linux-kernel - Re: [PATCH] RFC: x86/jump_label: Mark arguments as const to satisfy asm constraints

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Message-ID: <20210212094059.5f8d05e8@gandalf.local.home>
Date:   Fri, 12 Feb 2021 09:40:59 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jason Gerecke <killertofu@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Jason Gerecke <jason.gerecke@...om.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH] RFC: x86/jump_label: Mark arguments as const to satisfy
 asm constraints

On Thu, 11 Feb 2021 13:48:48 -0800
Jason Gerecke <killertofu@...il.com> wrote:

> When compiling an external kernel module with `-O0` or `-O1`, the following
> compile error may be reported:
> 
>     ./arch/x86/include/asm/jump_label.h:25:2: error: impossible constraint in ‘asm’
>        25 |  asm_volatile_goto("1:"
>           |  ^~~~~~~~~~~~~~~~~
> 
> It appears that these lower optimization levels prevent GCC from detecting
> that the key/branch arguments can be treated as constants and used as
> immediate operands. To work around this, explicitly add the `const` label.

Yes this makes sense. The "i" constraint needs to be a constant.

Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

-- Steve

> 
> Signed-off-by: Jason Gerecke <jason.gerecke@...om.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ard Biesheuvel <ardb@...nel.org>
> ---
> Marked RFC since I'm not familiar with this subsystem or the asm blocks that
> are impacted. Extra-close inspection would be appreciated.
> 
>  arch/x86/include/asm/jump_label.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
> index 06c3cc22a058..7f2006645d84 100644
> --- a/arch/x86/include/asm/jump_label.h
> +++ b/arch/x86/include/asm/jump_label.h
> @@ -20,7 +20,7 @@
>  #include <linux/stringify.h>
>  #include <linux/types.h>
>  
> -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:"
>  		".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
> @@ -36,7 +36,7 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
>  	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:"
>  		".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"