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

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Message-ID: <ac4e3ceb-7de8-2c3f-4689-1730d811bf3d@linux.vnet.ibm.com>
Date:   Wed, 25 May 2022 15:44:04 +0530
From:   Sathvika Vasireddy <sv@...ux.vnet.ibm.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     linux-kernel@...r.kernel.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,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>, 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 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.

- Sathvika