From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: Ilya Maximets <i.maximets@samsung.com>,
Maxime Coquelin <maxime.coquelin@redhat.com>,
dev@dpdk.org, jfreimann@redhat.com, tiwei.bie@intel.com,
zhihong.wang@intel.com, stable@dpdk.org
Subject: Re: [dpdk-stable] [1/5] vhost: enforce avail index and desc read ordering
Date: Thu, 6 Dec 2018 08:48:14 -0500 [thread overview]
Message-ID: <20181206083048-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <c9b62713-1cce-a93b-9f32-4e7cf95c1001@redhat.com>
On Thu, Dec 06, 2018 at 12:17:38PM +0800, Jason Wang wrote:
>
> On 2018/12/5 下午7:30, Ilya Maximets wrote:
> > On 05.12.2018 12:49, Maxime Coquelin wrote:
> > > A read barrier is required to ensure the ordering between
> > > available index and the descriptor reads is enforced.
> > >
> > > Fixes: 4796ad63ba1f ("examples/vhost: import userspace vhost application")
> > > Cc: stable@dpdk.org
> > >
> > > Reported-by: Jason Wang <jasowang@redhat.com>
> > > Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> > > ---
> > > 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 5e1a1a727..f11ebb54f 100644
> > > --- a/lib/librte_vhost/virtio_net.c
> > > +++ b/lib/librte_vhost/virtio_net.c
> > > @@ -791,6 +791,12 @@ 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();
> > > +
> > Hmm. This looks weird to me.
> > Could you please describe the bad scenario here? (It'll be good to have it
> > in commit message too)
> >
> > As I understand, you're enforcing the read of avail->idx to happen before
> > reading the avail->ring[avail_idx]. Is it correct?
> >
> > But we have following code sequence:
> >
> > 1. read avail->idx (avail_head).
> > 2. check that last_avail_idx != avail_head.
> > 3. read from the ring using last_avail_idx.
> >
> > So, there is a strict dependency between all 3 steps and the memory
> > transaction will be finished at the step #2 in any case. There is no
> > way to read the ring before reading the avail->idx.
> >
> > Am I missing something?
>
>
> Nope, I kind of get what you meaning now. And even if we will
>
> 4. read descriptor from descriptor ring using the id read from 3
>
> 5. read descriptor content according to the address from 4
>
> They still have dependent memory access. So there's no need for rmb.
I am pretty sure on some architectures there is a need for a barrier
here. This is an execution dependency since avail_head is not used as an
index. And reads can be speculated. So the read from the ring can be
speculated and execute before the read of avail_head and the check.
However SMP rmb is/should be free on x86. So unless someone on this
thread is actually testing performance on non-x86, you are both wasting
cycles discussing removal of nop macros and also risk pushing untested
software on users.
>
> >
> > > 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;
> > > @@ -1373,6 +1379,12 @@ 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();
> > > +
> > This one is strange too.
> >
> > free_entries = *((volatile uint16_t *)&vq->avail->idx) -
> > vq->last_avail_idx;
> > if (free_entries == 0)
> > return 0;
> >
> > The code reads the value of avail->idx and uses the value on the next
> > line even with any compiler optimizations. There is no way for CPU to
> > postpone the actual read.
>
>
> Yes.
>
> Thanks
>
>
> >
> > > VHOST_LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__);
> > > count = RTE_MIN(count, MAX_PKT_BURST);
> > >
next prev parent reply other threads:[~2018-12-06 13:48 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20181205094957.1938-1-maxime.coquelin@redhat.com>
2018-12-05 9:49 ` [dpdk-stable] [PATCH 1/5] " Maxime Coquelin
[not found] ` <CGME20181205113041eucas1p1943b9c13af2fb5b736ba4906b59a9cd5@eucas1p1.samsung.com>
2018-12-05 11:30 ` [dpdk-stable] [1/5] " Ilya Maximets
2018-12-06 4:17 ` Jason Wang
2018-12-06 12:48 ` Ilya Maximets
2018-12-06 13:25 ` Jason Wang
2018-12-06 13:48 ` Michael S. Tsirkin [this message]
2018-12-07 14:58 ` Ilya Maximets
2018-12-07 15:44 ` Michael S. Tsirkin
[not found] ` <CGME20181211103848eucas1p10c270ca8997fea8a2f55c2d94d02baea@eucas1p1.samsung.com>
2018-12-11 10:38 ` Ilya Maximets
2018-12-11 14:46 ` Maxime Coquelin
2018-12-05 9:49 ` [dpdk-stable] [PATCH 2/5] vhost: enforce desc flags and content " Maxime Coquelin
[not found] ` <CGME20181205133332eucas1p195b3864ed146403e314d7004d27be285@eucas1p1.samsung.com>
2018-12-05 13:33 ` [dpdk-stable] [2/5] " Ilya Maximets
2018-12-06 4:24 ` Jason Wang
2018-12-06 11:34 ` Ilya Maximets
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