On 2/18/2016 9:48 PM, Yuanhan Liu wrote:

Current virtio_dev_merge_rx() implementation just looks like the
old rte_vhost_dequeue_burst(), full of twisted logic, that you
can see same code block in quite many different places.

However, the logic of virtio_dev_merge_rx() is quite similar to
virtio_dev_rx().  The big difference is that the mergeable one
could allocate more than one available entries to hold the data.
Fetching all available entries to vec_buf at once makes the

[...]

-	}
+static inline uint32_t __attribute__((always_inline))
+copy_mbuf_to_desc_mergeable(struct virtio_net *dev, struct vhost_virtqueue *vq,
+			    uint16_t res_start_idx, uint16_t res_end_idx,
+			    struct rte_mbuf *m)
+{
+	struct virtio_net_hdr_mrg_rxbuf virtio_hdr = {{0, 0, 0, 0, 0, 0}, 0};
+	uint32_t vec_idx = 0;
+	uint16_t cur_idx = res_start_idx;
+	uint64_t desc_addr;
+	uint32_t mbuf_offset, mbuf_avail;
+	uint32_t desc_offset, desc_avail;
+	uint32_t cpy_len;
+	uint16_t desc_idx, used_idx;
+	uint32_t nr_used = 0;
 
-	cpy_len = RTE_MIN(vb_avail, seg_avail);
+	if (m == NULL)
+		return 0;

Is this inherited from old code? Let us remove the unnecessary check.
Caller ensures it is not NULL.

 
-	while (cpy_len > 0) {
-		/* Copy mbuf data to vring buffer */
-		rte_memcpy((void *)(uintptr_t)(vb_addr + vb_offset),
-			rte_pktmbuf_mtod_offset(pkt, const void *, seg_offset),
-			cpy_len);
+	LOG_DEBUG(VHOST_DATA,
+		"(%"PRIu64") Current Index %d| End Index %d\n",
+		dev->device_fh, cur_idx, res_end_idx);
 
-		PRINT_PACKET(dev,
-			(uintptr_t)(vb_addr + vb_offset),
-			cpy_len, 0);
+	desc_addr = gpa_to_vva(dev, vq->buf_vec[vec_idx].buf_addr);
+	rte_prefetch0((void *)(uintptr_t)desc_addr);
+
+	virtio_hdr.num_buffers = res_end_idx - res_start_idx;
+	LOG_DEBUG(VHOST_DATA, "(%"PRIu64") RX: Num merge buffers %d\n",
+		dev->device_fh, virtio_hdr.num_buffers);
 
-		seg_offset += cpy_len;
-		vb_offset += cpy_len;
-		seg_avail -= cpy_len;
-		vb_avail -= cpy_len;
-		entry_len += cpy_len;
-
-		if (seg_avail != 0) {
-			/*
-			 * The virtio buffer in this vring
-			 * entry reach to its end.
-			 * But the segment doesn't complete.
-			 */
-			if ((vq->desc[vq->buf_vec[vec_idx].desc_idx].flags &
-				VRING_DESC_F_NEXT) == 0) {
+	virtio_enqueue_offload(m, &virtio_hdr.hdr);
+	rte_memcpy((void *)(uintptr_t)desc_addr,
+		(const void *)&virtio_hdr, vq->vhost_hlen);
+	PRINT_PACKET(dev, (uintptr_t)desc_addr, vq->vhost_hlen, 0);
+
+	desc_avail  = vq->buf_vec[vec_idx].buf_len - vq->vhost_hlen;
+	desc_offset = vq->vhost_hlen;

As we know we are in merge-able path, use sizeof(virtio_net_hdr) to save
one load for the header len.

+
+	mbuf_avail  = rte_pktmbuf_data_len(m);
+	mbuf_offset = 0;
+	while (1) {
+		/* done with current desc buf, get the next one */
+

[...]

+		if (reserve_avail_buf_mergeable(vq, pkt_len, &start, &end) < 0)
+			break;
 
+		nr_used = copy_mbuf_to_desc_mergeable(dev, vq, start, end,
+						      pkts[pkt_idx]);

In which case couldn't we get nr_used from start and end?

 		rte_compiler_barrier();
 
 		/*
 		 * Wait until it's our turn to add our buffer
 		 * to the used ring.
 		 */
-		while (unlikely(vq->last_used_idx != res_base_idx))
+		while (unlikely(vq->last_used_idx != start))
 			rte_pause();
 
-		*(volatile uint16_t *)&vq->used->idx += entry_success;
-		vq->last_used_idx = res_cur_idx;
+		*(volatile uint16_t *)&vq->used->idx += nr_used;
+		vq->last_used_idx = end;
 	}
 
-merge_rx_exit:
 	if (likely(pkt_idx)) {
 		/* flush used->idx update before we read avail->flags. */
 		rte_mb();