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

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Message-ID: <CAAhV-H51AxAjhhge=w4Y=p0XnuW-RyuvA5dPpPc-F0N9YPYHrg@mail.gmail.com>
Date: Mon, 24 Mar 2025 12:09:58 +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

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.

Huacai

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