linux-kernel - Re: [PATCH 2/3] x86-64: Unify x86_*

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Date:	Sat, 27 Dec 2008 22:03:35 +1100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Brian Gerst <brgerst@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] x86-64: Unify x86_*_percpu() functions.

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.

    J
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