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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Wed, 22 Jul 2020 13:03:12 -0700
Message-ID: <CABCJKufW8rYG-R7b=ad8E5oRd+1xrVknWcTd2VFuvE7=SPtoTA@mail.gmail.com>
Subject: Re: [RFC][PATCH] objtool,x86_64: Replace recordmcount with objtool
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Will Deacon <will@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-kbuild <linux-kbuild@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, linux-pci@vger.kernel.org,
        X86 ML <x86@kernel.org>, Josh Poimboeuf <jpoimboe@redhat.com>
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On Wed, Jul 22, 2020 at 12:09 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Wed, 22 Jul 2020 20:41:37 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
>
> > > That said, Andi Kleen added an option to gcc called -mnop-mcount which
> > > will have gcc do both create the mcount section and convert the calls
> > > into nops. When doing so, it defines CC_USING_NOP_MCOUNT which will
> > > tell ftrace to expect the calls to already be converted.
> >
> > That seems like the much easier solution, then we can forget about
> > recordmcount / objtool entirely for this.
>
> Of course that was only for some gcc compilers, and I'm not sure if
> clang can do this.
>
> Or do you just see all compilers doing this in the future, and not
> worrying about record-mcount at all, and bothering with objtool?

Clang appears to only support -mrecord-mcount and -mnop-mcount for
s390, so we still need recordmcount / objtool for x86.

Sami