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    SubjectRe: [PATCH net-next 2/5] virtio-net: pack headroom into ctx for mergeable buffer
    On Mon, Jul 17, 2017 at 08:43:58PM +0800, Jason Wang wrote:
    > Pack headroom into ctx, then during XDP set, we could know the size of
    > headroom and copy if needed. This is required for avoiding reset on
    > XDP.

    Not really when XDP is set - it's when buffers are used.

    virtio-net: pack headroom into ctx for mergeable buffers

    Pack headroom into ctx - this way when we get a buffer we can figure out
    the actual headroom that was allocated for the buffer. Will be helpful
    to optimize switching between XDP and non-XDP modes which have different
    headroom requirements.


    >
    > Signed-off-by: Jason Wang <jasowang@redhat.com>
    > ---
    > drivers/net/virtio_net.c | 29 ++++++++++++++++++++++++-----
    > 1 file changed, 24 insertions(+), 5 deletions(-)
    >
    > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
    > index 1f8c15c..8fae9a8 100644
    > --- a/drivers/net/virtio_net.c
    > +++ b/drivers/net/virtio_net.c
    > @@ -270,6 +270,23 @@ static void skb_xmit_done(struct virtqueue *vq)
    > netif_wake_subqueue(vi->dev, vq2txq(vq));
    > }
    >
    > +#define MRG_CTX_HEADER_SHIFT 22
    > +static void *mergeable_len_to_ctx(unsigned int truesize,
    > + unsigned int headroom)
    > +{
    > + return (void *)(unsigned long)((headroom << MRG_CTX_HEADER_SHIFT) | truesize);
    > +}
    > +
    > +static unsigned int mergeable_ctx_to_headroom(void *mrg_ctx)
    > +{
    > + return (unsigned long)mrg_ctx >> MRG_CTX_HEADER_SHIFT;
    > +}
    > +
    > +static unsigned int mergeable_ctx_to_truesize(void *mrg_ctx)
    > +{
    > + return (unsigned long)mrg_ctx & ((1 << MRG_CTX_HEADER_SHIFT) - 1);
    > +}
    > +
    > /* Called from bottom half context */
    > static struct sk_buff *page_to_skb(struct virtnet_info *vi,
    > struct receive_queue *rq,
    > @@ -639,13 +656,14 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
    > }
    > rcu_read_unlock();
    >
    > - if (unlikely(len > (unsigned long)ctx)) {
    > + truesize = mergeable_ctx_to_truesize(ctx);
    > + if (unlikely(len > truesize)) {
    > pr_debug("%s: rx error: len %u exceeds truesize %lu\n",
    > dev->name, len, (unsigned long)ctx);
    > dev->stats.rx_length_errors++;
    > goto err_skb;
    > }
    > - truesize = (unsigned long)ctx;
    > +
    > head_skb = page_to_skb(vi, rq, page, offset, len, truesize);
    > curr_skb = head_skb;
    >
    > @@ -665,13 +683,14 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
    > }
    >
    > page = virt_to_head_page(buf);
    > - if (unlikely(len > (unsigned long)ctx)) {
    > +
    > + truesize = mergeable_ctx_to_truesize(ctx);
    > + if (unlikely(len > truesize)) {
    > pr_debug("%s: rx error: len %u exceeds truesize %lu\n",
    > dev->name, len, (unsigned long)ctx);
    > dev->stats.rx_length_errors++;
    > goto err_skb;
    > }
    > - truesize = (unsigned long)ctx;
    >
    > num_skb_frags = skb_shinfo(curr_skb)->nr_frags;
    > if (unlikely(num_skb_frags == MAX_SKB_FRAGS)) {
    > @@ -889,7 +908,7 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi,
    >
    > buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset;
    > buf += headroom; /* advance address leaving hole at front of pkt */
    > - ctx = (void *)(unsigned long)len;
    > + ctx = mergeable_len_to_ctx(len, headroom);
    > get_page(alloc_frag->page);
    > alloc_frag->offset += len + headroom;
    > hole = alloc_frag->size - alloc_frag->offset;
    > --
    > 2.7.4

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