linux-kernel - Re: [RFC][PATCH] objtool,x86_64: Replace recordmcount with objtool

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Date:   Thu, 23 Jul 2020 01:56:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        x86@...nel.org, Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [RFC][PATCH] objtool,x86_64: Replace recordmcount with objtool

On Wed, Jul 22, 2020 at 03:09:43PM -0400, Steven Rostedt wrote:
> On Wed, 22 Jul 2020 20:41:37 +0200
> Peter Zijlstra <peterz@...radead.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?

I got the GCC version wrong :/ Both -mnop-mcount and -mrecord-mcount
landed in GCC-5, where our minimum GCC is now at 4.9.

Anyway, what do you prefer, I suppose I can make objtool whatever we
need, that patch is trivial. Simply recording the sites and not
rewriting them should be simple enough.