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Subject: Re: [PATCH 03/12] powerpc/module: Create a special stub for
 ftrace_caller()
From: Michael Ellerman <mpe@ellerman.id.au>
To: Balbir Singh <bsingharora@gmail.com>, linuxppc-dev@ozlabs.org
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Date: Thu, 25 Feb 2016 21:48:15 +1100
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On Thu, 2016-02-25 at 11:08 +1100, Balbir Singh wrote:
> 
> On 25/02/16 01:28, Michael Ellerman wrote:
> > In order to support the new -mprofile-kernel ABI, we need to be able to
> > call from the module back to ftrace_caller() (in the kernel) without
> > using the module's r2. That is because the function in this module which
> > is calling ftrace_caller() may not have setup r2, if it doesn't
> > otherwise need it (ie. it accesses no globals).
> > 
> > To make that work we add a new stub which is used for calling
> > ftrace_caller(), which uses the kernel toc instead of the module toc.
> > 
> > diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> > index 9629966e614b..e711d40a3b8f 100644
> > --- a/arch/powerpc/kernel/module_64.c
> > +++ b/arch/powerpc/kernel/module_64.c
> > @@ -671,10 +671,56 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
> >  }
> >  
> >  #ifdef CONFIG_DYNAMIC_FTRACE
> > +
> > +#define PACATOC offsetof(struct paca_struct, kernel_toc)
> > +
> > +static unsigned long create_ftrace_stub(const Elf64_Shdr *sechdrs, struct module *me)
> > +{
> > +	struct ppc64_stub_entry *entry;
> > +	unsigned int i, num_stubs;

> 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)

Yeah I'll add some commentary.

I think the change log describes it fairly well but a comment is also good.

cheers