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

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Date:	Fri, 26 Feb 2016 21:35:47 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Torsten Duwe <duwe@....de>, Balbir Singh <bsingharora@...il.com>
Cc:	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, 2016-02-25 at 14:31 +0100, Torsten Duwe wrote:

> 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;

Yeah true. I originally wrote it with the address of ftrace_caller passed as an
argument, so it had to be computed at runtime, and getting it from the paca is
~= to getting it out of the GOT.

> 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.

Yeah that makes sense. I'll add a helper to do the + 32k.

cheers