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

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Date:	Wed, 04 Jun 2008 07:17:03 -0700
From:	Mike Travis <travis@....com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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

Jeremy Fitzhardinge wrote:
> 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

Incidentally, this is why the following load is needed in x86_64_start_kernel():

        pda = (struct x8664_pda *)__per_cpu_load;
        pda->data_offset = per_cpu_offset(0) = (unsigned long)pda;

        /* initialize boot cpu_pda data */
        pda_init(0);

pda_init() loads the %gs reg so early accesses to the static per_cpu section
can be executed before the percpu areas are allocated.

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