linux-kernel - Re: [PATCH v2] x86: fix early boot crash on gcc-10

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Date:   Wed, 22 Apr 2020 15:06:36 +0000 (UTC)
From:   Michael Matz <matz@...e.de>
To:     Martin Liška <mliska@...e.cz>
cc:     Jakub Jelinek <jakub@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sergei Trofimovich <slyfox@...too.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v2] x86: fix early boot crash on gcc-10

Hello,

On Wed, 22 Apr 2020, Martin Liška wrote:

> One possible solution can be usage of a GCC pragma that will disable the
> tail-call optimization:
> 
> $ cat tail.c
> int foo(int);
> 
> #pragma GCC push_options
> #pragma GCC optimize("-fno-optimize-sibling-calls")

As we determined upthread (and the reason why we even still have this 
thread): the optimize attribute (and pragma) reset flags from the command 
line (the case in point was -fno-omit-frame-pointer).  So, that's not a
solution for now.

> And as I talked to Boris, I would recommend to come up with a 
> "configure" check that a compiler does not optimize the key code 
> sequence:

Right.  I think the traditional asm (i.e. one without operands) is good 
enough for the forseeable future from GCCs side: it relies on documented 
behaviour of traditional asms, and hence would be very hard to change.

Ciao,
Michael.