netdev - Re: [PATCH] ipv4: IP defragmentation must be ECN aware

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Date:	Wed, 05 Jan 2011 18:41:01 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] ipv4: IP defragmentation must be ECN aware

Le mercredi 05 janvier 2011 à 09:13 -0800, Stephen Hemminger a écrit :
> On Wed, 05 Jan 2011 14:59:02 +0100
> Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
> > +
> > +static inline int ip4_frag_ecn(int tos)
> 
> Since tos is only a byte, this should be:
> 
> static inline u8 ip4_frag_ecn(u8 tos)
> 
> 

In fact, generated code is the same on x86, but some arches have faster
arithmetic on WORD units.

And I added the 'inline' because on x86_64 gcc, compiler chose _not_ to
inline this 6 instruction sequence ! Code was much larger.

31 d2                   xor    %edx,%edx
0f b6 40 01             movzbl 0x1(%rax),%eax
83 e0 03                and    $0x3,%eax
ff c0                   inc    %eax
a8 02                   test   $0x2,%al
0f 44 d0                cmove  %eax,%edx


We do roughly the same (working on WORD arith) in 

static inline int IP_ECN_set_ce(struct iphdr *iph)
{
u32 ecn = (iph->tos + 1) & INET_ECN_MASK;
...
}

What others think ? I have no real strong opinion.


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