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

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Date:	Mon, 23 Jun 2008 13:33:52 -0400
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Mike Travis <travis@....com>
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

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
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