linux-kernel - Re: [PATCH 3/3] x86/ftrace: Use text

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Date:   Wed, 23 Oct 2019 11:02:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Song Liu <songliubraving@...com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH 3/3] x86/ftrace: Use text_poke()

On Tue, Oct 22, 2019 at 09:20:27PM -0700, Andy Lutomirski wrote:
> Also, Alexei, are you testing on a CONFIG_FRAME_POINTER=y kernel?  The
> ftrace code has a somewhat nasty special case to make
> CONFIG_FRAME_POINTER=y work right, and your example trampoline does
> not but arguably should have exaclty the same fixup.  For good
> performance, you should be using CONFIG_FRAME_POINTER=n.

Trampolines and JITs (which both lack ORC data) should always have frame
pointers. That way, when ORC fails, it can fall back to FP and we can
still get sensible unwinds.

See:

  ae6a45a08689 ("x86/unwind/orc: Fall back to using frame pointers for generated code")