Re: [PATCH 1/2] powerpc: add Book E support to 64-bit hibernation

[Scott Wood <scottwood@freescale.com>]

On 06/17/2013 12:54:32 AM, Wang Dongsheng-B40534 wrote:
>=20
>=20
> > -----Original Message-----
> > From: Wood Scott-B07421
> > Sent: Friday, June 14, 2013 12:51 AM
> > To: Wang Dongsheng-B40534
> > Cc: Wood Scott-B07421; benh@kernel.crashing.org;
> > johannes@sipsolutions.net; anton@enomsg.org; =20
> galak@kernel.crashing.org;
> > linuxppc-dev@lists.ozlabs.org
> > Subject: Re: [PATCH 1/2] powerpc: add Book E support to 64-bit
> > hibernation
> >
> > On 06/13/2013 04:55:43 AM, Wang Dongsheng-B40534 wrote:
> > > > > +#else
> > > > > +	/* Save SPRGs */
> > > > > +	RESTORE_SPRG(0)
> > > > > +	RESTORE_SPRG(1)
> > > > > +	RESTORE_SPRG(2)
> > > > > +	RESTORE_SPRG(3)
> > > > > +	RESTORE_SPRG(4)
> > > > > +	RESTORE_SPRG(5)
> > > > > +	RESTORE_SPRG(6)
> > > > > +	RESTORE_SPRG(7)
> > > >
> > > > Why do we need this on book3e and not on book3s?
> > > >
> > > Book3e: SPRG1 used save paca, SPRG2 be defined
> > > SPRN_SPRG_TLB_EXFRAME,...
> > > I think those register should be save, even now some SPRG =20
> register not
> > > be use.
> >
> > Are those expected/allowed to change as a result of the restore?
> >
> Those registers are used by software, some allowed to change.
> Exception handling is used in some registers, see exception-64e.h
> These registers can be modified and saved.

I really doubt the exception scratch registers need to be saved -- =20
we're not trying to restore into the middle of an exception =20
prolog/epilog.

book3s has the PACA as well and they don't save it.  Don't we rely on =20
things like boot-time memory allocations happening in the same place =20
when we resume?  extlb is part of the PACA, so the same applies.

Granted, this isn't performance critical so it may seem better to =20
save/restore just in case, but there's value in not unnecessarily =20
deviating from what book3s does.

-Scott=