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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 2 Sep 2022 09:51:17 -0700
Message-ID: <CAHk-=whdvPcHmH5eG+FUrbQw1e-05n__EWobMqbxcCTP7dtZAg@mail.gmail.com>
Subject: Re: [PATCH v2 08/59] x86/build: Ensure proper function alignment
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        x86@kernel.org, Tim Chen <tim.c.chen@linux.intel.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Andrew Cooper <Andrew.Cooper3@citrix.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Johannes Wikner <kwikner@ethz.ch>,
        Alyssa Milburn <alyssa.milburn@linux.intel.com>,
        Jann Horn <jannh@google.com>, "H.J. Lu" <hjl.tools@gmail.com>,
        Joao Moreira <joao.moreira@intel.com>,
        Joseph Nuzman <joseph.nuzman@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Juergen Gross <jgross@suse.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        K Prateek Nayak <kprateek.nayak@amd.com>,
        Eric Dumazet <edumazet@google.com>
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On Fri, Sep 2, 2022 at 6:55 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> --- a/arch/x86/include/asm/linkage.h
> +++ b/arch/x86/include/asm/linkage.h
> @@ -14,9 +14,10 @@
>
>  #ifdef __ASSEMBLY__
>
> -#if defined(CONFIG_X86_64) || defined(CONFIG_X86_ALIGNMENT_16)
> -#define __ALIGN                .p2align 4, 0x90
> -#define __ALIGN_STR    __stringify(__ALIGN)
> +#if CONFIG_FUNCTION_ALIGNMENT == 16
> +#define __ALIGN                        .p2align 4, 0x90
> +#define __ALIGN_STR            __stringify(__ALIGN)
> +#define FUNCTION_ALIGNMENT     16
>  #endif

Ugh.

Why is this conditional on that alignment being 16?

Is it purely because the CONFIG variable was mis-designed, and is the
number of bytes, instead of being the shift? If so, just fix that, and
then do an unconditional

  #define __ALIGN                        .p2align
CONFIG_FUNCTION_ALIGNMENT_SHIFT, 0x90
  #define __ALIGN_STR            __stringify(__ALIGN)

(leave the asm-generic one conditional, since then the condition makes
sense - it's arch-specific).

           Linus