linux-kernel - Re: [PATCH v2] LoongArch: KGDB: Rework arch_kgdb

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Message-ID: <545ed081-bec3-395c-e0dd-a45146e00cd1@loongson.cn>
Date: Mon, 24 Mar 2025 09:42:38 +0800
From: Jinyang He <hejinyang@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>, WangYuli <wangyuli@...ontech.com>
Cc: kernel@...0n.name, guanwentao@...ontech.com, wentao@...ontech.com,
 loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
 zhoubinbin@...ngson.cn, lihui@...ngson.cn, rdunlap@...radead.org,
 chenhuacai@...ngson.cn, zhanjun@...ontech.com, niecheng1@...ontech.com,
 Tiezhu Yang <yangtiezhu@...ngson.cn>
Subject: Re: [PATCH v2] LoongArch: KGDB: Rework arch_kgdb_breakpoint()
 implementation

On 2025-03-22 20:51, Huacai Chen wrote:

> Hi, Tiezhu & Jinyang,
>
> On Fri, Mar 21, 2025 at 10:11 PM WangYuli <wangyuli@...ontech.com> wrote:
>> The arch_kgdb_breakpoint() function defines the kgdb_breakinst
>> symbol using inline assembly.
>>
>> There's a potential issue where the compiler might inline
>> arch_kgdb_breakpoint(), which would then define the kgdb_breakinst
>> symbol multiple times, leading to a linker error.
>>
>> To prevent this, declare arch_kgdb_breakpoint() as noline.
>>
>> Fix follow error with LLVM-19 *only* when LTO_CLANG_FULL:
>>      LD      vmlinux.o
>>    ld.lld-19: error: ld-temp.o <inline asm>:3:1: symbol 'kgdb_breakinst' is already defined
>>    kgdb_breakinst: break 2
>>    ^
>>
>> Additionally, remove "nop" here because it's meaningless for LoongArch
>> here.
>>
>> Fixes: e14dd076964e ("LoongArch: Add basic KGDB & KDB support")
>> Co-developed-by: Winston Wen <wentao@...ontech.com>
>> Signed-off-by: Winston Wen <wentao@...ontech.com>
>> Co-developed-by: Wentao Guan <guanwentao@...ontech.com>
>> Signed-off-by: Wentao Guan <guanwentao@...ontech.com>
>> Co-developed-by: Huacai Chen <chenhuacai@...ngson.cn>
>> Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
>> Tested-by: Yuli Wang <wangyuli@...ontech.com>
>> Tested-by: Binbin Zhou <zhoubinbin@...ngson.cn>
>> Signed-off-by: Yuli Wang <wangyuli@...ontech.com>
>> ---
>> Changelog:
>>   *v1->v2:
>>      1. Drop the nop which is no effect for LoongArch here.
>>      2. Add "STACK_FRAME_NON_STANDARD" for arch_kgdb_breakpoint() to
>> avoid the objtool warning.
>> ---
>>   arch/loongarch/kernel/kgdb.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/loongarch/kernel/kgdb.c b/arch/loongarch/kernel/kgdb.c
>> index 445c452d72a7..38bd0561d7d5 100644
>> --- a/arch/loongarch/kernel/kgdb.c
>> +++ b/arch/loongarch/kernel/kgdb.c
>> @@ -224,13 +224,13 @@ void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long pc)
>>          regs->csr_era = pc;
>>   }
>>
>> -void arch_kgdb_breakpoint(void)
>> +noinline void arch_kgdb_breakpoint(void)
>>   {
>>          __asm__ __volatile__ (                  \
>>                  ".globl kgdb_breakinst\n\t"     \
>> -               "nop\n"                         \
>>                  "kgdb_breakinst:\tbreak 2\n\t"); /* BRK_KDB = 2 */
>>   }
>> +STACK_FRAME_NON_STANDARD(arch_kgdb_breakpoint);
> Is there a better solution than STACK_FRAME_NON_STANDARD()? In the
> past we can use annotate_reachable() in arch_kgdb_breakpoint(), but
> annotate_reachable() is no longer exist.

Maybe we can parse the imm-code of `break` and set diffrent insn_type in 
objtool.
The BRK_KDB imply the PC will go head, while the BRK_BUG imply PC stop.
(arch/loongarch/include/uapi/asm/break.h)

Tiezhu, how do you think?


>
> Huacai
>
>>   /*
>>    * Calls linux_debug_hook before the kernel dies. If KGDB is enabled,
>> --
>> 2.49.0
>>