linux-kernel - Re: [PATCH v2] riscv: KGDB: Do not inline arch_kgdb

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Message-ID: <mhng-69513841-5068-441d-be8f-2aeebdc56a08@palmer-ri-x1c9a>
Date: Thu, 03 Apr 2025 09:14:43 -0700 (PDT)
From: Palmer Dabbelt <palmer@...belt.com>
To: alex@...ti.fr
CC: wangyuli@...ontech.com, Paul Walmsley <paul.walmsley@...ive.com>,
  aou@...s.berkeley.edu, chenhuacai@...nel.org, linux-riscv@...ts.infradead.org,
  linux-kernel@...r.kernel.org, vincent.chen@...ive.com, palmerdabbelt@...gle.com, zhanjun@...ontech.com,
  niecheng1@...ontech.com, guanwentao@...ontech.com, chenhuacai@...ngson.cn
Subject:     Re: [PATCH v2] riscv: KGDB: Do not inline arch_kgdb_breakpoint()

On Thu, 03 Apr 2025 02:58:50 PDT (-0700), alex@...ti.fr wrote:
> Hi WangYuli,
>
> On 02/04/2025 09:42, WangYuli wrote:
>> The arch_kgdb_breakpoint() function defines the kgdb_compiled_break
>> symbol using inline assembly.
>>
>> There's a potential issue where the compiler might inline
>> arch_kgdb_breakpoint(), which would then define the kgdb_breakinst
>
>
> I guess you meant kgdb_compiled_break.
>
>
>> symbol multiple times, leading to fail to link vmlinux.o.
>> This isn't merely a potential compilation problem. The intent here
>> is to determine the global symbol address of kgdb_compiled_break,
>> and if this function is inlined multiple times, it would logically
>> be a grave error.
>>
>> Link: https://lore.kernel.org/all/4b4187c1-77e5-44b7-885f-d6826723dd9a@sifive.com/
>> Fixes: fe89bd2be866 ("riscv: Add KGDB support")
>> Co-developed-by: Huacai Chen <chenhuacai@...ngson.cn>
>> Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
>> Signed-off-by: WangYuli <wangyuli@...ontech.com>
>> ---
>> Changelog:
>>   *v1->v2: Add the missing __ASSEMBLY__ check and substitute
>> ".option rvc/norvc" with ".option push/pop".
>> ---
>>   arch/riscv/include/asm/kgdb.h | 9 +--------
>>   arch/riscv/kernel/kgdb.c      | 8 ++++++++
>>   2 files changed, 9 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/riscv/include/asm/kgdb.h b/arch/riscv/include/asm/kgdb.h
>> index 46677daf708b..d9f6a8fc387f 100644
>> --- a/arch/riscv/include/asm/kgdb.h
>> +++ b/arch/riscv/include/asm/kgdb.h
>> @@ -19,16 +19,9 @@
>>
>>   #ifndef	__ASSEMBLY__
>>
>> +extern void arch_kgdb_breakpoint(void);
>
>
> The "extern" is not needed here.
>
>
>>   extern unsigned long kgdb_compiled_break;
>>
>> -static inline void arch_kgdb_breakpoint(void)
>> -{
>> -	asm(".global kgdb_compiled_break\n"
>> -	    ".option norvc\n"
>> -	    "kgdb_compiled_break: ebreak\n"
>> -	    ".option rvc\n");
>> -}
>> -
>>   #endif /* !__ASSEMBLY__ */
>>
>>   #define DBG_REG_ZERO "zero"
>> diff --git a/arch/riscv/kernel/kgdb.c b/arch/riscv/kernel/kgdb.c
>> index 2e0266ae6bd7..5873d3970360 100644
>> --- a/arch/riscv/kernel/kgdb.c
>> +++ b/arch/riscv/kernel/kgdb.c
>> @@ -254,6 +254,14 @@ void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long pc)
>>   	regs->epc = pc;
>>   }
>>
>> +noinline void arch_kgdb_breakpoint(void)
>> +{
>> +	asm(".global kgdb_compiled_break\n"
>> +	    ".option push\n"
>
>
> Here you forgot .option norvc. But this fix as mentioned by Samuel
> should be in a separate patch.

I don't think we actually need either here: we're just looking at the 
address of kgdb_compiled_break, so it's fine if it ends up as a 
c.ebreak.

> Thanks,
>
> Alex
>
>
>> +	    "kgdb_compiled_break: ebreak\n"
>> +	    ".option pop\n");
>> +}
>> +
>>   void kgdb_arch_handle_qxfer_pkt(char *remcom_in_buffer,
>>   				char *remcom_out_buffer)
>>   {