Re: [PATCH v5 0/9] ftrace with regs + live patching for ppc64 LE (ABI v2)

[Torsten Duwe]

On Thu, Jan 21, 2016 at 10:29:13PM +0100, Jiri Kosina wrote:
> On Thu, 21 Jan 2016, Torsten Duwe wrote:
> 
> > > mcount call sites looks normal on first sight...
> > 
> > Not quite.
> > LR is not saved on the stack before the call.
> > Argh!
> > 
> > Petr, this looks like 12 bytes offset for gcc-6.
> > I think I can work around the rest.
> 
> Are we sure that gcc is doing the right thing here?
> 
> I am far from claiming understanding of ppc64 ABI, but from what Vojtech 
> told me I understood that saving link register is necessary for (at least) 
> graph tracer to work properly.

It is held in R0 only, and saved right after _mcount. Thus, _mcount just
must not clobber R0 or save it the same way as it's done afterwards or
like gcc4 does it.

I'll make a v6 that's compiler agnostic. It's a few lines to change
for the kernel proper, and I'll have to have a look at the trampolines
for modules.

	Torsten