Re: [RFC PATCH 0/4] ftrace: Add use of -mfentry for x86_64

From: "H. Peter Anvin" <hpa@zytor.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
	Andrew Morton <akpm@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andi Kleen <andi@firstfloor.org>
Subject: Re: [RFC PATCH 0/4] ftrace: Add use of -mfentry for x86_64
Date: Tue, 07 Aug 2012 13:23:54 -0700	[thread overview]
Message-ID: <5021795A.6030902@zytor.com> (raw)
In-Reply-To: <20120807193845.146666357@goodmis.org>

On 08/07/2012 12:38 PM, Steven Rostedt wrote:
>
> This is an RFC patch set that makes gcc use the -mfentry option with
> -pg. This will set the ftrace 'hook' to the beginning of the function
> and also remove the requirement that -pg enables frame pointers.
>
> This has a couple of benefits (and probably more).
>
> 1) removal of the frame pointer requirement makes for smaller and faster code.
>
> 2) Having the function trace hook at the beginning of the function instead
>   of after the frame is set up, gives the function tracing callbacks access
>   to the parameters. This means that kprobes can take advantage of this.
>   When a kprobe is set on top of a ftrace hook (nop), it will automatically
>   use the function tracing callback. This makes it into an 'optimized' probe
>   as there's no need to hit a breakpoint and trigger the probe that way.
>   The function tracing code can do the work for it. Note, optimized probes
>   are only allowed with !PREEMPT, but a ftrace optimize probe is allowed
>   in any context (another benefit).
>
> This only implements fentry for x86_64.
>

Acked-by: H. Peter Anvin <hpa@linux.intel.com>

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.