Re: [PATCH 2/3] x86-64: Unify x86_*_percpu() functions.

[Jeremy Fitzhardinge]

Ingo Molnar wrote:
> * Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
>
>   
> > Brian Gerst wrote:
> >     
> > > Merge the 32-bit and 64-bit versions of these functions.  Unlike 32-bit,
> > > the segment base is the current cpu's PDA instead of the offset from the
> > > original per-cpu area.  This is because GCC hardcodes the stackprotector
> > > canary at %gs:40.  Since the assembler is incapable of relocating against
> > > multiple symbols, the code ends up looking like:
> > >
> > > 	movq $per_cpu__var, reg
> > > 	subq $per_cpu__pda, reg
> > > 	movq %gs:(reg), reg
> > >
> > > This is still atomic since the offset is a constant (just calculated at
> > > runtime) and not dependant on the cpu number.
> > >   
> > >       
> > Yeah, it's a real pity we can't convince the linker to do this simple 
> > computation as a single %gs:ADDR addressing mode.  On the other hand, if 
> > the compiler can reuse the computation of %reg 2-3 times, then the 
> > generated code could well end up being denser.
> >     
>
> There's a nice project for linker hackers?
>
> I'd like to see some kernel image size measurements done on x86 defconfig 
> to see how much real impact this has on code density. Unless the impact is 
> horribly unacceptable, removing ~200 lines of weird x86-specific APIs is a 
> definitive plus.

Yep, I'm all for it.  I don't think there'll be much of a size impact at 
all.

   J

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