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SubjectRe: [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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