linux-kernel - Re: [PATCH 12/17] x86-64: Use absolute displacements for per-cpu accesses.

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Message-ID: <20090118173820.GA14356@elte.hu>
Date:	Sun, 18 Jan 2009 18:38:20 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 12/17] x86-64: Use absolute displacements for per-cpu
	accesses.


* Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Brian Gerst <brgerst@...il.com> wrote:
> 
> > On Sat, Jan 17, 2009 at 11:22 PM, Tejun Heo <tj@...nel.org> wrote:
> > 
> > > Hello, Brian.
> > >
> > > Brian Gerst wrote:
> > > > Accessing memory through %gs should not use rip-relative addressing.
> > > > Adding a P prefix for the argument tells gcc to not add (%rip) to
> > > > the memory references.
> > >
> > > Nice catch.  I dind't know about the P prefix thing.  It also is used
> > > in other places too.  Hmmm... I can't find anything about the P
> > > argument prefix in the gcc info page (4.3).  Any ideas where I can
> > > find some information about it?  It's a bit weird that it's not a
> > > constraint prefix but an argument one.
> > 
> > The only place I could confirm that it works is in the gcc source 
> > itself, and even there it's not well documented.
> 
> does %P support go back as far as gcc 3.2 (the earliest GCC we still 
> support)?

update: Brian pointed it out off-list that switch_to() already uses %P, so 
we already rely on it.

	Ingo
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