Re: [Xen-devel] [PATCH net-next] xen-netback: add gso_segs calculation

[Eric Dumazet <eric.dumazet@xxxxxxxxx>]

On Mon, 2013-12-16 at 17:20 +0000, Paul Durrant wrote:
> netback already has code which parses IPv4 and v6 headers to set up checksum
> offsets and these are always applied to GSO packets being sent from
> frontends. It's therefore suboptimal that GSOs are being marked
> SKB_GSO_DODGY to defer the gso_segs calculation when netback already has all
> necessary information to hand to do the calculation. This patch adds that
> calculation.
> 
> Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
> Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
>  drivers/net/xen-netback/netback.c |   17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/xen-netback/netback.c 
> b/drivers/net/xen-netback/netback.c
> index 904e497..0af8d97 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -1043,10 +1043,7 @@ static int xenvif_set_skb_gso(struct xenvif *vif,
>       }
>  
>       skb_shinfo(skb)->gso_size = gso->u.gso.size;
> -
> -     /* Header must be checked, and gso_segs computed. */
> -     skb_shinfo(skb)->gso_type |= SKB_GSO_DODGY;
> -     skb_shinfo(skb)->gso_segs = 0;
> +     /* gso_segs will be calculated later */
>  
>       return 0;
>  }
> @@ -1582,6 +1579,18 @@ static int xenvif_tx_submit(struct xenvif *vif, int 
> budget)
>  
>               skb_probe_transport_header(skb, 0);
>  
> +             /* If the packet is GSO then we will have just set up the
> +              * transport header offset in checksum_setup so it's now
> +              * straightforward to calculate gso_segs.
> +              */
> +             if (skb_is_gso(skb)) {
> +                     int mss = skb_shinfo(skb)->gso_size;
> +                     int hdrlen = skb->transport_header + tcp_hdrlen(skb);

This means there is no headroom ?

This is not what I am seeing in this driver, as it uses :

skb_reserve(skb, NET_SKB_PAD + NET_IP_ALIGN);

> +
> +                     skb_shinfo(skb)->gso_segs =
> +                             DIV_ROUND_UP(skb->len - hdrlen, mss);
> +             }
> +
>               vif->dev->stats.rx_bytes += skb->len;
>               vif->dev->stats.rx_packets++;
>  



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