linux-kernel - Re: [PATCH v5 7/9] riscv/kprobe: Prepare detour buffer for optimized kprobe

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Message-ID: <87sfgqr7d4.fsf@all.your.base.are.belong.to.us>
Date:   Wed, 04 Jan 2023 10:12:23 +0100
From:   Björn Töpel <bjorn@...nel.org>
To:     "liaochang (A)" <liaochang1@...wei.com>,
        Chen Guokai <chenguokai17@...ls.ucas.ac.cn>,
        paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, rostedt@...dmis.org, mingo@...hat.com,
        sfr@...b.auug.org.au
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 7/9] riscv/kprobe: Prepare detour buffer for
 optimized kprobe

"liaochang (A)" <liaochang1@...wei.com> writes:

>>> +SYM_ENTRY(optprobe_template_insn, SYM_L_GLOBAL, SYM_A_NONE)
>>> +	/*
>>> +	 * Step3:
>>> +	 * NOPS will be replaced by the probed instruction, at worst case 3 RVC
>>> +	 * and 1 RVI instructions is about to execute out of line.
>>> +	 */
>>> +	nop
>> 
>> A nop here will be either a compressed nop or a non-compressed,
>> depending on the build (C-enabled or not), right? Maybe be explicit to
>> the assembler what you want?
>> 
>
> You are right, if CONFIG_RISCV_ISA_C is disabled, two NOP is enough for 2 RVI execute out of line,
> if CONFIG_RISCV_ISA_C is enabled, it needs eight C.NOP here for the worst case (3 RVC + 1 RVI).
>
> I will use {C}.NOP explicitly for different configure in next revision, thanks.

What I meant was that "nop" can expand to compressed instructions, and
you should be explicit. So you know how it's expanded by the
compiler/assembler.

An example:

$ cat bar.S
	.text
bar:
	nop
	nop
$ riscv64-linux-gnu-gcc -O2 -o bar.o -c bar.S && riscv64-linux-gnu-objdump -M no-aliases -d bar.o

bar.o:     file format elf64-littleriscv


Disassembly of section .text:

0000000000000000 <bar>:
   0:	0001                	c.addi	zero,0
   2:	0001                	c.addi	zero,0


vs

$ cat foo.S
	.text
foo:
	.option norvc
	nop
	nop

$ riscv64-linux-gnu-gcc -O2 -o foo.o -c foo.S && riscv64-linux-gnu-objdump -M no-aliases -d foo.o

foo.o:     file format elf64-littleriscv


Disassembly of section .text:

0000000000000000 <foo>:
   0:	00000013          	addi	zero,zero,0
   4:	00000013          	addi	zero,zero,0


Björn