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SubjectRe: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu area
Mike Travis wrote:
> 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
>

That looks OK to me. Does it work?

J


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