Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>
To:     "liaochang (A)" <liaochang1@huawei.com>,
        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
Subject: Re: [PATCH v5 7/9] riscv/kprobe: Prepare detour buffer for
 optimized kprobe
In-Reply-To: <088ed826-59a8-37a3-401e-2ed022a4a097@huawei.com>
References: <20221224114315.850130-1-chenguokai17@mails.ucas.ac.cn>
 <20221224114315.850130-8-chenguokai17@mails.ucas.ac.cn>
 <87r0wcvmn1.fsf@all.your.base.are.belong.to.us>
 <088ed826-59a8-37a3-401e-2ed022a4a097@huawei.com>
Date:   Wed, 04 Jan 2023 10:12:23 +0100
Message-ID: <87sfgqr7d4.fsf@all.your.base.are.belong.to.us>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

"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 R=
VC
>>> +	 * and 1 RVI instructions is about to execute out of line.
>>> +	 */
>>> +	nop
>>=20
>> 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?
>>=20
>
> 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, t=
hanks.

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


Bj=C3=B6rn