[PATCH v2] vhost: support CPU copy for small packets

[Wenwu Ma <wenwux.ma@intel.com>]

Offloading small packets to DMA degrades throughput 10%~20%,
and this is because DMA offloading is not free and DMA is not
good at processing small packets. In addition, control plane
packets are usually small, and assign those packets to DMA will
significantly increase latency, which may cause timeout like
TCP handshake packets. Therefore, this patch use CPU to perform
small copies in vhost.

Signed-off-by: Wenwu Ma <wenwux.ma@intel.com>
---
v2:
* fix CI build error
---
 lib/vhost/vhost.h      |  6 ++-
 lib/vhost/virtio_net.c | 87 +++++++++++++++++++++++++++---------------
 2 files changed, 61 insertions(+), 32 deletions(-)

diff --git a/lib/vhost/vhost.h b/lib/vhost/vhost.h
index 40fac3b7c6..b4523175a8 100644
--- a/lib/vhost/vhost.h
+++ b/lib/vhost/vhost.h
@@ -142,8 +142,10 @@ struct virtqueue_stats {
  * iovec
  */
 struct vhost_iovec {
-	void *src_addr;
-	void *dst_addr;
+	void *src_iov_addr;
+	void *dst_iov_addr;
+	void *src_virt_addr;
+	void *dst_virt_addr;
 	size_t len;
 };
 
diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
index 35fa4670fd..5ab30facc1 100644
--- a/lib/vhost/virtio_net.c
+++ b/lib/vhost/virtio_net.c
@@ -26,6 +26,8 @@
 
 #define MAX_BATCH_LEN 256
 
+#define CPU_COPY_THRESHOLD_LEN 256
+
 static __rte_always_inline uint16_t
 async_poll_dequeue_completed(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		struct rte_mbuf **pkts, uint16_t count, int16_t dma_id,
@@ -114,29 +116,36 @@ vhost_async_dma_transfer_one(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	int copy_idx = 0;
 	uint32_t nr_segs = pkt->nr_segs;
 	uint16_t i;
+	bool cpu_copy = true;
 
 	if (rte_dma_burst_capacity(dma_id, vchan_id) < nr_segs)
 		return -1;
 
 	for (i = 0; i < nr_segs; i++) {
-		copy_idx = rte_dma_copy(dma_id, vchan_id, (rte_iova_t)iov[i].src_addr,
-				(rte_iova_t)iov[i].dst_addr, iov[i].len, RTE_DMA_OP_FLAG_LLC);
-		/**
-		 * Since all memory is pinned and DMA vChannel
-		 * ring has enough space, failure should be a
-		 * rare case. If failure happens, it means DMA
-		 * device encounters serious errors; in this
-		 * case, please stop async data-path and check
-		 * what has happened to DMA device.
-		 */
-		if (unlikely(copy_idx < 0)) {
-			if (!vhost_async_dma_copy_log) {
-				VHOST_LOG_DATA(dev->ifname, ERR,
-					"DMA copy failed for channel %d:%u\n",
-					dma_id, vchan_id);
-				vhost_async_dma_copy_log = true;
+		if (iov[i].len > CPU_COPY_THRESHOLD_LEN) {
+			copy_idx = rte_dma_copy(dma_id, vchan_id, (rte_iova_t)iov[i].src_iov_addr,
+					(rte_iova_t)iov[i].dst_iov_addr,
+					iov[i].len, RTE_DMA_OP_FLAG_LLC);
+			/**
+			 * Since all memory is pinned and DMA vChannel
+			 * ring has enough space, failure should be a
+			 * rare case. If failure happens, it means DMA
+			 * device encounters serious errors; in this
+			 * case, please stop async data-path and check
+			 * what has happened to DMA device.
+			 */
+			if (unlikely(copy_idx < 0)) {
+				if (!vhost_async_dma_copy_log) {
+					VHOST_LOG_DATA(dev->ifname, ERR,
+						"DMA copy failed for channel %d:%u\n",
+						dma_id, vchan_id);
+					vhost_async_dma_copy_log = true;
+				}
+				return -1;
 			}
-			return -1;
+			cpu_copy = false;
+		} else {
+			rte_memcpy(iov[i].dst_virt_addr, iov[i].src_virt_addr, iov[i].len);
 		}
 	}
 
