linux-kernel - Re: [PATCH] ftrace: mcount record based dynamic tracing for ARM

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Message-ID: <20081121180929.GA13863@blackbean.org>
Date:	Fri, 21 Nov 2008 10:09:29 -0800
From:	Jim Radford <radford@...vanix.net>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sam Ravnborg <sam@...nborg.org>,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ftrace: mcount record based dynamic tracing for ARM

On Fri, Nov 21, 2008 at 03:38:27PM +0000, Russell King - ARM Linux wrote:
> On Fri, Nov 21, 2008 at 09:27:17AM -0500, Steven Rostedt wrote:
> > On Thu, 20 Nov 2008, Jim Radford wrote:
> > > -	select HAVE_FUNCTION_TRACER if (!XIP_KERNEL)
> > > +	select HAVE_FTRACE_MCOUNT_RECORD
> > > +	select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL)

> > Russell mentioned something about the code not being compatible
> > with Thumb2, is the above if statement enough?

> We've started on merging some of the pre-requisits, and it will mean
> that the instruction length is no longer constant.  (It may be a
> 16bit or 32bit instruction.)

The only instruction that matters for arm is "bl <func>" since that's
what's emitted by gcc to call mcount().  I suspect thumb will be easy
to support.  <func> isn't known when the file is compiled, so I assume
in that case the assembler will have to leave at least 4 bytes (even
in thumb) in case mcount() gets linked far away.

I haven't looked at the return tracing code yet.  That might be harder
to support, but given x86 works, I suspect it'll be doable.

-Jim
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