Re: [RFC] [PATCH] ip: skip IP checksum for skbs with CHECKSUM_UNNECESSARY set

[Stephen Hemminger <shemminger@vyatta.com>]

On Fri, 22 Aug 2008 17:36:25 -0700 (PDT)
David Miller <davem@davemloft.net> wrote:

> From: Stephen Hemminger <shemminger@vyatta.com>
> Date: Fri, 22 Aug 2008 18:33:20 -0400
> 
> > On Fri, 22 Aug 2008 22:24:31 +0300
> > Octavian Purdila <opurdila@ixiacom.com> wrote:
> > 
> > > 
> > > We are working on a completely in hardware LRO implementation and this patch 
> > > would simplify the hardware implementation. Is this acceptable?
> > > 
> > > --- a/net/ipv4/ip_input.c
> > > +++ b/net/ipv4/ip_input.c
> > > @@ -415,7 +415,8 @@ int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt,
> > >  
> > >         iph = ip_hdr(skb);
> > >  
> > > -       if (unlikely(ip_fast_csum((u8 *)iph, iph->ihl)))
> > > +       if (!skb_csum_unnecessary(skb) &&
> > > +           unlikely(ip_fast_csum((u8 *)iph, iph->ihl)))
> > >                 goto inhdr_error;
> > >  
> > >         len = ntohs(iph->tot_len);
> > > 
> > > --
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> > 
> > The overhead of the additional conditional might outweigh any benefit.
> 
> I don't think they want this for performance, they want to not have to
> compute the IP header checksum in their HW LRO implementation, which
> is just as silly :-)

And braindead for any other uses (like filtering or forwarding).