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Date:   Wed, 22 Jul 2020 20:41:37 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        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@googlegroups.com,
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        linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
        x86@kernel.org, Josh Poimboeuf <jpoimboe@redhat.com>,
        mhelsley@vmware.com
Subject: Re: [RFC][PATCH] objtool,x86_64: Replace recordmcount with objtool
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On Wed, Jul 22, 2020 at 01:55:42PM -0400, Steven Rostedt wrote:

> > Ha! it is trying to convert the "CALL __fentry__" into a NOP and not
> > finding the CALL -- because objtool already made it a NOP...
> > 
> > Weird, I thought recordmcount would also write NOPs, it certainly has
> > code for that. I suppose we can use CC_USING_NOP_MCOUNT to avoid those,
> > but I'd rather Steve explain this before I wreck things further.
> 
> The reason for not having recordmcount insert all the nops, is because
> x86 has more than one optimal nop which is determined by the machine it
> runs on, and not at compile time. So we figured just updated it then.
> 
> We can change it to be a nop on boot, and just modify it if it's not
> the optimal nop already. 

Right, I throught that's what we'd be doing already, anyway:

> 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.