On Thu, Jun 11, 2020 at 11:02:57AM +0800, Jason Wang wrote:
>
> On 2020/6/10 下午7:05, 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()
> fetch_buf()
> last_avail_idx++;
>
> Then we want do discard it:
> vhost_discard_avail_buf(1)
> unfetch_descs()
> last_avail_idx--;
> 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?
--
MST
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