linux-kernel - Re: [PATCH V2 4/9] tools/perf: Add support to capture and parse raw instruction in objdump

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Date: Tue, 7 May 2024 09:35:22 +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 4/9] tools/perf: Add support to capture and parse raw
 instruction in objdump



Le 06/05/2024 à 14:19, Athira Rajeev a écrit :
> Add support to capture and parse raw instruction in objdump.

What's the purpose of using 'objdump' for reading raw instructions ? 
Can't they be read directly without invoking 'objdump' ? It looks odd to 
me to use objdump to provide readable text and then parse it back.

> Currently, the perf tool infrastructure uses "--no-show-raw-insn" option
> with "objdump" while disassemble. Example from powerpc with this option
> for an instruction address is:

Yes and that makes sense because the purpose of objdump is to provide 
human readable annotations, not to perform automated analysis. Am I 
missing something ?

> 
> Snippet from:
> objdump  --start-address=<address> --stop-address=<address>  -d --no-show-raw-insn -C <vmlinux>
> 
> c0000000010224b4:	lwz     r10,0(r9)
> 
> This line "lwz r10,0(r9)" is parsed to extract instruction name,
> registers names and offset. Also to find whether there is a memory
> reference in the operands, "memory_ref_char" field of objdump is used.
> For x86, "(" is used as memory_ref_char to tackle instructions of the
> form "mov  (%rax), %rcx".
> 
> In case of powerpc, not all instructions using "(" are the only memory
> instructions. Example, above instruction can also be of extended form (X
> form) "lwzx r10,0,r19". Inorder to easy identify the instruction category
> and extract the source/target registers, patch adds support to use raw
> instruction. With raw instruction, macros are added to extract opcode
> and register fields.
> 
> "struct ins_operands" and "struct ins" is updated to carry opcode and
> raw instruction binary code (raw_insn). Function "disasm_line__parse"
> is updated to fill the raw instruction hex value and opcode in newly
> added fields. There is no changes in existing code paths, which parses
> the disassembled code. The architecture using the instruction name and
> present approach is not altered. Since this approach targets powerpc,
> the macro implementation is added for powerpc as of now.
> 
> Example:
> representation using --show-raw-insn in objdump gives result:
> 
> 38 01 81 e8     ld      r4,312(r1)
> 
> Here "38 01 81 e8" is the raw instruction representation. In powerpc,
> this translates to instruction form: "ld RT,DS(RA)" and binary code
> as:
> _____________________________________
> | 58 |  RT  |  RA |      DS       | |
> -------------------------------------
> 0    6     11    16              30 31
> 
> Function "disasm_line__parse" is updated to capture:
> 
> line:    38 01 81 e8     ld      r4,312(r1)
> opcode and raw instruction "38 01 81 e8"
> Raw instruction is used later to extract the reg/offset fields.
> 
> Signed-off-by: Athira Rajeev <atrajeev@...ux.vnet.ibm.com>
> ---