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

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

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On Fri, 2007-04-20 at 09:17 +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2007-04-16 at 01:23 -0500, Olof Johansson wrote:
> > Save the trap number in the case of getting a bad stack in an exception
> > handler. It is sometimes useful to know what exception it was that caused
> > this to happen. Without this, no trap is reported.
> > 
> > 
> > Signed-off-by: Olof Johansson <olof@lixom.net>
> > 
> > Index: linux-2.6/arch/powerpc/kernel/asm-offsets.c
> > ===================================================================
> > --- linux-2.6.orig/arch/powerpc/kernel/asm-offsets.c
> > +++ linux-2.6/arch/powerpc/kernel/asm-offsets.c
> > @@ -139,6 +139,7 @@ int main(void)
> >  	DEFINE(PACA_SYSTEM_TIME, offsetof(struct paca_struct, system_time));
> >  	DEFINE(PACA_SLBSHADOWPTR, offsetof(struct paca_struct, slb_shadow_ptr));
> >  	DEFINE(PACA_DATA_OFFSET, offsetof(struct paca_struct, data_offset));
> > +	DEFINE(PACA_TRAP_SAVE, offsetof(struct paca_struct, trap_save));
> >  
> >  	DEFINE(SLBSHADOW_STACKVSID,
> >  	       offsetof(struct slb_shadow, save_area[SLB_NUM_BOLTED - 1].vsid));
> > Index: linux-2.6/include/asm-powerpc/paca.h
> > ===================================================================
> > --- linux-2.6.orig/include/asm-powerpc/paca.h
> > +++ linux-2.6/include/asm-powerpc/paca.h
> > @@ -81,6 +81,8 @@ struct paca_struct {
> >  	u64 exslb[10];		/* used for SLB/segment table misses
> >   				 * on the linear mapping */
> >  
> > +	u64 trap_save;			/* Used when bad stack is encountered */
> 
> Can you put it elsewhere in the PACA (around less frequently used
> bits) ? This is a pretty hot area of the PACA...

It's only set in the bad_stack case though.

You should put it after cpu_start, there's a bit hole there, and
trap_save is readonly during normal operation, so it shouldn't be a
performance hit.

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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