linux-kernel - Re: [RFC PATCH 1/4] objtool: Add --mnop as an option to --mcount

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Message-ID: <Yo4UysC69UCwhlYp@hirez.programming.kicks-ass.net>
Date:   Wed, 25 May 2022 13:36:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Sathvika Vasireddy <sv@...ux.ibm.com>,
        "aik@...abs.ru" <aik@...abs.ru>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mbenes@...e.cz" <mbenes@...e.cz>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>
Subject: Re: [RFC PATCH 1/4] objtool: Add --mnop as an option to --mcount

On Tue, May 24, 2022 at 04:01:48PM +0530, Naveen N. Rao wrote:

> We need to know for sure either way. Nop'ing out the _mcount locations at
> boot allows us to discover existing long branch trampolines. If we want to
> avoid it, we need to note down those locations during build time.
> 
> Do you have a different approach in mind?

If you put _mcount in a separate section then the compiler cannot tell
where it is and is forced to always emit a long branch trampoline.

Does that help?