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

[Athira Rajeev]

> On 7 May 2024, at 3:05 PM, Christophe Leroy <christophe.leroy at csgroup.eu> wrote:
> 
> 
> 
> 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.

Hi Christophe,

Thanks for your review comments.

Current implementation for data type profiling on X86 uses "objdump" tool to get the disassembled code.
And then the objdump result lines are parsed to get the instruction name and register fields. The initial patchset I posted to enable the data type profiling feature in powerpc was using the same way by getting disassembled code from objdump and parsing the disassembled lines. But in V2, we are introducing change for powerpc to use "raw instruction" and fetch opcode, reg fields from the raw instruction.

I tried to explain below that current objdump uses option "--no-show-raw-insn" which doesn't capture raw instruction.  So to capture raw instruction, V2 patchset has changes to use default option "--show-raw-insn" and get the raw instruction [ for powerpc ] along with human readable annotation [ which is used by other archs ]. Since perf tool already has objdump implementation in place, I went in the direction to enhance it to use "--show-raw-insn" for powerpc purpose.

But as you mentioned, we can directly read raw instruction without using "objdump" tool.
perf has support to read object code. The dso open/read utilities and helper functions are already present in "util/dso.c" And "dso__data_read_offset" function reads data from dso file offset. We can use these functions and I can make changes to directly read binary instruction without using objdump.

Namhyung, Arnaldo, Christophe
Looking for your valuable feedback on this approach. Please suggest if this approach looks fine


Thanks
Athira
> 
>> 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 at linux.vnet.ibm.com>
>> ---