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

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Date:	Wed, 04 Jun 2008 13:59:55 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Mike Travis <travis@....com>
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

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]
>   

Unfortunately that doesn't actually work, because you can't have a reloc 
with two variables.

In something like:

	mov %gs:per_cpu__foo - 12345, %rax
	mov %gs:per_cpu__foo, %rax
	mov %gs:per_cpu__foo - 12345(%rip), %rax
	mov %gs:per_cpu__foo(%rip), %rax
	mov %gs:per_cpu__foo - __per_cpu_start, %rax
	mov %gs:per_cpu__foo - __per_cpu_start(%rip), %rax

the last two lines will not assemble:

t.S:5: Error: can't resolve `per_cpu__foo' {*UND* section} - `__per_cpu_start' {*UND* section}
t.S:6: Error: can't resolve `per_cpu__foo' {*UND* section} - `__per_cpu_start' {*UND* section}

Unfortunately, the only way I can think of fixing this is to compute the 
offset into a temp register, then use that:

	lea per_cpu__foo(%rip), %rax
	mov %gs:__per_cpu_offset(%rax), %rax

(where __per_cpu_offset is defined in the linker script as 
-__per_cpu_start).

This seems to be a general problem with zero-offset per-cpu.  And its 
unfortunate, because no-register access to per-cpu variables is nice to 
have.

The other alternative - and I have no idea whether this is practical or 
possible - is to define  a complete set of pre-offset per_cpu symbols.

    J
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