> -----Original Message----- > From: Wang, YuanX <[email protected]> > Sent: Monday, June 20, 2022 5:19 PM > To: Xia, Chenbo <[email protected]>; Hu, Jiayu <[email protected]>; > David Marchand <[email protected]>; [email protected] > Cc: [email protected]; He, Xingguang <[email protected]>; [email protected]; > Ling, WeiX <[email protected]>; [email protected]; > [email protected]; [email protected]; Heinrich Kuhn > <[email protected]> > Subject: RE: [PATCH v2] examples/vhost: fix retry logic on eth rx path > > > > > -----Original Message----- > > From: Xia, Chenbo <[email protected]> > > Sent: Monday, June 20, 2022 5:10 PM > > To: Hu, Jiayu <[email protected]>; David Marchand > > <[email protected]>; [email protected] > > Cc: Wang, YuanX <[email protected]>; [email protected]; He, Xingguang > > <[email protected]>; [email protected]; Ling, WeiX > > <[email protected]>; [email protected]; [email protected]; > > [email protected]; Heinrich Kuhn <[email protected]> > > Subject: RE: [PATCH v2] examples/vhost: fix retry logic on eth rx path > > > > > -----Original Message----- > > > From: Hu, Jiayu <[email protected]> > > > Sent: Monday, June 20, 2022 4:59 PM > > > To: Xia, Chenbo <[email protected]>; David Marchand > > > <[email protected]>; [email protected] > > > Cc: Wang, YuanX <[email protected]>; [email protected]; He, Xingguang > > > <[email protected]>; [email protected]; Ling, WeiX > > > <[email protected]>; [email protected]; [email protected]; > > > [email protected]; Heinrich Kuhn <[email protected]> > > > Subject: RE: [PATCH v2] examples/vhost: fix retry logic on eth rx path > > > > > > > > > > > > > -----Original Message----- > > > > From: Xia, Chenbo <[email protected]> > > > > Sent: Monday, June 20, 2022 3:49 PM > > > > To: David Marchand <[email protected]>; > > > > [email protected] > > > > Cc: Wang, YuanX <[email protected]>; [email protected]; Hu, Jiayu > > > > <[email protected]>; He, Xingguang <[email protected]>; > > > > [email protected]; Ling, WeiX <[email protected]>; > > > > [email protected]; [email protected]; > > > > [email protected]; Heinrich Kuhn <[email protected]> > > > > Subject: RE: [PATCH v2] examples/vhost: fix retry logic on eth rx > > > > path > > > > > > > > > -----Original Message----- > > > > > From: David Marchand <[email protected]> > > > > > Sent: Monday, June 20, 2022 3:36 PM > > > > > To: Xia, Chenbo <[email protected]>; > > [email protected] > > > > > Cc: Wang, YuanX <[email protected]>; [email protected]; Hu, Jiayu > > > > > <[email protected]>; He, Xingguang <[email protected]>; > > > > > [email protected]; Ling, WeiX <[email protected]>; > > > > > [email protected]; [email protected]; > > > > > [email protected]; Heinrich Kuhn > > > > > <[email protected]> > > > > > Subject: Re: [PATCH v2] examples/vhost: fix retry logic on eth rx > > > > > path > > > > > > > > > > On Mon, Jun 20, 2022 at 5:20 AM Xia, Chenbo <[email protected]> > > > > 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 <[email protected]> > > 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 <[email protected]> > > Reviewed-by: Maxime Coquelin <[email protected]> > > Tested-by: Qian Xu <[email protected]> > > > > It was introduced by this commit.
Yes, of course both last_XXX_idx are introduced for split ring. I was saying the story seems to be: At first, vhost usage is very trivial, get available and set used, so one idx is enough. Then dequeue_zero_copy comes (later removed) and you may not update used after you get avail in one func call. Then last_avail_idx is introduced but rte_vhost_avail_entries() is not updated. And when we introduced packed ring, this API is also not updated. Correct me if I misunderstand the story. Thanks, Chenbo > > commit 7202b0a8240158b317665c20525f81d55f16f602 > Author: Huawei Xie <[email protected]> > Date: Thu Oct 9 02:54:51 2014 +0800 > > vhost: get available vring entries > > Signed-off-by: Huawei Xie <[email protected]> > Acked-by: Changchun Ouyang <[email protected]> > [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 > > > > > > >
