linux-kernel - Re: [PATCH 3/4] x86_64: Fold pda into per cpu area

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Date:	Wed, 04 Jun 2008 14:58:09 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Mike Travis <travis@....com>
CC:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <clameter@....com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>,
	linux-kernel@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 3/4] x86_64: Fold pda into per cpu area

Mike Travis wrote:
> Jeremy Fitzhardinge wrote:
>   
>> Mike Travis wrote:
>>     
>>>   * Declare the pda as a per cpu variable.
>>>
>>>   * Make the x86_64 per cpu area start at zero.
>>>
>>>   * Since the pda is now the first element of the per_cpu area, cpu_pda()
>>>     is no longer needed and per_cpu() can be used instead.  This also
>>> makes
>>>     the _cpu_pda[] table obsolete.
>>>
>>>   * Since %gs is pointing to the pda, it will then also point to the
>>> per cpu
>>>     variables and can be accessed thusly:
>>>
>>>     %gs:[&per_cpu_xxxx - __per_cpu_start]
>>>
>>>       
>
>   
> The above is only a partial story (I folded the two patches but didn't
> update the comments correctly.]  The variables are already offset from
> __per_cpu_start by virtue of the .data.percpu section being based at
> zero.  Therefore only the %gs register needs to be set to the base of
> each cpu's percpu section to resolve the target address:
>
>         %gs:&per_cpu_xxxx
>   

Oh, good.  I'd played with trying to make that work at one point, and 
got lost in linker bugs and/or random version-specific strangeness. 

    J
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