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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Eric Dumazet [mailto:eric.dumazet@xxxxxxxxx]
> Sent: 16 December 2013 22:57
> To: Paul Durrant
> Cc: xen-devel@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Wei Liu; Ian Campbell;
> David Vrabel
> Subject: Re: [PATCH net-next] xen-netback: add gso_segs calculation
> 
> 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);
>

Good point. I'd misunderstood the value in transport_header.
 
> 
> You probably need to use :
> 
> hdrlen = skb_transport_header(skb) - skb_mac_header(skb) +
>        tcp_hdrlen(skb);
> 

Yes, that's what I need.

> (Assuming only TCP is supported)
> 

It is. The only possible GSOs are TCP.

  Paul
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