linux-kernel - Re: [RFC PATCH v2 0/7] objtool: Enable and implement --mcount option on powerpc

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Date:   Wed, 25 May 2022 17:39:24 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Sathvika Vasireddy <sv@...ux.vnet.ibm.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "aik@...abs.ru" <aik@...abs.ru>, "mbenes@...e.cz" <mbenes@...e.cz>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Sathvika Vasireddy <sv@...ux.ibm.com>
Subject: Re: [RFC PATCH v2 0/7] objtool: Enable and implement --mcount option
 on powerpc

Hi Sathvika,

Le 25/05/2022 à 12:14, Sathvika Vasireddy a écrit :
> Hi Christophe,
> 
> On 24/05/22 18:47, Christophe Leroy wrote:
>> This draft series adds PPC32 support to Sathvika's series.
>> Verified on pmac32 on QEMU.
>>
>> It should in principle also work for PPC64 BE but for the time being
>> something goes wrong. In the beginning I had a segfaut hence the first
>> patch. But I still get no mcount section in the files.
> Since PPC64 BE uses older elfv1 ABI, it prepends a dot to symbols.
> And so, the relocation records in case of PPC64BE point to "._mcount",
> rather than just "_mcount". We should be looking for "._mcount" to be
> able to generate mcount_loc section in the files.
> 
> Like:
> 
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 70be5a72e838..7da5bf8c7236 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -2185,7 +2185,7 @@ static int classify_symbols(struct objtool_file 
> *file)
>                          if (arch_is_retpoline(func))
>                                  func->retpoline_thunk = true;
> 
> -                       if ((!strcmp(func->name, "__fentry__")) || 
> (!strcmp(func->name, "_mcount")))
> +                       if ((!strcmp(func->name, "__fentry__")) || 
> (!strcmp(func->name, "_mcount")) || (!strcmp(func->name, "._mcount")))
>                                  func->fentry = true;
> 
>                          if (is_profiling_func(func->name))
> 
> 
> With this change, I could see __mcount_loc section being
> generated in individual ppc64be object files.
> 

Or should we implement an equivalent of arch_ftrace_match_adjust() in 
objtool ?

Christophe