linux-kernel - Re: [PATCH 03/12] powerpc/module: Create a special stub for ftrace

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Message-ID: <20160225133133.GE21932@lst.de>
Date:	Thu, 25 Feb 2016 14:31:33 +0100
From:	Torsten Duwe <duwe@....de>
To:	Balbir Singh <bsingharora@...il.com>
Cc:	Michael Ellerman <mpe@...erman.id.au>, linuxppc-dev@...abs.org,
	linux-kernel@...r.kernel.org, rostedt@...dmis.org,
	kamalesh@...ux.vnet.ibm.com, pmladek@...e.com, jeyu@...hat.com,
	jkosina@...e.cz, live-patching@...r.kernel.org, mbenes@...e.cz
Subject: Re: [PATCH 03/12] powerpc/module: Create a special stub for
	ftrace_caller()

On Thu, Feb 25, 2016 at 11:08:54AM +1100, Balbir Singh wrote:
> How about some comments on r2
> r2 is still pointing to the module's toc, will be saved by ftrace_caller and restored by the instruction following bl ftrace_caller (after patching _mcount/nop)

To be precise: ftrace_caller needs to save _and_ restore r2 in case of -mprofile-kernel.

> > +	/* Stub uses address relative to kernel_toc */
> > +	reladdr = (unsigned long)ftrace_caller - get_paca()->kernel_toc;

kernel_toc is a compile time constant; do you really want to look it up in
memory at runtime each time? It's a bit tricky to get the +- 0x8000 right
OTOH...

I wrote:
extern unsigned long __toc_start;
    reladdr = addr - ((unsigned long)(&__toc_start) + 0x8000UL);

looks a bit odd, but evaluates to a constant for ftrace_caller.

Either way is fine with me:

Signed-off-by: Torsten Duwe <duwe@...e.de>
Reviewed-by: Torsten Duwe <duwe@...e.de>

> Reviewed-by: Balbir Singh <bsingharora@...il.com>

	Torsten