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

[Jason Wang]

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: sta...@dpdk.org

Reported-by: Jason Wang <jasow...@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coque...@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);