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Date:   Thu, 5 Jan 2023 08:46:07 +0800
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Subject: Re: [PATCH v5 7/9] riscv/kprobe: Prepare detour buffer for optimized
 kprobe
To:     =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>,
        Chen Guokai <chenguokai17@mails.ucas.ac.cn>,
        <paul.walmsley@sifive.com>, <palmer@dabbelt.com>,
        <aou@eecs.berkeley.edu>, <rostedt@goodmis.org>, <mingo@redhat.com>,
        <sfr@canb.auug.org.au>
CC:     <linux-riscv@lists.infradead.org>, <linux-kernel@vger.kernel.org>
References: <20221224114315.850130-1-chenguokai17@mails.ucas.ac.cn>
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From:   "liaochang (A)" <liaochang1@huawei.com>
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在 2023/1/4 17:12, Björn Töpel 写道:
> "liaochang (A)" <liaochang1@huawei.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

Above examples are very clear, i will use these expaned instructions in next revision, thanks.

> 
> 
> Björn

-- 
BR,
Liao, Chang