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Date: Mon, 23 Jul 2012 19:36:11 +1000
From: Paul Mackerras <paulus@samba.org>
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Subject: Re: [PATCH -V3 09/11] arch/powerpc: Use 50 bits of VSID in slbmte
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On Mon, Jul 23, 2012 at 01:51:49PM +0530, Aneesh Kumar K.V wrote:
> Paul Mackerras <paulus@samba.org> writes:
> 
> > On Mon, Jul 09, 2012 at 06:43:39PM +0530, Aneesh Kumar K.V wrote:
> >> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
> >> 
> >> Increase the number of valid VSID bits in slbmte instruction.
> >> We will use the new bits when we increase valid VSID bits.
> >> 
> >> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> >> ---
> >>  arch/powerpc/mm/slb_low.S |    4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/arch/powerpc/mm/slb_low.S b/arch/powerpc/mm/slb_low.S
> >> index c355af6..c1fc81c 100644
> >> --- a/arch/powerpc/mm/slb_low.S
> >> +++ b/arch/powerpc/mm/slb_low.S
> >> @@ -226,7 +226,7 @@ _GLOBAL(slb_allocate_user)
> >>   */
> >>  slb_finish_load:
> >>  	ASM_VSID_SCRAMBLE(r10,r9,256M)
> >> -	rldimi	r11,r10,SLB_VSID_SHIFT,16	/* combine VSID and flags */
> >> +	rldimi	r11,r10,SLB_VSID_SHIFT,2	/* combine VSID and flags */
> >
> > You can't do that without either changing ASM_VSID_SCRAMBLE or masking
> > the VSID it generates to 36 bits, since the logic in ASM_VSID_SCRAMBLE
> > can leave non-zero bits in the high 28 bits of the result.  Similarly
> > for the 1T case.
> >
> 
> How about change ASM_VSID_SCRAMBLE to clear the high bits ? That would
> also make it close to vsid_scramble()

One more instruction in a hot path - I'd rather not.  How about
changing the rldimi instruction to:
	rldimi	r11,r10,SLB_VSID_SHIFT,(64-SLB_VSID_SHIFT-VSID_BITS_256M)

and similarly for the 1T case.  That will give the proper masking
when you change VSID_BITS_256M.

Paul.