linux-kernel - Re: [PATCH v2 08/59] x86/build: Ensure proper function alignment

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Message-ID: <YyA6gwtAKRwmJR53@hirez.programming.kicks-ass.net>
Date:   Tue, 13 Sep 2022 10:08:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v2 08/59] x86/build: Ensure proper function alignment

On Mon, Sep 12, 2022 at 09:44:05PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 12, 2022 at 10:09:38AM -0400, Linus Torvalds wrote:
> 
> > The patch looks sane, the only thing I worry a bit about is
> > 
> > > +config FUNCTION_ALIGNMENT
> > > +       int
> > > +       default 64 if FUNCTION_ALIGNMENT_64B
> > ..
> > > +       default 0
> > 
> > Is '0' even a valid value then for things like
> 
> At the time I thought to have read that 0 alignment effectively no-ops
> the statement, but now I can't find it in a hurry, happy to make it
> default to 4.

Found it: https://sourceware.org/binutils/docs-2.39/as/Balign.html

7.8 .balign[wl] [abs-expr[, abs-expr[, abs-expr]]]

Pad the location counter (in the current subsection) to a particular
storage boundary. The first expression (which must be absolute) is the
alignment request in bytes. ...  If the expression is omitted then a
default value of 0 is used, effectively disabling alignment
requirements.

(for some raisin google served me a very old binutils document last
night that doesn't have mention the 0 thing)