linux-kernel - Re: [RFC PATCH kernel] trace: Make FTRACE_MCOUNT_USE

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Message-ID: <5b021d73-51c1-164f-6136-483eeebef6fb@ozlabs.ru>
Date:   Wed, 16 Feb 2022 15:51:31 +1100
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Sami Tolvanen <samitolvanen@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>, llvm@...ts.linux.dev,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC PATCH kernel] trace: Make FTRACE_MCOUNT_USE_RECORDMCOUNT
 configurable



On 2/15/22 03:10, Sami Tolvanen wrote:
> On Fri, Feb 11, 2022 at 4:11 PM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
>>
>> + Sami, Kees
>>
>> Do you guys remember why we don't support
>> FTRACE_MCOUNT_USE_RECORDMCOUNT with LTO?
> 
> It's because recordmcount cannot distinguish between calls and other
> references to mcount/fentry, which is a problem with LTO where we have
> to process the entire vmlinux.o and cannot ignore individual
> translation units. Instead of resorting to workarounds, Peter was kind
> enough to implement a smarter mcount pass in objtool, which we now use
> on x86_64 with LTO.


So can x86_64 have both FTRACE_MCOUNT_USE_RECORDMCOUNT and LTO?

> Alexey, which architecture are you trying to fix here?


powerpc64le.

> 
> Sami