linux-kernel - Re: [RFC PATCH 0/4] ftrace: Add use of -mfentry for x86

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Date:	Tue, 07 Aug 2012 13:23:54 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [RFC PATCH 0/4] ftrace: Add use of -mfentry for x86_64

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@...ux.intel.com>

	-hpa


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

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