before:
> >              network xmit 7859051 recv 236420
> >      After:
> >              network xmit 7858610 recv 118136
> >      Signed-off-by: Rusty Russell<[email-masked]>
> > 
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 72bf8bc09014..21d9a62767af 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -87,8 +87,11 @@ static int vring_add_buf(struct virtqueue *_vq,
> >   	if (vq->num_free < out + in) {
> >   		pr_debug("Can't add buf len %i - avail = %i\n",
> >   			 out + in, vq->num_free);
> > -		/* We notify*even if*  VRING_USED_F_NO_NOTIFY is set here. */
> > -		vq->notify(&vq->vq);
> > +		/* FIXME: for historical reasons, we force a notify here if
> > +		 * there are outgoing parts to the buffer.  Presumably the
> > +		 * host should service the ring ASAP. */
> > +		if (out)
> > +			vq->notify(&vq->vq);
> >   		END_USE(vq);
> >   		return -ENOSPC;
> >   	}
> > 
> >