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

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Message-ID: <YCaebvpeLjY5oRAs@hirez.programming.kicks-ass.net>
Date:   Fri, 12 Feb 2021 16:27:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Jason Gerecke <killertofu@...il.com>, linux-kernel@...r.kernel.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 Fri, Feb 12, 2021 at 09:40:59AM -0500, Steven Rostedt wrote:
> 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.

Right, using -O[01] seems a little daft though. But yeah, that patch is
correct and won't cause harm.

I've queued it for after the next merge window.