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

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Date:   Wed, 16 Feb 2022 15:21:17 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Alexey Kardashevskiy <aik@...abs.ru>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>, llvm@...ts.linux.dev,
        Ingo Molnar <mingo@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sathvika Vasireddy <sv@...ux.ibm.com>
Subject: Re: [RFC PATCH kernel] trace: Make FTRACE_MCOUNT_USE_RECORDMCOUNT
 configurable

Alexey Kardashevskiy wrote:
> 
> 
> 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.

I had responded on a different thread about this:
http://lkml.kernel.org/r/1644579444.bal9kji6uc.naveen@linux.ibm.com

We are working on enabling 'objtool check --mcount' on powerpc.

> 
> 
> So can x86_64 have both FTRACE_MCOUNT_USE_RECORDMCOUNT and LTO?

No. x86_64 now supports HAVE_OBJTOOL_MCOUNT, which enables 
FTRACE_MCOUNT_USE_OBJTOOL. That allows LTO to be used.


- Naveen