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

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Message-ID: <5174DF58.7020506@linux.vnet.ibm.com>
Date:	Mon, 22 Apr 2013 12:27:28 +0530
From:	Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:	Michael Ellerman <michael@...erman.id.au>
CC:	linuxppc-dev@...abs.org, mikey@...ling.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 1/5] powerpc, perf: Add new BHRB related instructions
 for POWER8

On 04/22/2013 05:11 AM, Michael Ellerman 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" ?
> 

Sure, would add these descriptions in the change log.

>> 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.
> 
>> @@ -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 ?
> 

I would replace __PPC_RS(r) with __PPC_RT(r) which makes more sense from
instruction encoding point of view.

Regards
Anshuman

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