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

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Date:	Mon, 23 Jun 2008 11:04:57 -0700
From:	Mike Travis <travis@....com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Christoph Lameter <clameter@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu
 area

Jeremy Fitzhardinge wrote:
> Mike Travis wrote:
>> Eric W. Biederman wrote:
>>  
>>> Mike Travis <travis@....com> writes:
>>>
>>>    
>>>> Jeremy Fitzhardinge wrote:
>>>>      
>>>>> 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.
>>>>>
>>>>>         
>>>> ...
>>>> Here's where it's defined (in include/asm-generic/vmlinux.lds.h):
>>>>
>>>> #ifdef CONFIG_HAVE_ZERO_BASED_PER_CPU
>>>> #define
>>>> PERCPU(align)                                                   \
>>>>         . =
>>>> ALIGN(align);                                               \
>>>>         percpu : { }
>>>> :percpu                                            \
>>>>         __per_cpu_load =
>>>> .;                                             \
>>>>         .data.percpu 0 : AT(__per_cpu_load - LOAD_OFFSET)
>>>> {             \
>>>>                
>>>> *(.data.percpu.first)                                   \
>>>>                
>>>> *(.data.percpu.shared_aligned)                          \
>>>>                
>>>> *(.data.percpu)                                         \
>>>>                
>>>> *(.data.percpu.page_aligned)                            \
>>>>                 ____per_cpu_size =
>>>> .;                                   \
>>>>        
>>>> }                                                               \
>>>>         . = __per_cpu_load +
>>>> ____per_cpu_size;                          \
>>>>         data : { } :data
>>>> #else
>>>>
>>>> Can we generate a new symbol which would account for LOAD_OFFSET?
>>>>       
>>> Ouch.  Absolute symbols indeed.  On the 32bit kernel that may play havoc
>>> with the relocatable kernel, although we have had similar absolute logic
>>> for the last year. With __per_cpu_start and __per_cpu_end so it may
>>> not be a problem.
>>>
>>> To initialize the percpu data you do want to talk to the virtual address
>>> at __per_coup_load.  But it is absolute Ugh. 
>>> It might be worth saying something like.
>>> .data.percpu.start : AT(.data.percpu.dummy - LOAD_OFFSET) {
>>>     DATA(0)                       . = ALIGN(align);
>>>         __per_cpu_load = . ;                   }
>>> To make __per_cpu_load a relative symbol. ld has a bad habit of taking
>>> symbols out of empty sections and making them absolute.  Which is why
>>> I added the DATA(0).
>>>
>>> Still I don't think that would be the 64bit problem.
>>>
>>> Eric
>>>     
>>
>> I'm not sure I understand the linker lingo enough to fill in the rest
>> of the blanks... I've tried various versions around this framework and
>> none have been accepted yet.
>>
>> #ifdef CONFIG_HAVE_ZERO_BASED_PER_CPU
>> #define PERCPU(align)                                                   \
>>         .data.percpu.start : AT(.data.percpu.dummy - LOAD_OFFSET) {     \
>>                 DATA(0)                                                 \
>>                 . = ALIGN(align);                                       \
>>                 __per_cpu_load = .;                                     \
>>                 *(.data.percpu.first)                                   \
>>                 *(.data.percpu.shared_aligned)                          \
>>                 *(.data.percpu)                                         \
>>                 *(.data.percpu.page_aligned)                            \
>>                 ____per_cpu_size = . - __per_cpu_load                   \
>>         }                                                               \
>> #else
>>   
> 
> That looks OK to me.  Does it work?
> 
>    J

Nope, fighting undefines and/or syntax errors in the linker.
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