linux-kernel - Re: [PATCH 0/2] ftrace: fixes for PPC

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Date:	Mon, 20 Oct 2008 13:28:24 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Josh Boyer <jwboyer@...ux.vnet.ibm.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
	linuxppc-dev@...abs.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/2] ftrace: fixes for PPC


On Mon, 20 Oct 2008, Josh Boyer wrote:

> On Mon, Oct 20, 2008 at 12:30:33PM -0400, Steven Rostedt wrote:
> >> Anyway, if you want a tester let me know.  It seems 2.6.27.1 should be
> >> fine since FTRACE was disabled, but for .28-rc1 it would be cool if it
> >> worked :).
> >
> >Hi Josh,
> >
> >I've been looking deeper at the code for PPC. I realized that my PPC64 box 
> >that I've been testing on did not use modules. While looking at the module 
> >code it dawned on me the dynamic ftrace needs a bit of work. This is 
> >because the way modules are handled in PPC (and other architectures as 
> >well).  The jmps used by mcount is a 24 bit jump. Since the modules are 
> >loaded farther than 24bits away, a trampoline is needed.
> 
> Ah, indeed.
> 
> >A bit of rework is needed in the ftrace infrastructure to handle the 
> >trampoline. Too much work to go into 28. I'll start working on code that 
> >can hopefully be ready and tested for 29. It's not that major of a change, 
> >but since the merge window for 28 has already been opened, we would like 
> >to get a bit more testing in before we hand it over to Linus.
> 
> That seems like a good plan.  Should we mark dynamic ftrace as X86 only
> in Kconfig for .28 to prevent people from inadvertently setting it?

Well, we probably should disable it for PPC at least. I don't know if 
sparc has the same issue.

David?

-- Steve

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