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

["Hu, Jiayu" <jiayu.hu@intel.com>]

> -----Original Message-----
> From: Xia, Chenbo <chenbo.xia@intel.com>
> Sent: Monday, June 20, 2022 3:49 PM
> To: David Marchand <david.marchand@redhat.com>;
> maxime.coquelin@redhat.com
> Cc: Wang, YuanX <yuanx.wang@intel.com>; dev@dpdk.org; Hu, Jiayu
> <jiayu.hu@intel.com>; He, Xingguang <xingguang.he@intel.com>;
> stable@dpdk.org; Ling, WeiX <weix.ling@intel.com>; jin.liu@corigine.com;
> louis.peens@corigine.com; peng.zhang@corigine.com; Heinrich Kuhn
> <heinrich.kuhn@corigine.com>
> Subject: RE: [PATCH v2] examples/vhost: fix retry logic on eth rx path
> 
> > -----Original Message-----
> > From: David Marchand <david.marchand@redhat.com>
> > Sent: Monday, June 20, 2022 3:36 PM
> > To: Xia, Chenbo <chenbo.xia@intel.com>; maxime.coquelin@redhat.com
> > Cc: Wang, YuanX <yuanx.wang@intel.com>; dev@dpdk.org; Hu, Jiayu
> > <jiayu.hu@intel.com>; He, Xingguang <xingguang.he@intel.com>;
> > stable@dpdk.org; Ling, WeiX <weix.ling@intel.com>;
> > jin.liu@corigine.com; louis.peens@corigine.com;
> > peng.zhang@corigine.com; Heinrich Kuhn <heinrich.kuhn@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@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.

Same issue for rte_vhost_rx_queue_count(), and it is used in OVS.

But if look into the implementation of rte_vhost_avail_entries(), it calculates
the number of available descriptors by " vq->avail->idx - vq->last_used_idx".
This logic looks strange. Anyone knows the reason of this implementation?

Thanks,
Jiayu

> 
> Thanks,
> Chenbo
> 
> >
> >
> > --
> > David Marchand
>