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

Björn Töpel
2023-01-04T09:13:32Z

"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

Björn