[dpdk-dev] [PATCH v3] vhost: fix wrong async completion of multi-seg packets

[patrick.fu@intel.com]

From: Patrick Fu <patrick.fu@intel.com>

In async enqueue copy, a packet could be split into multiple copy
segments. When polling the copy completion status, current async data
path assumes the async device callbacks are aware of the packet
boundary and return completed segments only if all segments belonging
to the same packet are done. Such assumption are not generic to common
async devices and may degrees the copy performance if async callbacks
have to implement it in software manner.

This patch adds tracking of the completed copy segments at vhost side.
If async copy device reports partial completion of a packets, only
vhost internal record is updated and vring status keeps unchanged
until remaining segments of the packet are also finished. The async
copy device is no longer necessary to care about the packet boundary.

Fixes: cd6760da1076 ("vhost: introduce async enqueue for split ring")

Signed-off-by: Patrick Fu <patrick.fu@intel.com>
---
v2:
 - fix an issue that can stuck async poll when packets buffer is full
v3:
 - revise commit message and title
 - rename a local variable to better reflect its usage

 lib/librte_vhost/vhost.h      |  3 +++
 lib/librte_vhost/virtio_net.c | 27 +++++++++++++++++----------
 2 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
index 8c01cee42..0f7212f88 100644
--- a/lib/librte_vhost/vhost.h
+++ b/lib/librte_vhost/vhost.h
@@ -46,6 +46,8 @@
 
 #define MAX_PKT_BURST 32
 
+#define ASYNC_MAX_POLL_SEG 255
+
 #define VHOST_MAX_ASYNC_IT (MAX_PKT_BURST * 2)
 #define VHOST_MAX_ASYNC_VEC (BUF_VECTOR_MAX * 2)
 
@@ -225,6 +227,7 @@ struct vhost_virtqueue {
 	uint64_t	*async_pending_info;
 	uint16_t	async_pkts_idx;
 	uint16_t	async_pkts_inflight_n;
+	uint16_t	async_last_seg_n;
 
 	/* vq async features */
 	bool		async_inorder;
diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 1d0be3dd4..635113cb0 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -1631,8 +1631,9 @@ uint16_t rte_vhost_poll_enqueue_completed(int vid, uint16_t queue_id,
 {
 	struct virtio_net *dev = get_device(vid);
 	struct vhost_virtqueue *vq;
-	uint16_t n_pkts_cpl, n_pkts_put = 0, n_descs = 0;
+	uint16_t n_segs_cpl, n_pkts_put = 0, n_descs = 0;
 	uint16_t start_idx, pkts_idx, vq_size;
+	uint16_t n_inflight;
 	uint64_t *async_pending_info;
 
 	VHOST_LOG_DATA(DEBUG, "(%d) %s\n", dev->vid, __func__);
@@ -1646,46 +1647,52 @@ uint16_t rte_vhost_poll_enqueue_completed(int vid, uint16_t queue_id,
 
 	rte_spinlock_lock(&vq->access_lock);
 
+	n_inflight = vq->async_pkts_inflight_n;
 	pkts_idx = vq->async_pkts_idx;
 	async_pending_info = vq->async_pending_info;
 	vq_size = vq->size;
 	start_idx = virtio_dev_rx_async_get_info_idx(pkts_idx,
 		vq_size, vq->async_pkts_inflight_n);
 
-	n_pkts_cpl =
-		vq->async_ops.check_completed_copies(vid, queue_id, 0, count);
+	n_segs_cpl = vq->async_ops.check_completed_copies(vid, queue_id,
+		0, ASYNC_MAX_POLL_SEG - vq->async_last_seg_n) +
+		vq->async_last_seg_n;
 
 	rte_smp_wmb();
 
-	while (likely(((start_idx + n_pkts_put) & (vq_size - 1)) != pkts_idx)) {
+	while (likely((n_pkts_put < count) && n_inflight)) {
 		uint64_t info = async_pending_info[
 			(start_idx + n_pkts_put) & (vq_size - 1)];
 		uint64_t n_segs;
 		n_pkts_put++;
+		n_inflight--;
 		n_descs += info & ASYNC_PENDING_INFO_N_MSK;
 		n_segs = info >> ASYNC_PENDING_INFO_N_SFT;
 
 		if (n_segs) {
-			if (!n_pkts_cpl || n_pkts_cpl < n_segs) {
+			if (unlikely(n_segs_cpl < n_segs)) {
 				n_pkts_put--;
+				n_inflight++;
 				n_descs -= info & ASYNC_PENDING_INFO_N_MSK;
-				if (n_pkts_cpl) {
+				if (n_segs_cpl) {
 					async_pending_info[
 						(start_idx + n_pkts_put) &
 						(vq_size - 1)] =
-					((n_segs - n_pkts_cpl) <<
+					((n_segs - n_segs_cpl) <<
 					 ASYNC_PENDING_INFO_N_SFT) |
 					(info & ASYNC_PENDING_INFO_N_MSK);
-					n_pkts_cpl = 0;
+					n_segs_cpl = 0;
 				}
 				break;
 			}
-			n_pkts_cpl -= n_segs;
+			n_segs_cpl -= n_segs;
 		}
 	}
 
+	vq->async_last_seg_n = n_segs_cpl;
+
 	if (n_pkts_put) {
-		vq->async_pkts_inflight_n -= n_pkts_put;
+		vq->async_pkts_inflight_n = n_inflight;
 		__atomic_add_fetch(&vq->used->idx, n_descs, __ATOMIC_RELEASE);
 
 		vhost_vring_call_split(dev, vq);
-- 
2.18.4