linux-kernel - Re: [crash, bisected] Re: [PATCH 3/4] x86

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Message-ID: <Pine.LNX.4.64.0806201148430.16227@schroedinger.engr.sgi.com>
Date:	Fri, 20 Jun 2008 11:51:04 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
cc:	Mike Travis <travis@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu
 area

On Fri, 20 Jun 2008, Jeremy Fitzhardinge wrote:

> > The loader setup for the percpu section changes with zero basing. Maybe that
> > has bad side effects
> 
> How does it work?  The symbols in the percpu segment are 0-based, but where
> does the data for the sections which correspond to that segment go?

Its loaded at __per_cpu_load but the symbols have addresses starting at 0.

> So the question is what kernel virtual address is it being loaded to?
> __per_cpu_load is ffffffff808d1000, so ffffffff808d6000 is what you'd
> expect...

Correct.

> Hm, but what happens when this gets converted to bzImage?  Hm, looks OK, I
> think.
> 
> BTW, I think __per_cpu_load will cause trouble if you make a relocatable
> kernel, being an absolute symbol.  But I have relocation off at the moment.

Hmmm.... we could add the relocation offset to __per_cpu_load? 
__per_cpu_load is used very sparingly. Basically only useful during early 
boot and when a new per cpu area has to be setup. In that case we want to 
copy from __per_cpu_load to the newly allocated percpu area.
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