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 18EA51B54D; Wed, 19 Dec 2018 16:47:06 +0100 (CET) Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 654F6C068BD3; Wed, 19 Dec 2018 15:47:05 +0000 (UTC) Received: from redhat.com (ovpn-120-67.rdu2.redhat.com [10.10.120.67]) by smtp.corp.redhat.com (Postfix) with SMTP id 33D2D5D738; Wed, 19 Dec 2018 15:47:00 +0000 (UTC) Date: Wed, 19 Dec 2018 10:47:00 -0500 From: "Michael S. Tsirkin" To: Maxime Coquelin Cc: dev@dpdk.org, i.maximets@samsung.com, tiwei.bie@intel.com, zhihong.wang@intel.com, jasowang@redhat.com, stable@dpdk.org Message-ID: <20181219104410-mutt-send-email-mst@kernel.org> References: <20181219082113.24455-1-maxime.coquelin@redhat.com> <20181219082113.24455-2-maxime.coquelin@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181219082113.24455-2-maxime.coquelin@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Wed, 19 Dec 2018 15:47:05 +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 15:47:06 -0000 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 ... > 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