Re: [PATCH v2] riscv: KGDB: Do not inline arch_kgdb_breakpoint()

[Palmer Dabbelt]

On Thu, 03 Apr 2025 02:58:50 PDT (-0700), alex@xxxxxxxx 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@xxxxxxxxxx/
> > Fixes: fe89bd2be866 ("riscv: Add KGDB support")
> > Co-developed-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
> > Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
> > Signed-off-by: WangYuli <wangyuli@xxxxxxxxxxxxx>
> > ---
> > 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)
> >   {