Re: [PATCH v3 2/2] powerpc/8xx: use _PAGE_RO instead of _PAGE_RW

Joakim Tjernlund
2015-01-06T13:05:25Z

On Tue, 2015-01-06 at 08:03 +0100, leroy christophe wrote:
> Le 05/01/2015 19:12, Joakim Tjernlund a =E9crit :
> > On Mon, 2014-12-22 at 11:14 +0100, Christophe Leroy wrote:
> > > On powerpc 8xx, in TLB entries, 0x400 bit is set to 1 for read-only p=
ages
> > > and is set to 0 for RW pages. So we should use _PAGE_RO instead of _P=
AGE_RW
> > >=20
> > > Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
> > Hi Christophe, been meaning to look over all you recent 8xx MMU/TLB pat=
ches
> > but got so little time :(
> >=20
> > This is very cool (not sure if there will be a performance gain) but .=
.
> I think every saved cycle is worth it.
> Before I did any modification:
> * ITLBMiss was 28 instructions.
> * DTLBMiss was 32 instructions.
> Now, (No MODULES, no CPU6, no CPU15):
> * ITLBMiss is 15 instructions
> * DTLBMiss is 24 instructions

I only meant this patch, sorry for not being explicit about that.

> > >=20
> > >=20
> > > diff --git a/arch/powerpc/include/asm/pgtable-ppc32.h b/arch/powerpc/=
include/asm/pgtable-ppc32.h
> > > index caf094a..b4e0c3b 100644
> > > --- a/arch/powerpc/include/asm/pgtable-ppc32.h
> > > +++ b/arch/powerpc/include/asm/pgtable-ppc32.h
> > > @@ -178,9 +178,10 @@ static inline unsigned long pte_update(pte_t *p,
> > > andc %1,%0,%5\n\
> > > or %1,%1,%6\n\
> > > /* 0x200 =3D=3D Extended encoding, bit 22 */ \
> > > - /* Bit 22 has to be 1 if neither _PAGE_USER nor _PAGE_RW are =
set */ \
> > > + /* Bit 22 has to be 1 when _PAGE_USER is unset and _PAGE_RO i=
s set */ \
> > > rlwimi %1,%1,32-2,0x200\n /* get _PAGE_USER */ \
> > > - rlwinm %3,%1,32-1,0x200\n /* get _PAGE_RW */ \
> > > + rlwinm %3,%1,32-1,0x200\n /* get _PAGE_RO */ \
> > > + xori %3,%3,0x200\n \
> > > or %1,%3,%1\n\
> > > xori %1,%1,0x200\n"
> > > " stwcx. %1,0,%4\n\
> > ... here I expected to loose the existing xori insn instead of adding o=
ne?
> >=20
> >=20
> Well, I could have xored the PAGE_USER bit instead, but in that case, it
> is not anymore an 'or' but an 'and' that has to be performed between the
> bits, and then all other bits must be set to 1, or the result of the 'and=
' shall be inserted using 'rlwimi'. So it would be more modifications than =
just adding an xori, and not less instructions.
>=20

I see, thanks

Jocke=