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

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Message-ID: <d45030be-3f6b-ebeb-3d63-bf7a96d3ff3b@csgroup.eu>
Date:   Tue, 24 May 2022 10:20:17 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        "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



Le 24/05/2022 à 12:15, Naveen N. Rao a écrit :
> 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.
> 

But does it really matter whether _mcount is reachable or not ?

_mcount is never used, and the function we want to call in lieu of 
_mcount might be reachable while _mcount is not or might be unreachable 
while _mcount is.

Christophe