[PATCH v2] examples/vhost: fix retry logic on eth rx path

[Xia, Chenbo]

> -----Original Message-----
> From: David Marchand <david.marchand at redhat.com>
> Sent: Monday, June 20, 2022 3:36 PM
> To: Xia, Chenbo <chenbo.xia at intel.com>; maxime.coquelin at redhat.com
> Cc: Wang, YuanX <yuanx.wang at intel.com>; dev at dpdk.org; Hu, Jiayu
> <jiayu.hu at intel.com>; He, Xingguang <xingguang.he at intel.com>;
> stable at dpdk.org; Ling, WeiX <weix.ling at intel.com>; jin.liu at corigine.com;
> louis.peens at corigine.com; peng.zhang at corigine.com; Heinrich Kuhn
> <heinrich.kuhn at corigine.com>
> Subject: Re: [PATCH v2] examples/vhost: fix retry logic on eth rx path
> 
> On Mon, Jun 20, 2022 at 5:20 AM Xia, Chenbo <chenbo.xia at intel.com> wrote:
> > > drain_eth_rx() uses rte_vhost_avail_entries() to calculate
> > > the available entries to determine if a retry is required.
> > > However, this function only works with split rings, and
> > > calculating packed rings will return the wrong value and cause
> > > unnecessary retries resulting in a significant performance penalty.
> > >
> > > This patch fix that by using the difference between tx/rx burst
> > > as the retry condition.
> >
> > Does it mean we don't need the API rte_vhost_avail_entries() anymore?
> >
> > Jiayu/Yuan/Maxime, what do you think?
> 
> FWIW, I still see a user:
> virtio-forwarder/virtio_vhostuser.c:     * This check ensures that we
> do not call rte_vhost_avail_entries
> virtio-forwarder/virtio_worker.c:        try_rcv =
> rte_vhost_avail_entries((int)relay->vio.vio_dev,
> 
> Cc'd a few Corigine guys.

Thanks David for this info! Then I guess only split ring is used in this use case?
If we want to keep it, then this API should also be fixed as it's not supporting
packed ring.

Thanks,
Chenbo

> 
> 
> --
> David Marchand