linux-kernel - Re: [PATCH] x86: work around clang IAS bug referencing __force

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Date:   Fri, 07 Aug 2020 00:11:38 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org
Cc:     Arnd Bergmann <arnd@...db.de>, "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Zhenzhong Duan <zhenzhong.duan@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order

Arnd Bergmann <arnd@...db.de> writes:
> When using the clang integrated assembler, we get a reference
> to __force_order that should normally get ignored in a few
> rare cases:
>
> ERROR: modpost: "__force_order" [drivers/cpufreq/powernow-k6.ko] undefined!
>
> Add a 'static' definition so any file in which this happens can
> have a local copy.

That's a horrible hack.

And the only reason why it does not trigger -Wunused-variable warnings
all over the place is because it's "referenced" in unused inline
functions and then optimized out along with the unused inlines.

>   * It is not referenced from the code, but GCC < 5 with -fPIE would fail
>   * due to an undefined symbol. Define it to make these ancient GCCs
>   work.

Bah, we really should have moved straight to GCC5 instead of upping it
just to 4.9

> + *
> + * Clang sometimes fails to kill the reference to the dummy variable, so
> + * provide an actual copy.

Can that compiler be fixed instead?

Aside of that is there a reason to make this 'static' thing wrapped in
#ifdeffery? A quick check with GCC8.3 just works. But maybe 4.9 gets
unhappy. Can't say due to: -ENOANCIENTCOMPILER :)

Thanks,

        tglx