Re: [PATCH] [v2] powerpc: save trap number in bad_stack

[Michael Ellerman <michael@ellerman.id.au>]

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On Mon, 2007-04-23 at 23:22 +1000, Stephen Rothwell wrote:
> On Sun, 22 Apr 2007 18:38:21 -0500 olof@lixom.net (Olof Johansson) wrote:
> >
> > --- powerpc.orig/include/asm-powerpc/paca.h
> > +++ powerpc/include/asm-powerpc/paca.h
> > @@ -68,6 +68,7 @@ struct paca_struct {
> >  	void *emergency_sp;		/* pointer to emergency stack */
> >  	u64 data_offset;		/* per cpu data offset */
> >  	s16 hw_cpu_id;			/* Physical processor number */
> > +	u16 trap_save;			/* Used when bad stack is encountered */
> 
> Please read the comment at the top of the struct:
> 
>         /*
>          * Because hw_cpu_id, unlike other paca fields, is accessed
>          * routinely from other CPUs (from the IRQ code), we stick to
>          * read-only (after boot) fields in the first cacheline to
>          * avoid cacheline bouncing.
>          */
> 
> So please move this out of the first cache line.

The only time it's written to is when we detect a bad stack pointer,
after which we promptly panic, so it's essentially read-only (in fact
unused) during normal operation. Plus there's a massive gap there in
paca_struct before the start of the second cacheline.

cheers

-- 
Michael Ellerman
OzLabs, IBM Australia Development Lab

wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)

We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person

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