Date:	Mon, 05 Oct 2009 12:59:01 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.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

Masami Hiramatsu wrote:
> Frederic Weisbecker wrote:
>> On Fri, Oct 02, 2009 at 05:48:42PM -0400, Masami Hiramatsu wrote:
>>>    FETCHARGS	: Arguments. Each probe can have up to 128 args.
>>>     %REG	: Fetch register REG
>>> -  sN	: Fetch Nth entry of stack (N>= 0)
>>> -  sa	: Fetch stack address.
>>>     @ADDR	: Fetch memory at ADDR (ADDR should be in kernel)
>>>     @SYM[+|-offs]	: Fetch memory at SYM +|- offs (SYM should be a data symbol)
>>> -  aN	: Fetch function argument. (N>= 0)(*)
>>> -  rv	: Fetch return value.(**)
>>> -  ra	: Fetch return address.(**)
>>> +  $sN	: Fetch Nth entry of stack (N>= 0)
>>> +  $sa	: Fetch stack address.
>>> +  $aN	: Fetch function argument. (N>= 0)(*)
>>> +  $rv	: Fetch return value.(**)
>>> +  $ra	: Fetch return address.(**)
>>
>>
>>
>> I feel uncomfortable with that, because of bash scripts that
>> may use it and always need to escape it with antislashes or
>> use single quotes for it to not be replaced by a random variable
>> value. If one uses double quotes without antislashes to protect
>> the command line, the result is unpredictable, depending of
>> the current set of variables...
>>
>> May be we can use # instead of $ ? That's a kind of pity because
>> $ suggested a variable whereas # suggests a constant, we are then
>> losing this self-explainable characteristic for kprobes
>> "specific variable fetchs". But that will work without ambiguity.
>
> Hmm, # is widely used for comment, including some kernel pseudo
> file interfaces, kprobe_events too. Comments are useful if a
> probe list is restored from a file.
>
> For accessing local variables, kprobe-tracer needs to support *at least*
> below variables:
> - Registers
> - Stack address (if a register points stack address, this isn't needed)
>
> Below special vars are complementary aliases.
> - Function arguments
> - Return value
> - Return address
>
> and I'd like perf-probe to have a transparent syntax with kprobe-tracer.
>
> This means, if we can remove special vars except registers, or rename it
> non-conflictable name with registers, we just need to separate name spaces
> of
> - Regsiters
> - Local variables
>
> Here, local variables will support fields of data structs, and it will
> use '->' expression. Since'>' means redirection in bash, local variables
> need to be *escaped* in this case. Thus, I think we can use '$' prefix
> for it. (I'm OK, because this is similar syntax to systemtap:-).
>
> So, if you don't like %regs, $svars and locals, we can use regs and $locals :-)

As far as I can see in arch/*/include/asm/ptrace.h, all registers start with
alphabets :-). So, I'd like to suggest renaming sp-vars to '_sp-vars'.

Then, we will have;
- $local-vars
- @global-symbol
- regs
- _sp-vars
- +|-Offs(ARG)

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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