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

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Message-Id: <1653564857.f06fbbl3vg.naveen@linux.ibm.com>
Date:   Thu, 26 May 2022 17:21:02 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "aik@...abs.ru" <aik@...abs.ru>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "mbenes@...e.cz" <mbenes@...e.cz>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        Sathvika Vasireddy <sv@...ux.ibm.com>
Subject: Re: [RFC PATCH 1/4] objtool: Add --mnop as an option to --mcount

Peter Zijlstra wrote:
> 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?

That's an interesting thought. Depending on the type of trampoline the 
compiler emits, I might be able to use this approach. We will still need 
objtool on powerpc  so that we can note down those trampoline locations.


Thanks,
Naveen