linux-kernel - Re: [PATCH] powerpc 32: Provides VIRT_CPU

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Message-ID: <1398881677.24575.197.camel@snotra.buserror.net>
Date:	Wed, 30 Apr 2014 13:14:37 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC:	Christophe Leroy <christophe.leroy@....fr>,
	Paul Mackerras <paulus@...ba.org>,
	<linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] powerpc 32: Provides VIRT_CPU_ACCOUNTING

On Wed, 2014-04-30 at 14:56 +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2014-03-19 at 17:05 -0500, Scott Wood wrote:
> > On Wed, 2014-03-19 at 22:52 +0100, Christophe Leroy wrote:
> > > This patch provides VIRT_CPU_ACCOUTING to PPC32 architecture.
> > > Unlike PPC64, PPC32 doesn't provide the PACA register. Therefore the
> > > implementation is similar to the one done in the IA64 architecture.
> > > It is based on additional information added to the Task Info structure.
> > 
> > PACA isn't a register -- just a convention for how Linux uses a GPR.
> > Maybe it's time to use it on PPC32 as well?
> 
> PACA is actually a data structure and you really really don't want it
> on ppc32 :-) Having a register point to current works, having a register
> point to per-cpu data instead works too (ie, change what we do today),
> but don't introduce a PACA *please* :-)

What is special about 64-bit that warrants doing things differently from
32-bit?

What is the difference between PACA and "per-cpu data", other than the
obscure name?
 
-Scott


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