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Subject: Re: Re: [RFC][PATCH 2/4 v4] ftrace/x86: Add save_regs for i386
 function calls
From: Steven Rostedt <rostedt@goodmis.org>
To: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
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>,
        "H. Peter Anvin" <hpa@zytor.com>, yrl.pp-manager.tt@hitachi.com
Date: Mon, 16 Jul 2012 23:05:02 -0400
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On Tue, 2012-07-17 at 11:08 +0900, Masami Hiramatsu wrote:

> > I found that regs_get_register() doesn't honor this either. Thus,
> > kprobes in tracing gets this:
> > 
> >  # echo 'p:ftrace sys_read+4 s=%sp' > /debug/tracing/kprobe_events
> >  # echo 1 > /debug/tracing/events/kprobes/enable
> >  # cat trace
> >             sshd-1345  [000] d...   489.117168: ftrace: (sys_read+0x4/0x70) s=b7e96768
> >             sshd-1345  [000] d...   489.117191: ftrace: (sys_read+0x4/0x70) s=b7e96768
> >              cat-1447  [000] d...   489.117392: ftrace: (sys_read+0x4/0x70) s=5a7
> >              cat-1447  [001] d...   489.118023: ftrace: (sys_read+0x4/0x70) s=b77ad05f
> >             less-1448  [000] d...   489.118079: ftrace: (sys_read+0x4/0x70) s=b7762e06
> >             less-1448  [000] d...   489.118117: ftrace: (sys_read+0x4/0x70) s=b7764970
> > 
> 
> Yes, that is by design, since I made it so. :)
> Instead of %sp, kprobe tracer provides $stack special argument
> for stack address, because "sp" is not always means the stack
> address on every arch.

But is that useful? Wouldn't the actual stack pointer be more
informative?

-- Steve