> -----Original Message-----
> From: Xia, Chenbo <chenbo....@intel.com>
> Sent: Monday, June 20, 2022 5:10 PM
> To: Hu, Jiayu <jiayu...@intel.com>; David Marchand
> <david.march...@redhat.com>; maxime.coque...@redhat.com
> Cc: Wang, YuanX <yuanx.w...@intel.com>; dev@dpdk.org; He, Xingguang
> <xingguang...@intel.com>; sta...@dpdk.org; Ling, WeiX
> <weix.l...@intel.com>; jin....@corigine.com; louis.pe...@corigine.com;
> peng.zh...@corigine.com; Heinrich Kuhn <heinrich.k...@corigine.com>
> Subject: RE: [PATCH v2] examples/vhost: fix retry logic on eth rx path
>
> > -----Original Message-----
> > From: Hu, Jiayu <jiayu...@intel.com>
> > Sent: Monday, June 20, 2022 4:59 PM
> > To: Xia, Chenbo <chenbo....@intel.com>; David Marchand
> > <david.march...@redhat.com>; maxime.coque...@redhat.com
> > Cc: Wang, YuanX <yuanx.w...@intel.com>; dev@dpdk.org; He, Xingguang
> > <xingguang...@intel.com>; sta...@dpdk.org; Ling, WeiX
> > <weix.l...@intel.com>; jin....@corigine.com; louis.pe...@corigine.com;
> > peng.zh...@corigine.com; Heinrich Kuhn <heinrich.k...@corigine.com>
> > Subject: RE: [PATCH v2] examples/vhost: fix retry logic on eth rx path
> >
> >
> >
> > > -----Original Message-----
> > > From: Xia, Chenbo <chenbo....@intel.com>
> > > Sent: Monday, June 20, 2022 3:49 PM
> > > To: David Marchand <david.march...@redhat.com>;
> > > maxime.coque...@redhat.com
> > > Cc: Wang, YuanX <yuanx.w...@intel.com>; dev@dpdk.org; Hu, Jiayu
> > > <jiayu...@intel.com>; He, Xingguang <xingguang...@intel.com>;
> > > sta...@dpdk.org; Ling, WeiX <weix.l...@intel.com>;
> > > jin....@corigine.com; louis.pe...@corigine.com;
> > > peng.zh...@corigine.com; Heinrich Kuhn <heinrich.k...@corigine.com>
> > > Subject: RE: [PATCH v2] examples/vhost: fix retry logic on eth rx
> > > path
> > >
> > > > -----Original Message-----
> > > > From: David Marchand <david.march...@redhat.com>
> > > > Sent: Monday, June 20, 2022 3:36 PM
> > > > To: Xia, Chenbo <chenbo....@intel.com>;
> maxime.coque...@redhat.com
> > > > Cc: Wang, YuanX <yuanx.w...@intel.com>; dev@dpdk.org; Hu, Jiayu
> > > > <jiayu...@intel.com>; He, Xingguang <xingguang...@intel.com>;
> > > > sta...@dpdk.org; Ling, WeiX <weix.l...@intel.com>;
> > > > jin....@corigine.com; louis.pe...@corigine.com;
> > > > peng.zh...@corigine.com; Heinrich Kuhn
> > > > <heinrich.k...@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....@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?
>
> I was not in the history, but as I checked the git log. Seems it's because in
> this
> commit, this API was not improved (This API is introduced before the
> commit).
>
> commit f6be82d7259ee35683721092d61283d99a47aff1
> Author: Yuanhan Liu <yuanhan....@linux.intel.com>
> Date: Sun Oct 9 15:27:56 2016 +0800
>
> vhost: introduce last available index for dequeue
>
> So far, we retrieve both the used ring and avail ring idx by the var
> last_used_idx; it won't be a problem because the used ring is updated
> immediately after those avail entries are consumed.
>
> But that's not true when dequeue zero copy is enabled, that used ring is
> updated only when the mbuf is consumed. Thus, we need use another var
> to
> note the last avail ring idx we have consumed.
>
> Therefore, last_avail_idx is introduced.
>
> Signed-off-by: Yuanhan Liu <yuanhan....@linux.intel.com>
> Reviewed-by: Maxime Coquelin <maxime.coque...@redhat.com>
> Tested-by: Qian Xu <qian.q...@intel.com>
>
It was introduced by this commit.
commit 7202b0a8240158b317665c20525f81d55f16f602
Author: Huawei Xie <huawei....@intel.com>
Date: Thu Oct 9 02:54:51 2014 +0800
vhost: get available vring entries
Signed-off-by: Huawei Xie <huawei....@intel.com>
Acked-by: Changchun Ouyang <changchun.ouy...@intel.com>
[Thomas: split patch]
Check for the define of VQ, it is obvious for split ring.
struct vhost_virtqueue {
struct vring_desc *desc; /**< Virtqueue
descriptor ring. */
struct vring_avail *avail; /**< Virtqueue
available ring. */
struct vring_used *used; /**< Virtqueue used
ring. */
uint32_t size; /**< Size of descriptor
ring. */
int backend; /**< Backend value to
determine if device should started/stopped. */
uint16_t vhost_hlen; /**< Vhost header
length (varies depending on RX merge buffers. */
volatile uint16_t last_used_idx; /**< Last index used on
the available ring */
volatile uint16_t last_used_idx_res; /**< Used for multiple
devices reserving buffers. */
#define VIRTIO_INVALID_EVENTFD (-1)
#define VIRTIO_UNINITIALIZED_EVENTFD (-2)
int callfd; /**< Used to notify the
guest (trigger interrupt). */
int kickfd; /**< Currently unused
as polling mode is enabled. */
int enabled;
uint64_t log_guest_addr; /**< Physical address
of used ring, for logging */
uint64_t reserved[15]; /**< Reserve some
spaces for future extension. */
struct buf_vector buf_vec[BUF_VECTOR_MAX]; /**< for
scatter RX. */
} __rte_cache_aligned;
Thanks,
Yuan
> Thanks,
> Chenbo
>
> >
> > Thanks,
> > Jiayu
> >
> > >
> > > Thanks,
> > > Chenbo
> > >
> > > >
> > > >
> > > > --
> > > > David Marchand
> > >
> >
