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SubjectRe: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu area
Eric W. Biederman wrote:
> Mike Travis <travis@sgi.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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