linux-kernel - Re: [PATCH v8 1/8] ppc64 (le): prepare for -mprofile-kernel

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Message-ID: <20160217113022.GA9578@lst.de>
Date:	Wed, 17 Feb 2016 12:30:22 +0100
From:	Torsten Duwe <duwe@....de>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	Jiri Kosina <jkosina@...e.cz>, Miroslav Benes <mbenes@...e.cz>,
	Petr Mladek <pmladek@...e.com>, Jessica Yu <jeyu@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	live-patching@...r.kernel.org
Subject: Re: [PATCH v8 1/8] ppc64 (le): prepare for -mprofile-kernel

On Wed, Feb 17, 2016 at 09:55:40PM +1100, Michael Ellerman wrote:
> On Wed, 2016-02-10 at 17:21 +0100, Torsten Duwe wrote:
> 
> > --- a/arch/powerpc/kernel/module_64.c
> > +++ b/arch/powerpc/kernel/module_64.c
> > @@ -476,17 +474,44 @@ static unsigned long stub_for_addr(Elf64_Shdr *sechdrs,
> >  	return (unsigned long)&stubs[i];
> >  }
> >  
> > +#ifdef CC_USING_MPROFILE_KERNEL
> > +static int is_early_mcount_callsite(u32 *instruction)
> > +{
> > +	/* -mprofile-kernel sequence starting with
> > +	 * mflr r0 and maybe std r0, LRSAVE(r1).
> > +	 */
> > +	if ((instruction[-3] == PPC_INST_MFLR &&
> > +	     instruction[-2] == PPC_INST_STD_LR) ||
> > +	    instruction[-2] == PPC_INST_MFLR) {
> > +		/* Nothing to be done here, it's an _mcount
> > +		 * call location and r2 will have to be
> > +		 * restored in the _mcount function.
> > +		 */
> > +		return 1;
> > +	}
> > +	return 0;
> > +}
> 
> On a kernel built with the 2 instruction version this will fault when the
> function we're looking at is located at the beginning of a page. Because
> instruction[-3] goes off the front of the mapping.
> 
> We can probably fix that. But it's still a bit dicey.

Not necessarily. Now that it's a separate function, it can be nested a bit deeper,
so we don't take chances on compiler optimisation:

if (instruction[-2] == PPC_INST_STD_LR) /* where should R0 come from? there must be... */
  {
    if (instruction[-3] == PPC_INST_MFLR)
      return 1;
  }
else if (instruction[-2] == PPC_INST_MFLR)
    return 1;
return 0;

> I'm wondering if we want to just say we only support the 2 instruction version.
> Currently that means GCC 6 only, or a distro compiler with the backport of
> e95d0248dace. But we could also ask GCC to backport it to 4.9 and 5.
> 
> Thoughts?

IMHO that's a too weak reason for a too strong limitation. OTOH getting everyone
to use the 2 insn version sounds appealing...

Is e95d0248dace self-sufficient or does it depend on other improvements?

	Torsten