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    Subject[RFC PATCH 6/8] virtio-net: unmark scatterlist ending after virtqueue_add_buf
    Date
    Prepare for when virtqueue_add_buf will use sg_next instead of
    ignoring ending markers.

    Note that for_each_sg (and thus virtqueue_add_buf) allows you
    to pass a "truncated" scatterlist that does not have a marker
    on the last item. We rely on this in add_recvbuf_mergeable.

    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    ---
    This is the only part that survived of Rusty's ideas. :)

    drivers/net/virtio_net.c | 21 ++++++++++++++++-----
    1 files changed, 16 insertions(+), 5 deletions(-)

    diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
    index 35c00c5..ce08b54 100644
    --- a/drivers/net/virtio_net.c
    +++ b/drivers/net/virtio_net.c
    @@ -440,13 +440,17 @@ static int add_recvbuf_small(struct receive_queue *rq, gfp_t gfp)

    hdr = skb_vnet_hdr(skb);
    sg_set_buf(rq->sg, &hdr->hdr, sizeof hdr->hdr);
    -
    skb_to_sgvec(skb, rq->sg + 1, 0, skb->len);

    err = virtqueue_add_buf(rq->vq, rq->sg, 0, 2, skb, gfp);
    if (err < 0)
    dev_kfree_skb(skb);

    + /*
    + * An optimization: clear the end bit set by skb_to_sgvec, so
    + * we can simply re-use rq->sg[] next time.
    + */
    + sg_unmark_end(rq->sg + 1);
    return err;
    }

    @@ -505,8 +509,7 @@ static int add_recvbuf_mergeable(struct receive_queue *rq, gfp_t gfp)
    if (!page)
    return -ENOMEM;

    - sg_init_one(rq->sg, page_address(page), PAGE_SIZE);
    -
    + sg_set_page(rq->sg, page, PAGE_SIZE, 0);
    err = virtqueue_add_buf(rq->vq, rq->sg, 0, 1, page, gfp);
    if (err < 0)
    give_pages(rq, page);
    @@ -671,6 +674,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
    const unsigned char *dest = ((struct ethhdr *)skb->data)->h_dest;
    struct virtnet_info *vi = sq->vq->vdev->priv;
    unsigned num_sg;
    + int ret;

    pr_debug("%s: xmit %p %pM\n", vi->dev->name, skb, dest);

    @@ -710,8 +714,15 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
    sg_set_buf(sq->sg, &hdr->hdr, sizeof hdr->hdr);

    num_sg = skb_to_sgvec(skb, sq->sg + 1, 0, skb->len) + 1;
    - return virtqueue_add_buf(sq->vq, sq->sg, num_sg,
    - 0, skb, GFP_ATOMIC);
    + ret = virtqueue_add_buf(sq->vq, sq->sg, num_sg,
    + 0, skb, GFP_ATOMIC);
    +
    + /*
    + * An optimization: clear the end bit set by skb_to_sgvec, so
    + * we can simply re-use sq->sg[] next time.
    + */
    + sg_unmark_end(&sq->sg[num_sg-1]);
    + return ret;
    }

    static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
    --
    1.7.1



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