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

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Message-Id: <1653386854.o7nss9hzc9.naveen@linux.ibm.com>
Date:   Tue, 24 May 2022 15:45:20 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Sathvika Vasireddy <sv@...ux.ibm.com>
Cc:     "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>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>
Subject: Re: [RFC PATCH 1/4] objtool: Add --mnop as an option to --mcount

Christophe Leroy wrote:
> 
> 
> Le 23/05/2022 à 19:55, Sathvika Vasireddy a écrit :
>> Architectures can select HAVE_NOP_MCOUNT if they choose
>> to nop out mcount call sites. If that config option is
>> selected, then --mnop is passed as an option to objtool,
>> along with --mcount.
>> 
> 
> Is there a reason not to nop out mcount call sites on powerpc as well ?

Yes, if there are functions that are out of range of _mcount(), then the 
linker would have inserted long branch trampolines. We detect such cases 
during boot. But, if we nop out the _mcount call sites during build 
time, we will need some other way to identify these.

- Naveen