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

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Message-ID: <CAAhV-H7vg=Vh40phqu4btsZU39o+Rfo0mAsrKF9P0nX=UvaO_g@mail.gmail.com>
Date: Mon, 24 Mar 2025 12:53:22 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Jinyang He <hejinyang@...ngson.cn>
Cc: WangYuli <wangyuli@...ontech.com>, 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 Mon, Mar 24, 2025 at 12:31 PM Jinyang He <hejinyang@...ngson.cn> wrote:
>
> On 2025-03-24 12:09, Huacai Chen wrote:
>
> > Hi, Jinyang,
> >
> > On Mon, Mar 24, 2025 at 9:42 AM Jinyang He <hejinyang@...ngson.cn> wrote:
> >> 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?
> > Touching objtool may be a little complicated, is ANNOTATE_REACHABLE()
> > the successor of annotate_reachable()? I tried
> > ANNOTATE_REACHABLE(kgdb_breakinst) but there was still a warning.
> Should it annotate to the next insn of `break`?
> Like,
>
>      kgdb_breakinst:
>      break 2
>      another_label:
>
> ANNOTATE_REACHABLE(another_label)
Still no help.

Huacai

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