Re: [PATCH RFC v6 02/11] vhost: use batched get_vq_desc version

[Jason Wang]

On 2020/6/11 äå5:06, Michael S. Tsirkin wrote:
> On Thu, Jun 11, 2020 at 11:02:57AM +0800, Jason Wang wrote:
> > On 2020/6/10 äå7:05, Michael S. Tsirkin wrote:
> > > > > +EXPORT_SYMBOL_GPL(vhost_get_vq_desc);
> > > > >     /* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
> > > > >     void vhost_discard_vq_desc(struct vhost_virtqueue *vq, int n)
> > > > >     {
> > > > > +	unfetch_descs(vq);
> > > > >     	vq->last_avail_idx -= n;
> > > > So unfetch_descs() has decreased last_avail_idx.
> > > > Can we fix this by letting unfetch_descs() return the number and then we can
> > > > do:
> > > >
> > > > int d = unfetch_descs(vq);
> > > > vq->last_avail_idx -= (n > d) ? n - d: 0;
> > > >
> > > > Thanks
> > > That's intentional I think - we need both.
> >
> > Yes, but:
> >
> >
> > > Unfetch_descs drops the descriptors in the cache that were
> > > *not returned to caller*  through get_vq_desc.
> > >
> > > vhost_discard_vq_desc drops the ones that were returned through get_vq_desc.
> > >
> > > Did I miss anything?
> > We could count some descriptors twice, consider the case e.g we only cache
> > on descriptor:
> >
> > fetch_descs()
> >  ÂÂÂ fetch_buf()
> >  ÂÂÂ ÂÂÂ last_avail_idx++;
> >
> > Then we want do discard it:
> > vhost_discard_avail_buf(1)
> >  ÂÂÂ unfetch_descs()
> >  ÂÂÂ ÂÂÂ last_avail_idx--;
> >  ÂÂÂ last_avail_idx -= 1;
> >
> > Thanks
>
> I don't think that happens. vhost_discard_avail_buf(1) is only called
> after get vhost_get_avail_buf. vhost_get_avail_buf increments
> first_desc.  unfetch_descs only counts from first_desc to ndescs.
>
> If I'm wrong, could you show values of first_desc and ndescs in this
> scenario?


You're right, fetch_descriptor could not be called directly from the 
device code.

Thanks


>