linux-kernel - Re: [PATCH v5 1/1] x86/cpu: Make sure flag_is_changeable

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Message-ID: <0EF5D1F5-6D70-4BEA-B5E9-458836B80221@zytor.com>
Date: Fri, 08 Nov 2024 18:28:54 +0100
From: "H. Peter Anvin" <hpa@...or.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
CC: Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        Justin Stitt <justinstitt@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH v5 1/1] x86/cpu: Make sure flag_is_changeable_p() is always being used

On November 8, 2024 5:29:52 PM GMT+01:00, Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
>On Fri, Nov 08, 2024 at 04:48:16PM +0100, H. Peter Anvin wrote:
>> On November 8, 2024 4:41:16 PM GMT+01:00, Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
>> >On Fri, Nov 08, 2024 at 04:35:17PM +0100, H. Peter Anvin wrote:
>> >> On November 8, 2024 4:30:10 PM GMT+01:00, Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
>
>...
>
>> >> >See also commit 6863f5643dd7 ("kbuild: allow Clang to find unused static
>> >> >inline functions for W=1 build").
>
>^^^ (1)
>
>...
>
>> >> But another question: why the hell does clang complain about an unused static inline function?!
>> >
>> >Does (1) shed a bit of light to this?
>> 
>> How on earth is that supposed to work?! We have static inline functions in
>> headers all over the place that are only used in certain circumstances.
>> 
>> Is this a good thing, really? Or is it noise?
>
>This is a good question and IIRC somebody else already asked something similar.
>
>Clang people are Cc'ed here, I leave this to them to comment on,
>I am not an expert here.
>

So there are two questions:

1. How? How does this work, technically? 
2. Why? Is this really the right thing to do, or will it result in a bunch of unnecessary #ifdeffery instead?