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: 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 | 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;
