linux-kernel - Re: [PATCH] x86: Fix early boot crash on gcc-10, next try

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Message-ID: <20200425185313.GD24294@zn.tnic>
Date:   Sat, 25 Apr 2020 20:53:13 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Jakub Jelinek <jakub@...hat.com>, jgross@...e.com,
        x86@...nel.org, Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linuxppc-dev@...ts.ozlabs.org, Michael Matz <matz@...e.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Sergei Trofimovich <slyfox@...too.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Ingo Molnar <mingo@...hat.com>,
        Paul Mackerras <paulus@...ba.org>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Frédéric Pierret (fepitre) 
        <frederic.pierret@...es-os.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Martin Liška <mliska@...e.cz>,
        boris.ostrovsky@...cle.com
Subject: Re: [PATCH] x86: Fix early boot crash on gcc-10, next try

On Sat, Apr 25, 2020 at 01:37:01PM -0500, Segher Boessenkool wrote:
> That is a lot more typing then
> 	asm("");

That's why a macro with a hopefully more descriptive name would be
telling more than a mere asm("").

> but more seriously, you probably should explain why you do not want a
> tail call *anyway*, and in such a comment you can say that is what the
> asm is for.

Yes, the final version will have a comment and the whole spiel. This
diff is just me polling the maintainers: "do you want this for your arch
too?" Well, the PPC maintainers only, actually.

The other call in init/main.c would be for everybody.

> I don't see anything that prevents the tailcall in current code either,
> fwiw.

Right, and I don't see a reason why gcc-10 would do that optimization on
x86 only but I better ask first.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette