Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
MIME-Version: 1.0
References: <20220211014313.1790140-1-aik@ozlabs.ru> <CAKwvOdkKsoYw9KOfwdHYg7OkTz0TQ6YgZZcqQ5PNg+_O1Q5bgg@mail.gmail.com>
In-Reply-To: <CAKwvOdkKsoYw9KOfwdHYg7OkTz0TQ6YgZZcqQ5PNg+_O1Q5bgg@mail.gmail.com>
From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Mon, 14 Feb 2022 08:10:16 -0800
Message-ID: <CABCJKudj5VJ-f+QU0qmnODoadEzpVME2rXhtUZbzrEV8o69hqQ@mail.gmail.com>
Subject: Re: [RFC PATCH kernel] trace: Make FTRACE_MCOUNT_USE_RECORDMCOUNT configurable
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Kees Cook <keescook@chromium.org>, llvm@lists.linux.dev,
        Alexey Kardashevskiy <aik@ozlabs.ru>,
        LKML <linux-kernel@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Ingo Molnar <mingo@redhat.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Fri, Feb 11, 2022 at 4:11 PM Nick Desaulniers
<ndesaulniers@google.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