vhost: optimize dequeue for small packets

A virtio driver normally uses at least 2 desc buffers for Tx: the
first for storing the header, and the others for storing the data.

Therefore, we could fetch the first data desc buf before the main
loop, and do the copy first before the check of "are we done yet?".
This could save one check for small packets that just have one data
desc buffer and need one mbuf to store it.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Huawei Xie <huawei.xie@intel.com>
Tested-by: Rich Lane <rich.lane@bigswitch.com>

diff --git a/lib/librte_vhost/vhost_rxtx.c b/lib/librte_vhost/vhost_rxtx.c
index 52f87c1..237fb64 100644 (file)
--- a/lib/librte_vhost/vhost_rxtx.c
+++ b/lib/librte_vhost/vhost_rxtx.c
@@ -756,18 +756,49 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
                return -1;
 
        desc_addr = gpa_to_vva(dev, desc->addr);
-       rte_prefetch0((void *)(uintptr_t)desc_addr);
-
-       /* Retrieve virtio net header */
        hdr = (struct virtio_net_hdr *)((uintptr_t)desc_addr);
-       desc_avail  = desc->len - dev->vhost_hlen;
-       desc_offset = dev->vhost_hlen;
+       rte_prefetch0(hdr);
+
+       /*
+        * A virtio driver normally uses at least 2 desc buffers
+        * for Tx: the first for storing the header, and others
+        * for storing the data.
+        */
+       if (likely((desc->len == dev->vhost_hlen) &&
+                  (desc->flags & VRING_DESC_F_NEXT) != 0)) {
+               desc = &vq->desc[desc->next];
+
+               desc_addr = gpa_to_vva(dev, desc->addr);
+               rte_prefetch0((void *)(uintptr_t)desc_addr);
+
+               desc_offset = 0;
+               desc_avail  = desc->len;
+               nr_desc    += 1;
+
+               PRINT_PACKET(dev, (uintptr_t)desc_addr, desc->len, 0);
+       } else {
+               desc_avail  = desc->len - dev->vhost_hlen;
+               desc_offset = dev->vhost_hlen;
+       }
 
        mbuf_offset = 0;
        mbuf_avail  = m->buf_len - RTE_PKTMBUF_HEADROOM;
-       while (desc_avail != 0 || (desc->flags & VRING_DESC_F_NEXT) != 0) {
+       while (1) {
+               cpy_len = RTE_MIN(desc_avail, mbuf_avail);
+               rte_memcpy(rte_pktmbuf_mtod_offset(cur, void *, mbuf_offset),
+                       (void *)((uintptr_t)(desc_addr + desc_offset)),
+                       cpy_len);
+
+               mbuf_avail  -= cpy_len;
+               mbuf_offset += cpy_len;
+               desc_avail  -= cpy_len;
+               desc_offset += cpy_len;
+
                /* This desc reaches to its end, get the next one */
                if (desc_avail == 0) {
+                       if ((desc->flags & VRING_DESC_F_NEXT) == 0)
+                               break;
+
                        if (unlikely(desc->next >= vq->size ||
                                     ++nr_desc >= vq->size))
                                return -1;
@@ -803,16 +834,6 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
                        mbuf_offset = 0;
                        mbuf_avail  = cur->buf_len - RTE_PKTMBUF_HEADROOM;
                }
-
-               cpy_len = RTE_MIN(desc_avail, mbuf_avail);
-               rte_memcpy(rte_pktmbuf_mtod_offset(cur, void *, mbuf_offset),
-                       (void *)((uintptr_t)(desc_addr + desc_offset)),
-                       cpy_len);
-
-               mbuf_avail  -= cpy_len;
-               mbuf_offset += cpy_len;
-               desc_avail  -= cpy_len;
-               desc_offset += cpy_len;
        }
 
        prev->data_len = mbuf_offset;