linux-kernel - Re: [PATCH 1/2] arm64: jump_label: mark arguments as const to satisfy asm constraints

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Date:   Thu, 6 Oct 2022 16:46:16 +0800
From:   Jisheng Zhang <jszhang@...nel.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] arm64: jump_label: mark arguments as const to
 satisfy asm constraints

On Thu, Oct 06, 2022 at 10:17:44AM +0200, Ard Biesheuvel wrote:
> On Thu, 6 Oct 2022 at 10:05, Jisheng Zhang <jszhang@...nel.org> wrote:
> >
> > Inspired by x86 commit 864b435514b2("x86/jump_label: Mark arguments as
> > const to satisfy asm constraints"), mark arch_static_branch()'s and
> > arch_static_branch_jump()'s arguments as const to satisfy asm
> > constraints. And Steven in [1] also pointed out that "The "i"
> > constraint needs to be a constant."
> >
> > Tested with building a simple external kernel module with "O0".
> >
> > [1]https://lore.kernel.org/all/20210212094059.5f8d05e8@gandalf.local.home/
> >
> > Signed-off-by: Jisheng Zhang <jszhang@...nel.org>
> > ---
> >  arch/arm64/include/asm/jump_label.h | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/jump_label.h b/arch/arm64/include/asm/jump_label.h
> > index cea441b6aa5d..48ddc0f45d22 100644
> > --- a/arch/arm64/include/asm/jump_label.h
> > +++ b/arch/arm64/include/asm/jump_label.h
> > @@ -15,8 +15,8 @@
> >
> >  #define JUMP_LABEL_NOP_SIZE            AARCH64_INSN_SIZE
> >
> > -static __always_inline bool arch_static_branch(struct static_key *key,
> > -                                              bool branch)
> > +static __always_inline bool arch_static_branch(struct static_key * const key,
> > +                                              const bool branch)
> >  {
> >         asm_volatile_goto(
> >                 "1:     nop                                     \n\t"
> 
> Is this still necessary if we specify the constraints in a more
> reasonable manner:
> 
>  "      .quad           %c0 - . + %1            \n\t"
>  :  :  "i"(key), "i"(branch) :  : l_yes);

Just tried this locally with the simple external kernel module, the
asm operand 0 probably does not match constraints can still be
reproduced with "O0".

Thanks

> 
> instead of the horrid hack with the char* cast and using a bool as an
> array index?
> 
> 
> 
> 
> > @@ -32,8 +32,8 @@ static __always_inline bool arch_static_branch(struct static_key *key,
> >         return true;
> >  }
> >
> > -static __always_inline bool arch_static_branch_jump(struct static_key *key,
> > -                                                   bool branch)
> > +static __always_inline bool arch_static_branch_jump(struct static_key * const key,
> > +                                                   const bool branch)
> >  {
> >         asm_volatile_goto(
> >                 "1:     b               %l[l_yes]               \n\t"
> > --
> > 2.37.2
> >