virtio-net: avoid unnecessary sg initialzation

Usually an skb does not have up to MAX_SKB_FRAGS frags. So no need to
initialize the unuse part of sg. This patch initialize the sg based on
the real number it will used:

- during xmit, it could be inferred from nr_frags and can_push.
- for small receive buffer, it will also be 2.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 9b950f2..d8838de 100644 (file)
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -540,7 +540,7 @@ static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
        skb_put(skb, GOOD_PACKET_LEN);
 
        hdr = skb_vnet_hdr(skb);
-       sg_init_table(rq->sg, MAX_SKB_FRAGS + 2);
+       sg_init_table(rq->sg, 2);
        sg_set_buf(rq->sg, hdr, vi->hdr_len);
        skb_to_sgvec(skb, rq->sg + 1, 0, skb->len);
 
@@ -893,7 +893,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
        if (vi->mergeable_rx_bufs)
                hdr->num_buffers = 0;
 
-       sg_init_table(sq->sg, MAX_SKB_FRAGS + 2);
+       sg_init_table(sq->sg, skb_shinfo(skb)->nr_frags + (can_push ? 1 : 2));
        if (can_push) {
                __skb_push(skb, hdr_len);
                num_sg = skb_to_sgvec(skb, sq->sg, 0, skb->len);