From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by dpdk.org (Postfix) with ESMTP id 35ECC1B59F; Wed, 19 Dec 2018 16:50:43 +0100 (CET) Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 844268667D; Wed, 19 Dec 2018 15:50:42 +0000 (UTC) Received: from [10.36.112.26] (ovpn-112-26.ams2.redhat.com [10.36.112.26]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 932191001F3C; Wed, 19 Dec 2018 15:50:37 +0000 (UTC) To: "Michael S. Tsirkin" Cc: dev@dpdk.org, i.maximets@samsung.com, tiwei.bie@intel.com, zhihong.wang@intel.com, jasowang@redhat.com, stable@dpdk.org References: <20181219082113.24455-1-maxime.coquelin@redhat.com> <20181219082113.24455-2-maxime.coquelin@redhat.com> <20181219104410-mutt-send-email-mst@kernel.org> From: Maxime Coquelin Message-ID: <2dfb7495-7922-d3ad-bef9-7c48065e7396@redhat.com> Date: Wed, 19 Dec 2018 16:50:35 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1 MIME-Version: 1.0 In-Reply-To: <20181219104410-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Wed, 19 Dec 2018 15:50:42 +0000 (UTC) Subject: Re: [dpdk-dev] [PATCH v2 1/4] vhost: enforce avail index and desc read ordering X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 19 Dec 2018 15:50:43 -0000 On 12/19/18 4:47 PM, Michael S. Tsirkin wrote: > On Wed, Dec 19, 2018 at 09:21:10AM +0100, Maxime Coquelin wrote: >> A read barrier is required to ensure the ordering between >> available index and the descriptor reads is enforced. >> >> 1. read avail_head = avail->idx >> 2. read cur_idx = last_avail_idx >> if (cur_idx != avail_head) { >> 3. read idx = avail->ring[cur_idx] >> 4. read desc[idx] >> } >> >> There is a control dependency between step 1 and steps 3 & 4, >> 3 could be speculatively executed before 1, which could result >> in 'idx' to not being updated yet. >> >> Fixes: 4796ad63ba1f ("examples/vhost: import userspace vhost application") >> Cc: stable@dpdk.org >> >> Reported-by: Jason Wang >> Signed-off-by: Maxime Coquelin >> Acked-by: Ilya Maximets > > Acked-by: Michael S. Tsirkin > > BTW Ilya do you see a performance degradation from RMBs with these patches? > > >> --- >> lib/librte_vhost/virtio_net.c | 12 ++++++++++++ >> 1 file changed, 12 insertions(+) >> >> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c >> index 8c657a101..7f37bbbed 100644 >> --- a/lib/librte_vhost/virtio_net.c >> +++ b/lib/librte_vhost/virtio_net.c >> @@ -752,6 +752,12 @@ virtio_dev_rx_split(struct virtio_net *dev, struct vhost_virtqueue *vq, >> rte_prefetch0(&vq->avail->ring[vq->last_avail_idx & (vq->size - 1)]); >> avail_head = *((volatile uint16_t *)&vq->avail->idx); >> >> + /* >> + * The ordering between avail index and >> + * desc reads needs to be enforced. >> + */ >> + rte_smp_rmb(); >> + > > I'd guess you want to put the RMB before the prefetch. No? > Otherwise I think you are either stalling until that completes > or discard the prefetch ... Yeah, this is actually done in patch 3. Thanks, Maxime >> for (pkt_idx = 0; pkt_idx < count; pkt_idx++) { >> uint32_t pkt_len = pkts[pkt_idx]->pkt_len + dev->vhost_hlen; >> uint16_t nr_vec = 0; >> @@ -1334,6 +1340,12 @@ virtio_dev_tx_split(struct virtio_net *dev, struct vhost_virtqueue *vq, >> if (free_entries == 0) >> return 0; >> >> + /* >> + * The ordering between avail index and >> + * desc reads needs to be enforced. >> + */ >> + rte_smp_rmb(); >> + >> VHOST_LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__); >> >> count = RTE_MIN(count, MAX_PKT_BURST); >> -- >> 2.17.2