linux-kernel - Re: [PATCH V3 1/5] powerpc, perf: Add new BHRB related instructions for POWER8

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Date:	Mon, 22 Apr 2013 11:13:43 +1000
From:	Michael Neuling <mikey@...ling.org>
To:	Michael Ellerman <michael@...erman.id.au>
cc:	Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
	linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 1/5] powerpc, perf: Add new BHRB related instructions for POWER8

Michael Ellerman <michael@...erman.id.au> wrote:

> On Thu, Apr 18, 2013 at 05:56:12PM +0530, Anshuman Khandual wrote:
> > This patch adds new POWER8 instruction encoding for reading
> > the BHRB buffer entries and also clearing it. Encoding for
> > "clrbhrb" instruction is straight forward.
> 
> Which is "clear branch history rolling buffer" ?
> 
> > But "mfbhrbe"
> > encoding involves reading a certain index of BHRB buffer
> > into a particular GPR register.
> 
> And "Move from branch history rolling buffer entry" ?
> 
> > diff --git a/arch/powerpc/include/asm/ppc-opcode.h b/arch/powerpc/include/asm/ppc-opcode.h
> > index 8752bc8..93ae5a1 100644
> > --- a/arch/powerpc/include/asm/ppc-opcode.h
> > +++ b/arch/powerpc/include/asm/ppc-opcode.h
> > @@ -82,6 +82,7 @@
> >  #define	__REGA0_R31	31
> >  
> >  /* sorted alphabetically */
> > +#define PPC_INST_BHRBE			0x7c00025c
> 
> I don't think you really need this, just use the literal value below.

The rest of the defines in this file do this, so Anshuman's right. 

> > @@ -297,6 +298,12 @@
> >  #define PPC_NAP			stringify_in_c(.long PPC_INST_NAP)
> >  #define PPC_SLEEP		stringify_in_c(.long PPC_INST_SLEEP)
> >  
> > +/* BHRB instructions */
> > +#define PPC_CLRBHRB		stringify_in_c(.long 0x7c00035c)
> > +#define PPC_MFBHRBE(r, n)	stringify_in_c(.long PPC_INST_BHRBE | \
> > +						__PPC_RS(r) | \
> > +							(((n) & 0x1f) << 11))
> 
> Why are you not using ___PPC_RB(n) here ?

Actually, this is wrong.  The number field should be 10 bits (0x3ff),
not 5 (0x1f)  Anshuman please fix.  

Mikey
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