[dpdk-dev] [PATCH 4/5] vhost: do not use rte_memcpy for virtio_hdr copy

[Yuanhan Liu]

First of all, rte_memcpy() is mostly useful for coping big packets
by leveraging hardware advanced instructions like AVX. But for virtio
net hdr, which is 12 bytes at most, invoking rte_memcpy() will not
introduce any performance boost.

And, to my suprise, rte_memcpy() is huge. Since rte_memcpy() is
inlined, it takes more space every time we call it at a different
place.  Replacing the two rte_memcpy() with directly copy saves
nearly 12K bytes of code size!

    # without this patch
    $ size /path/to/vhost_rxtx.o
       text    data     bss     dec     hex filename
      36171       0       0   36171    8d4b /path/to/vhost_rxtx.o

    # with this patch
    $ size /path/to/vhost_rxtx.o
       text    data     bss     dec     hex filename
      24179       0       0   24179    5e73 /path/to/vhost_rxtx.o

Signed-off-by: Yuanhan Liu <yuanhan.liu at linux.intel.com>
---
 lib/librte_vhost/vhost_rxtx.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/lib/librte_vhost/vhost_rxtx.c b/lib/librte_vhost/vhost_rxtx.c
index 7464b6b..1e0a24e 100644
--- a/lib/librte_vhost/vhost_rxtx.c
+++ b/lib/librte_vhost/vhost_rxtx.c
@@ -70,14 +70,17 @@ copy_mbuf_to_desc(struct virtio_net *dev, struct 
vhost_virtqueue *vq,
        uint32_t cpy_len;
        struct vring_desc *desc;
        uint64_t desc_addr;
-       struct virtio_net_hdr_mrg_rxbuf virtio_hdr = {{0, 0, 0, 0, 0, 0}, 0};
+       struct virtio_net_hdr_mrg_rxbuf hdr = {{0, 0, 0, 0, 0, 0}, 0};

        desc = &vq->desc[desc_idx];
        desc_addr = gpa_to_vva(dev, desc->addr);
        rte_prefetch0((void *)(uintptr_t)desc_addr);

-       rte_memcpy((void *)(uintptr_t)desc_addr,
-                       (const void *)&virtio_hdr, vq->vhost_hlen);
+       if (vq->vhost_hlen == sizeof(struct virtio_net_hdr_mrg_rxbuf)) {
+               *(struct virtio_net_hdr_mrg_rxbuf *)(uintptr_t)desc_addr = hdr;
+       } else {
+               *(struct virtio_net_hdr *)(uintptr_t)desc_addr = hdr.hdr;
+       }
        PRINT_PACKET(dev, (uintptr_t)desc_addr, vq->vhost_hlen, 0);

        desc_offset = vq->vhost_hlen;
@@ -340,7 +343,7 @@ 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 *pkt)
 {
-       struct virtio_net_hdr_mrg_rxbuf virtio_hdr = {{0, 0, 0, 0, 0, 0}, 0};
+       struct virtio_net_hdr_mrg_rxbuf hdr = {{0, 0, 0, 0, 0, 0}, 0};
        uint32_t vec_idx = 0;
        uint16_t cur_idx = res_start_idx;
        uint64_t desc_addr;
@@ -361,13 +364,16 @@ copy_mbuf_to_desc_mergeable(struct virtio_net *dev, 
struct vhost_virtqueue *vq,

        rte_prefetch0((void *)(uintptr_t)desc_addr);

-       virtio_hdr.num_buffers = res_end_idx - res_start_idx;
+       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);

-       rte_memcpy((void *)(uintptr_t)desc_addr,
-               (const void *)&virtio_hdr, vq->vhost_hlen);
+       if (vq->vhost_hlen == sizeof(struct virtio_net_hdr_mrg_rxbuf)) {
+               *(struct virtio_net_hdr_mrg_rxbuf *)(uintptr_t)desc_addr = hdr;
+       } else {
+               *(struct virtio_net_hdr *)(uintptr_t)desc_addr = hdr.hdr;
+       }
        PRINT_PACKET(dev, (uintptr_t)desc_addr, vq->vhost_hlen, 0);

        desc_avail  = vq->buf_vec[vec_idx].buf_len - vq->vhost_hlen;
-- 
1.9.0