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

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Message-ID: <20160203112449.GA27247@lst.de>
Date:	Wed, 3 Feb 2016 12:24:49 +0100
From:	Torsten Duwe <duwe@....de>
To:	Jiri Kosina <jikos@...nel.org>
Cc:	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	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, Feb 03, 2016 at 09:55:11AM +0100, Jiri Kosina wrote:
> On Wed, 3 Feb 2016, AKASHI Takahiro wrote:
> > those efforts, we are proposing[1] a new *generic* gcc option, -fprolog-add=N.
> > This option will insert N nop instructions at the beginning of each function.

> The interesting part of the story with ppc64 is that you indeed want to 
> create the callsite before the *most* of the prologue, but not really :) 

I was silently assuming that GCC would do this right on ppc64le; add the NOPs
right after the TOC load. Or after TOC load and LR save? ...

> The part of the prologue where TOC pointer is saved needs to happen before 
> the fentry/profiling call.

Yes, any call, to any profiler/tracer/live patcher is potentially global
and needs the _new_ TOC value.

This proposal, if implemented in a too naive fashion, will worsen the problem
we currently discuss: a few NOPs _never_ cause any global reference. GCC might
be even more inclined to not load a new TOC value. That change would need to be
fairly smart on ppc64le.

	Torsten