[dpdk-stable] [PATCH] vhost: fix packed ring descriptor update in async enqueue

[Jiayu Hu]

For the packet using multiple descriptors, the flag of first
descriptor cannot be updated until finishing updating the flag
of remaining descriptors. However, if the packet's descriptor
index is wrapped, the first descriptor's flag is updated earlier
than the others.

Fixes: 873e8dad6f49 ("vhost: support packed ring in async datapath")
Cc: stable at dpdk.org

Signed-off-by: Jiayu Hu <jiayu.hu at intel.com>
---
 lib/vhost/virtio_net.c | 122 ++++++++++++++++++-----------------------
 1 file changed, 54 insertions(+), 68 deletions(-)

diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
index cef4bcf15c..b3d954aab4 100644
--- a/lib/vhost/virtio_net.c
+++ b/lib/vhost/virtio_net.c
@@ -1549,60 +1549,6 @@ virtio_dev_rx_async_submit_split(struct virtio_net *dev,
 	return pkt_idx;
 }
 
-static __rte_always_inline void
-vhost_update_used_packed(struct vhost_virtqueue *vq,
-			struct vring_used_elem_packed *shadow_ring,
-			uint16_t count)
-{
-	int i;
-	uint16_t used_idx = vq->last_used_idx;
-	uint16_t head_idx = vq->last_used_idx;
-	uint16_t head_flags = 0;
-
-	if (count == 0)
-		return;
-
-	/* Split loop in two to save memory barriers */
-	for (i = 0; i < count; i++) {
-		vq->desc_packed[used_idx].id = shadow_ring[i].id;
-		vq->desc_packed[used_idx].len = shadow_ring[i].len;
-
-		used_idx += shadow_ring[i].count;
-		if (used_idx >= vq->size)
-			used_idx -= vq->size;
-	}
-
-	/* The ordering for storing desc flags needs to be enforced. */
-	rte_atomic_thread_fence(__ATOMIC_RELEASE);
-
-	for (i = 0; i < count; i++) {
-		uint16_t flags;
-
-		if (vq->shadow_used_packed[i].len)
-			flags = VRING_DESC_F_WRITE;
-		else
-			flags = 0;
-
-		if (vq->used_wrap_counter) {
-			flags |= VRING_DESC_F_USED;
-			flags |= VRING_DESC_F_AVAIL;
-		} else {
-			flags &= ~VRING_DESC_F_USED;
-			flags &= ~VRING_DESC_F_AVAIL;
-		}
-
-		if (i > 0) {
-			vq->desc_packed[vq->last_used_idx].flags = flags;
-		} else {
-			head_idx = vq->last_used_idx;
-			head_flags = flags;
-		}
-
-		vq_inc_last_used_packed(vq, shadow_ring[i].count);
-	}
-
-	vq->desc_packed[head_idx].flags = head_flags;
-}
 
 static __rte_always_inline int
 vhost_enqueue_async_packed(struct virtio_net *dev,
@@ -1819,23 +1765,63 @@ write_back_completed_descs_packed(struct vhost_virtqueue *vq,
 				uint16_t n_buffers)
 {
 	struct vhost_async *async = vq->async;
-	uint16_t nr_left = n_buffers;
-	uint16_t from, to;
+	uint16_t from = async->last_buffer_idx_packed;
+	uint16_t used_idx = vq->last_used_idx;
+	uint16_t head_idx = vq->last_used_idx;
+	uint16_t head_flags = 0;
+	uint16_t i;
 
-	do {
-		from = async->last_buffer_idx_packed;
-		to = (from + nr_left) % vq->size;
-		if (to > from) {
-			vhost_update_used_packed(vq, async->buffers_packed + from, to - from);
-			async->last_buffer_idx_packed += nr_left;
-			nr_left = 0;
+	/* Split loop in two to save memory barriers */
+	for (i = 0; i < n_buffers; i++) {
+		vq->desc_packed[used_idx].id = async->buffers_packed[from].id;
+		vq->desc_packed[used_idx].len = async->buffers_packed[from].len;
+
+		used_idx += async->buffers_packed[from].count;
+		if (used_idx >= vq->size)
+			used_idx -= vq->size;
+
+		from++;
+		if (from >= vq->size)
+			from = 0;
+	}
+
+	/* The ordering for storing desc flags needs to be enforced. */
+	rte_atomic_thread_fence(__ATOMIC_RELEASE);
+
+	from = async->last_buffer_idx_packed;
+
+	for (i = 0; i < n_buffers; i++) {
+		uint16_t flags;
+
+		if (async->buffers_packed[from].len)
+			flags = VRING_DESC_F_WRITE;
+		else
+			flags = 0;
+
+		if (vq->used_wrap_counter) {
+			flags |= VRING_DESC_F_USED;
+			flags |= VRING_DESC_F_AVAIL;
 		} else {
-			vhost_update_used_packed(vq, async->buffers_packed + from,
-				vq->size - from);
-			async->last_buffer_idx_packed = 0;
-			nr_left -= vq->size - from;
+			flags &= ~VRING_DESC_F_USED;
+			flags &= ~VRING_DESC_F_AVAIL;
 		}
-	} while (nr_left > 0);
+
+		if (i > 0) {
+			vq->desc_packed[vq->last_used_idx].flags = flags;
+		} else {
+			head_idx = vq->last_used_idx;
+			head_flags = flags;
+		}
+
+		vq_inc_last_used_packed(vq, async->buffers_packed[from].count);
+
+		from++;
+		if (from == vq->size)
+			from = 0;
+	}
+
+	vq->desc_packed[head_idx].flags = head_flags;
+	async->last_buffer_idx_packed = from;
 }
 
 static __rte_always_inline uint16_t
-- 
2.25.1