Re: [dpdk-dev] [1/5] vhost: enforce avail index and desc read ordering

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 12/11/18 11:38 AM, 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(+)
>>
> 
> I'd like to have a bit more details about a bad scenario in a commit
> message because it's not an obvious change at a first glance.

Sure, I'll rework the commit message in the v2.

> Otherwise,
> Acked-by: Ilya Maximets <i.maximets@samsung.com>

Thanks,
Maxime

> 
>> 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();
>> +
>>   	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();
>> +
>>   	VHOST_LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__);
>>   
>>   	count = RTE_MIN(count, MAX_PKT_BURST);
>>