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

[Jeremy Fitzhardinge]

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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