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

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Date:	Thu, 4 Feb 2016 18:31:40 +0900
From:	AKASHI Takahiro <takahiro.akashi@...aro.org>
To:	Torsten Duwe <duwe@....de>, Jiri Kosina <jikos@...nel.org>
Cc:	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

Jiri, Torsten

Thank you for your explanation.

On 02/03/2016 08:24 PM, Torsten Duwe wrote:
> 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? ...

On arm/arm64, link register must be saved before any function call. So anyhow
we will have to add something, 3 instructions at the minimum, like:
    save lr
    branch _mcount
    restore lr
    <prologue>
    ...
    <body>
    ...

>> 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.

I don't want to bother you, but for my better understandings, could you show me
an example of asm instructions for a function prologue under -mprofile-kernel, please?

-Takahiro AKASHI

> 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
>