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

[Eric Dumazet <eric.dumazet@xxxxxxxxx>]

On Mon, 2013-12-16 at 10:23 -0800, Eric Dumazet wrote:
> 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);


You probably need to use :

hdrlen = skb_transport_header(skb) - skb_mac_header(skb) +
         tcp_hdrlen(skb);

(Assuming only TCP is supported)



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