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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-dev] [2/5] vhost: enforce desc flags and content read ordering
Date: Thu, 6 Dec 2018 12:24:48 +0800	[thread overview]
Message-ID: <b3156ec2-8514-a4ae-e526-f5bb2f1ac435@redhat.com> (raw)
In-Reply-To: <16a4b2fd-c701-7822-3215-62e431e3f339@samsung.com>


On 2018/12/5 下午9:33, Ilya Maximets wrote:
> On 05.12.2018 12:49, Maxime Coquelin wrote:
>> A read barrier is required to ensure that the ordering between
>> descriptor's flags and content reads is enforced.
>>
>> Fixes: 2f3225a7d69b ("vhost: add vector filling support for packed ring")
>> 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 | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
>> index f11ebb54f..68b72e7a5 100644
>> --- a/lib/librte_vhost/virtio_net.c
>> +++ b/lib/librte_vhost/virtio_net.c
>> @@ -520,6 +520,12 @@ fill_vec_buf_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
>>   	if (unlikely(!desc_is_avail(&descs[avail_idx], wrap_counter)))
>>   		return -1;
>>   
>> +	/*
>> +	 * The ordering between desc flags and desc
>> +	 * content reads need to be enforced.
>> +	 */
>> +	rte_smp_rmb();
>> +
> Same here. 'desc_is_avail' reads and uses the flags. i.e.
> no way for reordering,
> Writes must be ordered on the virtio side by the write barrier.
> This means that if flags are updated (desc_is_avail() == true)
> than the whole descriptor already updated and the data is written.
> No need to have any read barriers here.


In fact , the sequence might be:


flag = read desc[avail_idx].flag [1]

if(flag is not avail) {

     read desc[avail_idx].id [2]

}


There's no data dependency but control dependency here, so 2 could be 
done before 1 without a rmb.

Thanks


>
>>   	*desc_count = 0;
>>   	*len = 0;
>>   
>>

  reply	other threads:[~2018-12-06  4:25 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-05  9:49 [dpdk-dev] [PATCH 0/5] vhost: add missing barriers, remove useless volatiles Maxime Coquelin
2018-12-05  9:49 ` [dpdk-dev] [PATCH 1/5] vhost: enforce avail index and desc read ordering Maxime Coquelin
     [not found]   ` <CGME20181205113041eucas1p1943b9c13af2fb5b736ba4906b59a9cd5@eucas1p1.samsung.com>
2018-12-05 11:30     ` [dpdk-dev] [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
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-dev] [PATCH 2/5] vhost: enforce desc flags and content " Maxime Coquelin
     [not found]   ` <CGME20181205133332eucas1p195b3864ed146403e314d7004d27be285@eucas1p1.samsung.com>
2018-12-05 13:33     ` [dpdk-dev] [2/5] " Ilya Maximets
2018-12-06  4:24       ` Jason Wang [this message]
2018-12-06 11:34         ` Ilya Maximets
2018-12-05  9:49 ` [dpdk-dev] [PATCH 3/5] vhost: prefetch descriptor after the read barrier Maxime Coquelin
2018-12-05  9:49 ` [dpdk-dev] [PATCH 4/5] vhost: remove useless prefetch for packed ring descriptor Maxime Coquelin
2018-12-05  9:49 ` [dpdk-dev] [PATCH 5/5] vhost: remove useless casts to volatile Maxime Coquelin
     [not found]   ` <CGME20181205135231eucas1p1c89281f6525a0fedab4a2fc0d2e21393@eucas1p1.samsung.com>
2018-12-05 13:52     ` [dpdk-dev] [5/5] " Ilya Maximets
2018-12-06 16:59       ` Maxime Coquelin
2018-12-07 11:16         ` Ilya Maximets

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