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

[Jeremy Fitzhardinge]

Mike Travis wrote:
> Eric W. Biederman wrote:
>   
> > Mike Travis <travis@xxxxxxx> 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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