netdev - Re: [patch 1/4] network dev read

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Message-ID: <45F8F8DC.3010302@cosmosbay.com>
Date:	Thu, 15 Mar 2007 08:42:20 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
CC:	bcrl@...ck.org, shemminger@...ux-foundation.org,
	netdev@...r.kernel.org
Subject: Re: [patch 1/4] network dev read_mostly

David Miller a écrit :
> From: Eric Dumazet <dada1@...mosbay.com>
> Date: Thu, 15 Mar 2007 07:28:35 +0100
> 
>> One problem with your patch is that all read_always() of pointers are going to 
>> use 3 bytes more of code, thus raising icache pressure.
>>
>> 48 b8 c3 08 e8 8c af    mov    $0x71af8ce808c3,%rax
>> 71 00 00
>>
>> instead of %rip relative addressing
>>
>> 48 8b 05 99 f3 09 00    mov    652185(%rip),%rax
> 
> Could we obtain %rip relative addressing with the ELF
> relocation approach I mentioned?

I dont think so, because 32 bits relative to %rip is not enough to cover all 
the addresses that a 64bits kernel can use :)

%rip relative addressing works only because all the x86_64 kernel (text+ 
static/bss data) are in the 0xffffffff80000000 - fffffffffff00000  2^31 quadrant

but vmalloc() range for example  is ffffc20000000000 - ffffe1ffffffffff (45 bits)


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