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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 22 Apr 2020 11:47:27 -0700
Message-ID: <CAKwvOd=1CDhfgkcXJ1Q8bZcy0UKWFFoP4QCeCzt4OE7LTGq6yA@mail.gmail.com>
Subject: Re: [PATCH v2] x86: fix early boot crash on gcc-10
To:     Borislav Petkov <bp@alien8.de>
Cc:     =?UTF-8?Q?Martin_Li=C5=A1ka?= <mliska@suse.cz>,
        Jakub Jelinek <jakub@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Michael Matz <matz@suse.de>,
        Sergei Trofimovich <slyfox@gentoo.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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On Wed, Apr 22, 2020 at 9:53 AM Borislav Petkov <bp@alien8.de> wrote:
>
> On Wed, Apr 22, 2020 at 04:16:53PM +0200, Martin Li=C5=A1ka wrote:
> > And as I talked to Boris, I would recommend to come up with a "configur=
e" check
> > that a compiler does not optimize the key code sequence:
> >
> > $ cat asm-detect.c
> > int foo(int a);
> > int bar(int a)
> > {
> >   int r =3D foo(a);
> >   asm ("");
> >   return r;
> > }
> >
> > $ gcc -O2 -c asm-detect.c -S -o/dev/stdout | grep jmp
> > [no output]
>
> That is a good test to run at the beginning of the compilation I guess.
>
> Without the asm("") it produces:
>
> bar:
> .LFB0:
>         .cfi_startproc
>         jmp     foo@PLT
>         .cfi_endproc
>
> I'd like for LLVM folks to confirm that this is a good test for LLVM too
> Trying that here with clang gives:
>
> bar:                                    # @bar
>         .cfi_startproc
> # %bb.0:
>         jmp     foo                     # TAILCALL
> .Lfunc_end0:
>
> so this *looks* like it would work with LLVM too but I might be missing
> something...


LGTM https://godbolt.org/z/ExtHx7
--=20
Thanks,
~Nick Desaulniers