linux-kernel - Re: [PATCH 02/10][v6] powerpc/Power7: detect load/store instructions

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Date:	Wed, 16 Oct 2013 08:39:29 -0700
From:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
To:	Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:	David Laight <David.Laight@...LAB.COM>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Michael Ellerman <michaele@....ibm.com>,
	linux-kernel@...r.kernel.org,
	Stephane Eranian <eranian@...gle.com>, linuxppc-dev@...abs.org,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH 02/10][v6] powerpc/Power7: detect load/store instructions

Anshuman Khandual [khandual@...ux.vnet.ibm.com] wrote:
| On 10/16/2013 01:55 PM, David Laight wrote:
| >> Implement instr_is_load_store_2_06() to detect whether a given instruction
| >> is one of the fixed-point or floating-point load/store instructions in the
| >> POWER Instruction Set Architecture v2.06.
| > ...
| 
| The op code encoding is dependent on the ISA version ? Does the basic load
| and store instructions change with newer ISA versions ?

TBH, I don't know whether the encoding is dependent on the ISA version.

We need this for a very narrow/specific purpose on Power7 _and_ did not
want to set up expectations that it will work with all versions. Hence
the horribly named function :-)

| BTW we have got a
| newer version for the ISA "PowerISA_V2.07_PUBLIC.pdf" here at power.org
| 
| https://www.power.org/documentation/power-isa-version-2-07/

Yes, but on Power8 there is a bit in the SIER that tells us whether it
is a load or store instruction. We use that and don't need to determine
in software.

Power7 does not have such a bit and we need this only for Power7. We are
not targetting this "memory hierarchy" feature for Power6 or older processors.

| 
| Does not sound like a good idea to analyse the instructions with functions
| names which specify ISA version number. Besides, this function does not
| belong to specific processor or platform. It has to be bit generic.
| 
| >> +int instr_is_load_store_2_06(const unsigned int *instr)
| >> +{
| >> +	unsigned int op, upper, lower;
| >> +
| >> +	op = instr_opcode(*instr);
| >> +
| >> +	if ((op >= 32 && op <= 58) || (op == 61 || op == 62))
| >> +		return true;
| >> +
| >> +	if (op != 31)
| >> +		return false;
| >> +
| >> +	upper = op >> 5;
| >> +	lower = op & 0x1f;
| >> +
| >> +	/* Short circuit as many misses as we can */
| >> +	if (lower < 3 || lower > 23)
| >> +		return false;
| >> +
| >> +	if (lower == 3) {
| >> +		if (upper >= 16)
| >> +			return true;
| >> +
| >> +		return false;
| >> +	}
| >> +
| >> +	if (lower == 7 || lower == 12)
| >> +		return true;
| >> +
| >> +	if (lower >= 20) /* && lower <= 23 (implicit) */
| >> +		return true;
| >> +
| >> +	return false;
| >> +}
| > 
| > I can't help feeling the code could do with some comments about
| > which actual instructions are selected where.
| 
| Yeah, I agree. At least which category of load-store instructions are
| getting selected in each case.

Like I mentioned in the other message, how about adding a couple
of lines in the function header ?

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