[RFC PATCH 1/2]powerpc: foundation code to handle CR5 for local_t

[Benjamin Herrenschmidt]

On Fri, 2014-11-28 at 10:53 +0000, David Laight wrote:
> From: Benjamin Herrenschmidt
> > On Fri, 2014-11-28 at 08:45 +0530, Madhavan Srinivasan wrote:
> > > > Can't we just unconditionally clear at as long as we do that after we've
> > > > saved it ? In that case, it's just a matter for the fixup code to check
> > > > the saved version rather than the actual CR..
> > > >
> > > I use CR bit setting in the interrupt return path to enter the fixup
> > > section search. If we unconditionally clear it, we will have to enter
> > > the fixup section for every kernel return nip right?
> > 
> > As I said above. Can't we look at the saved version ?
> > 
> > IE.
> > 
> >  - On interrupt entry:
> > 
> > 	* Save CR to CCR(r1)
> > 	* clear CR5
> > 
> >  - On exit
> > 
> > 	* Check CCR(r1)'s CR5 field
> > 	* restore CR
> 
> Actually there is no real reason why the 'fixup' can't be done
> during interrupt entry.

Other than if we crash, we get the wrong PC in the log etc... unlikely
but I tend to prefer this. Also if we ever allow something like a local
atomic on a faulting (uesrspace) address, we want a precise PC on entry.

Generally, we have a lot more entry path than exit path, it's easier to
keep the entry path simpler.

Cheers,
Ben.

> 	David
> 
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