linux-kernel - Re: [PATCH V2 6/9] tools/perf: Update instruction tracking for powerpc

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Message-ID: <d26d353b-2f89-42fc-a247-4c0a072990de@csgroup.eu>
Date: Tue, 7 May 2024 09:52:50 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Athira Rajeev <atrajeev@...ux.vnet.ibm.com>, "acme@...nel.org"
	<acme@...nel.org>, "jolsa@...nel.org" <jolsa@...nel.org>,
	"adrian.hunter@...el.com" <adrian.hunter@...el.com>, "irogers@...gle.com"
	<irogers@...gle.com>, "namhyung@...nel.org" <namhyung@...nel.org>,
	"segher@...nel.crashing.org" <segher@...nel.crashing.org>
CC: "linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"maddy@...ux.ibm.com" <maddy@...ux.ibm.com>, "kjain@...ux.ibm.com"
	<kjain@...ux.ibm.com>, "disgoel@...ux.vnet.ibm.com"
	<disgoel@...ux.vnet.ibm.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "akanksha@...ux.ibm.com"
	<akanksha@...ux.ibm.com>
Subject: Re: [PATCH V2 6/9] tools/perf: Update instruction tracking for
 powerpc



Le 06/05/2024 à 14:19, Athira Rajeev a écrit :
> Add instruction tracking function "update_insn_state_powerpc" for
> powerpc. Example sequence in powerpc:
> 
> ld      r10,264(r3)
> mr      r31,r3
> <<after some sequence>
> ld      r9,312(r31)

Your approach looks fragile.

mr is a simplified instruction which in fact is  "or r31, r3, r3"

By the way, the compiler sometimes does it different, like below:

lwz	r10,264(r3)
addi	r31, r3, 312
lwz	r9, 0(r31)

And what about sequences with lwzu ?

> 
> Consider ithe sample is pointing to: "ld r9,312(r31)".
> Here the memory reference is hit at "312(r31)" where 312 is the offset
> and r31 is the source register. Previous instruction sequence shows that
> register state of r3 is moved to r31. So to identify the data type for r31
> access, the previous instruction ("mr") needs to be tracked and the
> state type entry has to be updated. Current instruction tracking support
> in perf tools infrastructure is specific to x86. Patch adds this for
> powerpc and adds "mr" instruction to be tracked.
> 
> Signed-off-by: Athira Rajeev <atrajeev@...ux.vnet.ibm.com>