linux-kernel - Re: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this is for x86

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Date:	Thu, 17 Feb 2011 21:37:06 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.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>,
	"H. Peter Anvin" <hpa@...or.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this is
 for x86_64 right now)

(2011/02/10 5:02), Steven Rostedt wrote:
> Thanks to Andi Kleen gcc 4.6.0 now supports -mfentry with the -pg option
> to place a call to __fentry__ at the very beginning of the function
> instead of after the fact.
> 
> The old way:
> 
> 00000000000000c4 <atomic_long_add>:
>       c4:       55                      push   %rbp
>       c5:       48 89 e5                mov    %rsp,%rbp
>       c8:       e8 00 00 00 00          callq  cd <atomic_long_add+0x9>
>                         c9: R_X86_64_PC32       mcount-0x4
>       cd:       f0 48 01 3e             lock add %rdi,(%rsi)
>       d1:       c9                      leaveq 
>       d2:       c3                      retq   
> 
> The new way:
> 
> 000000000000009e <atomic_long_add>:
>       9e:       e8 00 00 00 00          callq  a3 <atomic_long_add+0x5>
>                         9f: R_X86_64_PC32       __fentry__-0x4
>       a3:       55                      push   %rbp
>       a4:       48 89 e5                mov    %rsp,%rbp
>       a7:       f0 48 01 3e             lock add %rdi,(%rsi)
>       ab:       5d                      pop    %rbp
>       ac:       c3                      retq   
> 
> Note, with -mfentry, frame pointers is no longer required
> by the function tracer. But this patch series still requires
> FRAME_POINTER to be set, since I need to figure out a good way to
> enable FRAME_POINTER only if gcc doesn't support this. But that can
> come later.
> 
> With the new __fentry__, we could possible record the parameters
> of a function call. This may take some work, and perhaps be
> a little like kprobes. But it is doable.
> 
> This is still just RFC. I only wrote the code to support x86_64
> even though gcc 4.6.0 also supports i386. I figured I would post
> this first to get peoples reactions before converting
> i386 too. Other archs can soon follow.

Oops! with this change, current kprobes might not be able to probe
the entry of functions, because that is always reserved by ftrace!
I think we need to have some new interface for replacing each other
safely...

Thank you,

-- 
Masami HIRAMATSU
2nd Dept. Linux Technology Center
Hitachi, Ltd., Systems Development Laboratory
E-mail: masami.hiramatsu.pt@...achi.com
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