linux-kernel - Re: [PATCH tracing/kprobes v2 1/5] tracing/kprobes: Rename special variables syntax

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Message-ID: <20091005205826.GE6071@nowhere>
Date:	Mon, 5 Oct 2009 22:58:28 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>,
	lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ibm.com>,
	"Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [PATCH tracing/kprobes v2 1/5] tracing/kprobes: Rename special
	variables syntax

On Mon, Oct 05, 2009 at 04:18:39PM -0400, Masami Hiramatsu wrote:
> Frederic Weisbecker wrote:
>> For the function arguments, I guess we don't need to worry
>> anymore about r0, r1, etc... but we can deal with the true var
>> name, without any kind of prefixes.
>
> This depends on ABI, function argument from ABI doesn't need
> debuginfo, but it will be unstable on some arch, e.g. x86-32
> with/without asmlinkage.
>
> Thus, I think that we can just describe where function arguments
> will be(e.g. arg0 is ax) as a note for each architecture
> in Documents/trace/kprobetrace.txt.


Yeah that may help. Although everyone can look at the calling convention
ABI for a given arch but that would still help.


>> What about @return :-) ?
>
> Hmm, it might conflict with global symbol... Maybe, we can remove this
> because retprobe already shows return address in the head of entry.


It won't conflict since "return" is a reserved word and can't then be
used as a symbol.

But yeah, if it's an embeded field, we should remove it.


>> What if we take the following:
>>
>> [Ftrace and perf probe side]
>>
>> %reg = registers, we can also play with deref and offsets like (%esp), 8(%esp), etc.
>
> Hmm, on x86-32, sp at intr context is not pointing the top of stack. actually &sp is
> the real address of the stack :(
> Perhaps, on x86-32, we can translate %sp to stack address in kprobe-tracer.



Oh? You mean in the saved registers while triggering an int 3?



> > %return = return value
>
> or %retval? :)


Yeah, better!


>
>> @return = return addr
>
> I'd like to remove it, because it's already shown.


Ok.



>> arg(n) = arg number, where n is the number
>
> How about %N? or just adds a note in documents.
>


Hmm, the problem is that %1, %2, etc. is not very self-explainable.

May be %arg1, %arg2, etc.. But would that sound confusing since we
have % for registers?



>> [Perf probe only]
>>
>> var = variable name, global or local, we can deal with shadow issues later
>>        through variable scope: func_name:var, filename:var, whatever for now
>>        it's not a problem. Local also includes argument names.
>
> That's fine to me. :-)
>


Great :)
Thanks!

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