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

On Wed, 2016-02-17 at 12:30 +0100, Torsten Duwe wrote:

On Wed, Feb 17, 2016 at 09:55:40PM +1100, Michael Ellerman wrote:

quoted

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;

Yeah true that should work in practice.

It's still trivial to construct a module that will oops the loader, but I guess
that's always been true.

quoted

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.

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

Fair enough. I'm just trying to manage the complexity explosion.

I'd certainly advocate that you backport it to your toolchain.

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

AFAIK it's self sufficient, it just deletes a single line. I'll ask the GCC
guys tomorrow if they can backport it if you don't beat me to it :)

cheers