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Message-ID: <20160127134502.GC32095@lst.de>
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
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