From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
virtualization@lists.linux-foundation.org,
netdev@vger.kernel.org, eperezma@redhat.com
Subject: Re: [PATCH RFC v6 02/11] vhost: use batched get_vq_desc version
Date: Thu, 11 Jun 2020 05:06:38 -0400 [thread overview]
Message-ID: <20200611050416-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <76b14132-407a-48bf-c4d5-9d0b2c700bb0@redhat.com>
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
next prev parent reply other threads:[~2020-06-11 9:06 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-08 12:52 [PATCH RFC v6 00/11] vhost: ring format independence Michael S. Tsirkin
2020-06-08 12:52 ` [PATCH RFC v6 01/11] vhost: option to fetch descriptors through an independent struct Michael S. Tsirkin
2020-06-08 12:52 ` [PATCH RFC v6 02/11] vhost: use batched get_vq_desc version Michael S. Tsirkin
2020-06-10 3:14 ` Jason Wang
2020-06-10 11:05 ` Michael S. Tsirkin
2020-06-11 3:02 ` Jason Wang
2020-06-11 9:06 ` Michael S. Tsirkin [this message]
2020-06-15 2:43 ` Jason Wang
2020-06-10 11:24 ` Eugenio Perez Martin
2020-06-08 12:52 ` [PATCH RFC v6 03/11] vhost/net: pass net specific struct pointer Michael S. Tsirkin
2020-06-08 12:53 ` [PATCH RFC v6 04/11] vhost: reorder functions Michael S. Tsirkin
2020-06-08 12:53 ` [PATCH RFC v6 05/11] vhost: format-independent API for used buffers Michael S. Tsirkin
2020-06-08 12:53 ` [PATCH RFC v6 06/11] vhost/net: convert to new API: heads->bufs Michael S. Tsirkin
2020-06-08 12:53 ` [PATCH RFC v6 07/11] vhost/net: avoid iov length math Michael S. Tsirkin
2020-06-08 12:53 ` [PATCH RFC v6 08/11] vhost/test: convert to the buf API Michael S. Tsirkin
2020-06-08 12:53 ` [PATCH RFC v6 09/11] vhost/scsi: switch to buf APIs Michael S. Tsirkin
2020-06-08 12:53 ` [PATCH RFC v6 10/11] vhost/vsock: switch to the buf API Michael S. Tsirkin
2020-06-08 17:08 ` Stefano Garzarella
2020-06-08 12:53 ` [PATCH RFC v6 11/11] vhost: drop head based APIs Michael S. Tsirkin
2020-06-08 17:30 ` [PATCH RFC v6 00/11] vhost: ring format independence Stefano Garzarella
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