linux-kernel - Re: [patch 00/41] cpu alloc / cpu ops v3: Optimize per cpu access

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Message-ID: <484D8F7E.4070104@cosmosbay.com>
Date:	Mon, 09 Jun 2008 22:15:58 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Christoph Lameter <clameter@....com>, Mike Travis <travis@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [patch 00/41] cpu alloc / cpu ops v3: Optimize per cpu access

Andi Kleen a écrit :
> Christoph Lameter <clameter@....com> writes:
> 
>> The problem is that offsets relative to %gs or %fs are limited by the 
>> small memory model that is chosen.
> 
> Actually they are not. If you really want you can do 
> movabs $64bit,%reg ; op ...,%gs:(%reg) 
> It's just not very efficient compared to small (or rather kernel) model
> and also older binutils didn't support large model.
> 

I am not sure Christoph was refering to actual instructions.

I was suggesting using for static percpu (vmlinux or modules) :

vmlinux : (offset31 computed by linker at vmlinux link edit time)
incl  %gs:offset31

modules : (offset31 computed at module load time by module loader)
incl %gs:offset31

(If we make sure all this stuff is allocated in first chunk)

And for dynamic percpu :

movq   field(%rdi),%rax
incl    %gs:(%rax)   /* full 64bits 'offsets' */

I understood (but might be wrong again) that %gs itself could not be used with an offset > 2GB, because
the way %gs segment is setup. So in the 'dynamic percpu' case, %rax should not exceed 2^31





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