[dpdk-dev] [PATCH] virtio: fix idx in used ring retrieved only once

[Huawei Xie <huawei.xie@intel.com>]

In the following loop:
    while (vq->vq_used_cons_idx != vq->vq_ring.used->idx) {
            ...
    }
There is no external function call or any explict memory barrier
in the loop, the re-read of used->idx would be optimized and would
only be retrieved once.

use of voaltile normally should be prohibited, and access_once
is Linux kernel's style to handle this issue; Once we have that
macro in DPDK, we could change to that style.

virtio_recv_mergable_pkts might have the same issue, so will be fixed.

Fixes: 823ad647950a ("virtio: support multiple queues")
Fixes: 13ce5e7eb94f ("virtio: mergeable buffers")

Signed-off-by: Huawei Xie <huawei.xie@intel.com>
---
 drivers/net/virtio/virtio_ethdev.c      | 4 ++--
 drivers/net/virtio/virtio_ring.h        | 2 +-
 drivers/net/virtio/virtio_rxtx_simple.c | 3 +--
 3 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/drivers/net/virtio/virtio_ethdev.c b/drivers/net/virtio/virtio_ethdev.c
index ea7a48e..6a51948 100644
--- a/drivers/net/virtio/virtio_ethdev.c
+++ b/drivers/net/virtio/virtio_ethdev.c
@@ -219,12 +219,12 @@ virtio_send_command(struct virtnet_ctl *cvq, struct virtio_pmd_ctrl *ctrl,
 	virtqueue_notify(vq);
 
 	rte_rmb();
-	while (vq->vq_used_cons_idx == vq->vq_ring.used->idx) {
+	while (VIRTQUEUE_NUSED(vq) == 0) {
 		rte_rmb();
 		usleep(100);
 	}
 
-	while (vq->vq_used_cons_idx != vq->vq_ring.used->idx) {
+	while (VIRTQUEUE_NUSED(vq)) {
 		uint32_t idx, desc_idx, used_idx;
 		struct vring_used_elem *uep;
 
diff --git a/drivers/net/virtio/virtio_ring.h b/drivers/net/virtio/virtio_ring.h
index 447760a..fcecc16 100644
--- a/drivers/net/virtio/virtio_ring.h
+++ b/drivers/net/virtio/virtio_ring.h
@@ -79,7 +79,7 @@ struct vring_used_elem {
 
 struct vring_used {
 	uint16_t flags;
-	uint16_t idx;
+	volatile uint16_t idx;
 	struct vring_used_elem ring[0];
 };
 
diff --git a/drivers/net/virtio/virtio_rxtx_simple.c b/drivers/net/virtio/virtio_rxtx_simple.c
index 7b50119..a0ef8d2 100644
--- a/drivers/net/virtio/virtio_rxtx_simple.c
+++ b/drivers/net/virtio/virtio_rxtx_simple.c
@@ -184,8 +184,7 @@ virtio_recv_pkts_vec(void *rx_queue, struct rte_mbuf **rx_pkts,
 	if (unlikely(nb_pkts < RTE_VIRTIO_DESC_PER_LOOP))
 		return 0;
 
-	nb_used = *(volatile uint16_t *)&vq->vq_ring.used->idx -
-		vq->vq_used_cons_idx;
+	nb_used = vq->vq_ring.used->idx - vq->vq_used_cons_idx;
 
 	rte_compiler_barrier();
 
-- 
1.8.1.4