linux-kernel - Re: [PATCH RFC v7 03/14] vhost: use batched get_vq

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Date:   Wed, 10 Jun 2020 11:13:36 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Eugenio Perez Martin <eperezma@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm list <kvm@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH RFC v7 03/14] vhost: use batched get_vq_desc version

On Wed, Jun 10, 2020 at 02:37:50PM +0200, Eugenio Perez Martin wrote:
> > +/* This function returns a value > 0 if a descriptor was found, or 0 if none were found.
> > + * A negative code is returned on error. */
> > +static int fetch_descs(struct vhost_virtqueue *vq)
> > +{
> > +       int ret;
> > +
> > +       if (unlikely(vq->first_desc >= vq->ndescs)) {
> > +               vq->first_desc = 0;
> > +               vq->ndescs = 0;
> > +       }
> > +
> > +       if (vq->ndescs)
> > +               return 1;
> > +
> > +       for (ret = 1;
> > +            ret > 0 && vq->ndescs <= vhost_vq_num_batch_descs(vq);
> > +            ret = fetch_buf(vq))
> > +               ;
> 
> (Expanding comment in V6):
> 
> We get an infinite loop this way:
> * vq->ndescs == 0, so we call fetch_buf() here
> * fetch_buf gets less than vhost_vq_num_batch_descs(vq); descriptors. ret = 1
> * This loop calls again fetch_buf, but vq->ndescs > 0 (and avail_vq ==
> last_avail_vq), so it just return 1

That's what
	 [PATCH RFC v7 08/14] fixup! vhost: use batched get_vq_desc version
is supposed to fix.

-- 
MST