linux-kernel - Re: [PATCH v1] objtool/LoongArch: Mark special atomic instruction as INSN

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Message-ID: <CAHirt9jbY4yX84Kfzax71rCvPY--SQpDKrjqWsDCHMVnuUuD0g@mail.gmail.com>
Date: Tue, 16 Sep 2025 17:02:19 +0800
From: WANG Rui <wangrui@...ngson.cn>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Huacai Chen <chenhuacai@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	rust-for-linux@...r.kernel.org, loongarch@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] objtool/LoongArch: Mark special atomic instruction as
 INSN_BUG type

On Tue, Sep 16, 2025 at 2:11 PM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>
> When compiling with LLVM and CONFIG_RUST is set, there exists the
> following objtool warning:
>
>   rust/compiler_builtins.o: warning: objtool: __rust__unordsf2(): unexpected end of section .text.unlikely.
>
> objdump shows that the end of section .text.unlikely is a atomic
> instruction:
>
>   amswap.w        $zero, $ra, $zero
>
> According to the LoongArch Reference Manual, if the amswap.w atomic
> memory access instruction has the same register number as rd and rj,
> the execution will trigger an Instruction Non-defined Exception, so
> mark the above instruction as INSN_BUG type to fix the warning.

LLVM lowers `llvm.trap()` to `amswap.w R0, R1, R0`. For x86, it lowers
to `ud2`, and objtool marks it as INSN_BUG.

https://github.com/llvm/llvm-project/blob/788a25a0f71bfa5e5e1c12ad093993b115d10e7a/llvm/lib/Target/LoongArch/LoongArchInstrInfo.td#L1376-L1381

LLVM and GCC handle this differently, GCC lowers it to a break
instruction. Since the break instruction has other uses, can objtool
mark all break instructions as INSN_BUG? or it should mark different
types based on the break immediate code.

https://github.com/torvalds/linux/blob/46a51f4f5edade43ba66b3c151f0e25ec8b69cb6/tools/objtool/arch/loongarch/decode.c#L315-L317

>
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> ---
>  tools/arch/loongarch/include/asm/inst.h | 12 ++++++++++++
>  tools/objtool/arch/loongarch/decode.c   | 21 +++++++++++++++++++++
>  2 files changed, 33 insertions(+)
>
> diff --git a/tools/arch/loongarch/include/asm/inst.h b/tools/arch/loongarch/include/asm/inst.h
> index c25b5853181d..d68fad63c8b7 100644
> --- a/tools/arch/loongarch/include/asm/inst.h
> +++ b/tools/arch/loongarch/include/asm/inst.h
> @@ -51,6 +51,10 @@ enum reg2i16_op {
>         bgeu_op         = 0x1b,
>  };
>
> +enum reg3_op {
> +       amswapw_op      = 0x70c0,
> +};
> +
>  struct reg0i15_format {
>         unsigned int immediate : 15;
>         unsigned int opcode : 17;
> @@ -96,6 +100,13 @@ struct reg2i16_format {
>         unsigned int opcode : 6;
>  };
>
> +struct reg3_format {
> +       unsigned int rd : 5;
> +       unsigned int rj : 5;
> +       unsigned int rk : 5;
> +       unsigned int opcode : 17;
> +};
> +
>  union loongarch_instruction {
>         unsigned int word;
>         struct reg0i15_format   reg0i15_format;
> @@ -105,6 +116,7 @@ union loongarch_instruction {
>         struct reg2i12_format   reg2i12_format;
>         struct reg2i14_format   reg2i14_format;
>         struct reg2i16_format   reg2i16_format;
> +       struct reg3_format      reg3_format;
>  };
>
>  #define LOONGARCH_INSN_SIZE    sizeof(union loongarch_instruction)
> diff --git a/tools/objtool/arch/loongarch/decode.c b/tools/objtool/arch/loongarch/decode.c
> index b6fdc68053cc..707f339b1840 100644
> --- a/tools/objtool/arch/loongarch/decode.c
> +++ b/tools/objtool/arch/loongarch/decode.c
> @@ -278,6 +278,25 @@ static bool decode_insn_reg2i16_fomat(union loongarch_instruction inst,
>         return true;
>  }
>
> +static bool decode_insn_reg3_fomat(union loongarch_instruction inst,
> +                                  struct instruction *insn)
> +{
> +       switch (inst.reg3_format.opcode) {
> +       case amswapw_op:
> +               if (inst.reg3_format.rd == LOONGARCH_GPR_ZERO &&
> +                   inst.reg3_format.rk == LOONGARCH_GPR_RA &&
> +                   inst.reg3_format.rj == LOONGARCH_GPR_ZERO) {
> +                       /* amswap.w $zero, $ra, $zero */
> +                       insn->type = INSN_BUG;
> +               }
> +               break;
> +       default:
> +               return false;
> +       }
> +
> +       return true;
> +}
> +
>  int arch_decode_instruction(struct objtool_file *file, const struct section *sec,
>                             unsigned long offset, unsigned int maxlen,
>                             struct instruction *insn)
> @@ -309,6 +328,8 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
>                 return 0;
>         if (decode_insn_reg2i16_fomat(inst, insn))
>                 return 0;
> +       if (decode_insn_reg3_fomat(inst, insn))
> +               return 0;
>
>         if (inst.word == 0)
>                 insn->type = INSN_NOP;
> --
> 2.42.0
>
>