@@ -144,7 +153,13 @@ vhost_async_dma_transfer_one(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	 * Only store packet completion flag address in the last copy's
 	 * slot, and other slots are set to NULL.
 	 */
-	dma_info->pkts_cmpl_flag_addr[copy_idx & ring_mask] = &vq->async->pkts_cmpl_flag[flag_idx];
+	if (cpu_copy == false) {
+		dma_info->pkts_cmpl_flag_addr[copy_idx & ring_mask] =
+			&vq->async->pkts_cmpl_flag[flag_idx];
+	} else {
+		vq->async->pkts_cmpl_flag[flag_idx] = true;
+		nr_segs = 0;
+	}
 
 	return nr_segs;
 }
@@ -1008,7 +1023,7 @@ async_iter_initialize(struct virtio_net *dev, struct vhost_async *async)
 
 static __rte_always_inline int
 async_iter_add_iovec(struct virtio_net *dev, struct vhost_async *async,
-		void *src, void *dst, size_t len)
+		void *src_iova, void *dst_iova, void *src_addr, void *dst_addr, size_t len)
 {
 	struct vhost_iov_iter *iter;
 	struct vhost_iovec *iovec;
@@ -1027,8 +1042,10 @@ async_iter_add_iovec(struct virtio_net *dev, struct vhost_async *async,
 	iter = async->iov_iter + async->iter_idx;
 	iovec = async->iovec + async->iovec_idx;
 
-	iovec->src_addr = src;
-	iovec->dst_addr = dst;
+	iovec->src_iov_addr = src_iova;
+	iovec->dst_iov_addr = dst_iova;
+	iovec->src_virt_addr = src_addr;
+	iovec->dst_virt_addr = dst_addr;
 	iovec->len = len;
 
 	iter->nr_segs++;
@@ -1064,12 +1081,13 @@ async_iter_reset(struct vhost_async *async)
 static __rte_always_inline int
 async_fill_seg(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		struct rte_mbuf *m, uint32_t mbuf_offset,
-		uint64_t buf_iova, uint32_t cpy_len, bool to_desc)
+		uint64_t buf_iova, uint64_t buf_addr, uint32_t cpy_len, bool to_desc)
 {
 	struct vhost_async *async = vq->async;
 	uint64_t mapped_len;
 	uint32_t buf_offset = 0;
-	void *src, *dst;
+	void *src_iova, *dst_iova;
+	void *src_addr, *dst_addr;
 	void *host_iova;
 
 	while (cpy_len) {
@@ -1083,14 +1101,21 @@ async_fill_seg(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		}
 
 		if (to_desc) {
-			src = (void *)(uintptr_t)rte_pktmbuf_iova_offset(m, mbuf_offset);
-			dst = host_iova;
+			src_iova = (void *)(uintptr_t)rte_pktmbuf_iova_offset(m, mbuf_offset);
+			dst_iova = host_iova;
+
+			src_addr = rte_pktmbuf_mtod_offset(m, void *, mbuf_offset);
+			dst_addr = (void *)(uintptr_t)(buf_addr + buf_offset);
 		} else {
-			src = host_iova;
-			dst = (void *)(uintptr_t)rte_pktmbuf_iova_offset(m, mbuf_offset);
+			src_iova = host_iova;
+			dst_iova = (void *)(uintptr_t)rte_pktmbuf_iova_offset(m, mbuf_offset);
+
+			src_addr = (void *)(uintptr_t)(buf_addr + buf_offset);
+			dst_addr = rte_pktmbuf_mtod_offset(m, void *, mbuf_offset);
 		}
 
-		if (unlikely(async_iter_add_iovec(dev, async, src, dst, (size_t)mapped_len)))
+		if (unlikely(async_iter_add_iovec(dev, async, src_iova, dst_iova,
+						src_addr, dst_addr, (size_t)mapped_len)))
 			return -1;
 
 		cpy_len -= (uint32_t)mapped_len;
@@ -1239,7 +1264,8 @@ mbuf_to_desc(struct virtio_net *dev, struct vhost_virtqueue *vq,
 
 		if (is_async) {
 			if (async_fill_seg(dev, vq, m, mbuf_offset,
-					   buf_iova + buf_offset, cpy_len, true) < 0)
+					   buf_iova + buf_offset, buf_addr + buf_offset,
+					   cpy_len, true) < 0)
 				goto error;
 		} else {
 			sync_fill_seg(dev, vq, m, mbuf_offset,
@@ -2737,7 +2763,8 @@ desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
 
 		if (is_async) {
 			if (async_fill_seg(dev, vq, cur, mbuf_offset,
-					   buf_iova + buf_offset, cpy_len, false) < 0)
+					   buf_iova + buf_offset, buf_addr + buf_offset,
+					   cpy_len, false) < 0)
 				goto error;
 		} else if (likely(hdr && cur == m)) {
 			rte_memcpy(rte_pktmbuf_mtod_offset(cur, void *, mbuf_offset),
-- 
2.25.1