From: Jason Wang <jasowang@redhat.com>
To: 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
Cc: stable@dpdk.org, "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [dpdk-stable] [1/5] vhost: enforce avail index and desc read ordering
Date: Thu, 6 Dec 2018 12:17:38 +0800 [thread overview]
Message-ID: <c9b62713-1cce-a93b-9f32-4e7cf95c1001@redhat.com> (raw)
In-Reply-To: <e98693c8-d667-d0f2-5123-3f9e26818e2b@samsung.com>
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.
>
>> 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 4:17 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 [this message]
2018-12-06 12:48 ` Ilya Maximets
2018-12-06 13:25 ` Jason Wang
2018-12-06 13:48 ` Michael S. Tsirkin
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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