Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu area

[ebiederm@xmission.com (Eric W. Biederman)]

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