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

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Message-ID: <485FD56B.1090303@sgi.com>
Date:	Mon, 23 Jun 2008 09:55:07 -0700
From:	Mike Travis <travis@....com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	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

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

Thanks,
Mike
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