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

[Jason Wang <jasowang@redhat.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);
>>