linux-kernel - Re: [GIT RFC] percpu: use dynamic percpu allocator as the default percpu allocator

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Date:	Wed, 01 Apr 2009 17:53:42 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
CC:	Ingo Molnar <mingo@...e.hu>, rusty@...tcorp.com.au,
	tglx@...utronix.de, x86@...nel.org, linux-kernel@...r.kernel.org,
	hpa@...or.com, Paul Mundt <lethal@...ux-sh.org>,
	rmk@....linux.org.uk, starvik@...s.com, ralf@...ux-mips.org,
	davem@...emloft.net, cooloney@...nel.org, kyle@...artin.ca,
	matthew@....cx, grundler@...isc-linux.org, takata@...ux-m32r.org,
	benh@...nel.crashing.org, rth@...ddle.net,
	ink@...assic.park.msu.ru, heiko.carstens@...ibm.com
Subject: Re: [GIT RFC] percpu: use dynamic percpu allocator as the default
 percpu allocator

Hello, Martin.

Martin Schwidefsky wrote:
> Yes, @GOTENT is a relocation against the GOT slot that contains the
> address of the symbol. It is a special version of @GOT that uses larl
> to locate the got slot directly without the need of a got base pointer.
> 
> The code sequence with @GOT:
> 
> 	larl	%r12,_GLOBAL_OFFSET_TABLE_
> 	lg	%r1,symbol@GOT(%r12)
> 
> is equivalent to:
> 
> 	larl	%r1,symbol@...ENT
> 	lg	%r1,0(%r1)
> 
> The advantage of the second code sequence is that it need a single
> register and the size of the GOT is not limited to 4K as in the first
> example (the offset in an RX format instruction is limited to 12 bits -
> but that is probably something you don't want to know ;-).

Maybe we can build indirection pointer manually by twiddling with
DEFINE_PER_CPU() in such a way that it doesn't have to distinguish
symbols and variables?

-- 
tejun
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