linux-kernel - Re: [RFC][PATCH] ftrace/x86: Remove mcount support

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Message-ID: <20190509223051.710a8f4e@gandalf.local.home>
Date:   Thu, 9 May 2019 22:30:51 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        live-patching@...r.kernel.org, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC][PATCH] ftrace/x86: Remove mcount support

On Thu, 9 May 2019 15:14:30 -0500
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> > Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> > ---
> >  arch/x86/include/asm/ftrace.h    |  8 +++----
> >  arch/x86/include/asm/livepatch.h |  3 ---
> >  arch/x86/kernel/ftrace_32.S      | 36 +++++---------------------------
> >  arch/x86/kernel/ftrace_64.S      | 28 +------------------------
> >  4 files changed, 9 insertions(+), 66 deletions(-)  
> 
> I was wondering why you didn't do s/mcount/fentry/ everywhere, but I
> guess it's because mcount is still used by other arches, so it still has
> a generic meaning tree-wide, right?

Yes, fentry works nicely when you have a single instruction that pushes
the return address on the stack and then jumps to another location.
It's much trickier to implement with link registers. There's a few
different implementations for other archs, but mcount happens to be the
one supported by most.

> 
> Anyway it's nice to finally see this cruft disappear.
> 
> Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>

Thanks!

-- Steve