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
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