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

[David Miller <davem@davemloft.net>]

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 :-)