netdev - Re: [PATCH] eth: Declare an optimized compare_ether_addr

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Message-ID: <492A4962.1080400@cosmosbay.com>
Date:	Mon, 24 Nov 2008 07:27:46 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
CC:	netdev@...r.kernel.org
Subject: Re: [PATCH] eth: Declare an optimized compare_ether_addr_64bits()
 function

David Miller a écrit :
> From: Eric Dumazet <dada1@...mosbay.com>
> Date: Sat, 22 Nov 2008 08:19:17 +0100
> 
>> This patch implements a compare_ether_addr_64bits() function, that
>> handles the case of x86 cpus, but might be used on other arches as
>> well, if their potential misaligned long word reads are not
>> expensive.
> 
> We have a test for this, HAVE_EFFICIENT_UNALIGNED_ACCESS
> 
> Please use that instead of CONFIG_X86 and I'll apply this
> to net-next-2.6
> 

Excellent !

I missed this cool feature.

Thanks David

[PATCH] eth: Declare an optimized compare_ether_addr_64bits() function

Linus mentioned we could try to perform long word operations, even
on potentially unaligned addresses, on x86 at least. David mentioned
the HAVE_EFFICIENT_UNALIGNED_ACCESS test to handle this on all
arches that have efficient unailgned accesses.

I tried this idea and got nice assembly on 32 bits:

158:   33 82 38 01 00 00       xor    0x138(%edx),%eax
15e:   33 8a 34 01 00 00       xor    0x134(%edx),%ecx
164:   c1 e0 10                shl    $0x10,%eax
167:   09 c1                   or     %eax,%ecx
169:   74 0b                   je     176 <eth_type_trans+0x87>

And very nice assembly on 64 bits of course (one xor, one shl)

Nice oprofile improvement in eth_type_trans(), 0.17 % instead of 0.41 %,
expected since we remove 8 instructions on a fast path.

This patch implements a compare_ether_addr_64bits() function,
that uses the CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS ifdef
and get_unaligned() macro to efficiently perform the 6 bytes
comparison on all capable arches.

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
---
 include/linux/etherdevice.h |   44 ++++++++++++++++++++++++++++++++++
 net/ethernet/eth.c          |    6 ++--
 2 files changed, 47 insertions(+), 3 deletions(-)

View attachment "ether.patch" of type "text/plain" (2839 bytes)