linux-kernel - Re: [PATCH v6 1/9] ppc64 (le): prepare for -mprofile-kernel

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Date:	Wed, 27 Jan 2016 14:45:02 +0100
From:	Torsten Duwe <duwe@....de>
To:	Alan Modra <amodra@...il.com>
Cc:	Michael Ellerman <mpe@...erman.id.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	Anton Blanchard <anton@...ba.org>,
	Jiri Kosina <jkosina@...e.cz>, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: Re: [PATCH v6 1/9] ppc64 (le): prepare for -mprofile-kernel

On Wed, Jan 27, 2016 at 11:28:09PM +1030, Alan Modra wrote:
> On Wed, Jan 27, 2016 at 09:19:27PM +1100, Michael Ellerman wrote:
> > 
> > Can we use r11 instead? eg:
> > 
> > _GLOBAL(_mcount)
> > 	mflr	r11
> > 	mtctr	r11
> > 	mtlr	r0
> > 	bctr
> 
> Depends on what you need to support.  As Torsten says, the code to
> call _mcount when -mprofile-kernel is emitted before the prologue of a
> function (similar to -m32), but after the ELFv2 global entry point
> code.  If you trash r11 here you're killing the static chain pointer,
> used by C nested functions or other languages that use a static chain,
> eg. Pascal.  r11 has *not* been saved for ELFv2.

Even if nested functions aren't supported in the Linux kernel(?), I think
it was an earlier version of mcount when r11 usage ruined my day.

> r12 might be a better choice for a temp reg.

Good idea. r12 holds the function entry point and is used to calculate the
new TOC value just _before_ the call. It should be available.
I'll try, thanks for the hint.

	Torsten