Re: [dpdk-dev] [PATCH v2 1/4] vhost: enforce avail index and desc read ordering

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

On 12/19/18 4:47 PM, Michael S. Tsirkin wrote:
> 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 <jasowang@redhat.com>
>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
>> Acked-by: Ilya Maximets <i.maximets@samsung.com>
> 
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> 
> 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 ...

Yeah, this is actually done in patch 3.

Thanks,
Maxime
>>   	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