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

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Date:	Fri, 20 Jun 2008 12:04:55 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Christoph Lameter <clameter@....com>
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

Christoph Lameter wrote:
> 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.
>   

Yes, which leads to an odd-looking ELF file where the Phdrs aren't 
sorted by virtual address order.  I'm wondering what would happen if a 
bootloader that actually understood ELF files tried to load it as an 
actual ELF file...

>> 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.
>   

Well, reading back from that address got zeros, so something is amiss.

>> 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.
>   

Yes, it should be fairly easy to manually relocate it by applying the 
(load - link) offset to it.


    J
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