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

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Date:   Mon, 14 Feb 2022 08:10:16 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>, llvm@...ts.linux.dev,
        Alexey Kardashevskiy <aik@...abs.ru>,
        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 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.

Alexey, which architecture are you trying to fix here?

Sami