Re: [PATCH v2 2/2] kernel/trace: Remove function callback casts

[Oscar Carter <oscar.carter@gmx.com>]

Hi Steven,

On Mon, Jul 27, 2020 at 09:53:06AM -0400, Steven Rostedt wrote:
> On Sun, 26 Jul 2020 17:52:42 +0200
> Oscar Carter <oscar.carter@gmx.com> wrote:
>
> > > If I try to do this I will need some help. Some info that point me to the
> > > right direction would be greatly appreciated. Some advice about what
> > > functions I will need to implement would be really helpfull. Or point me
> > > to the right piece of code that I can pick as base point.
> >
> > I've been searching and reading the code as much as possible. I've found
> > two patches that I think can be useful to guide me. One [1] adds support
> > for ftrace_ops to the riscv architecture. The other one [2] adds support
> > for ftrace_ops to the parisc architecture.
> >
> > [1] commit 71e736a7d655 ("riscv/ftrace: Add ARCH_SUPPORTS_FTRACE_OPS support")
> > [2] commit d562aca37a54 ("parisc/ftrace: Add ARCH_SUPPORTS_FTRACE_OPS support")
> >
> > Due to powerpc arch calls the needed functions from assembly, I based my
> > idea on the patch for the RISCV arch.
> >
> > Can something like the following work?
> >
> > diff --git a/arch/powerpc/include/asm/ftrace.h b/arch/powerpc/include/asm/ftrace.h
> > index bc76970b6ee5..1c51ff5afae1 100644
> > --- a/arch/powerpc/include/asm/ftrace.h
> > +++ b/arch/powerpc/include/asm/ftrace.h
> > @@ -61,9 +61,8 @@ struct dyn_arch_ftrace {
> >  };
> >  #endif /* __ASSEMBLY__ */
> >
> > -#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> >  #define ARCH_SUPPORTS_FTRACE_OPS 1
> > -#endif
> > +
> >  #endif /* CONFIG_FUNCTION_TRACER */
> >
> >  #ifndef __ASSEMBLY__
> > diff --git a/arch/powerpc/kernel/trace/ftrace_32.S b/arch/powerpc/kernel/trace/ftrace_32.S
> > index e023ae59c429..e69a4e945986 100644
> > --- a/arch/powerpc/kernel/trace/ftrace_32.S
> > +++ b/arch/powerpc/kernel/trace/ftrace_32.S
> > @@ -29,6 +29,10 @@ _GLOBAL(ftrace_caller)
> >         MCOUNT_SAVE_FRAME
> >         /* r3 ends up with link register */
> >         subi    r3, r3, MCOUNT_INSN_SIZE
> > +
> > +       /* Set ftrace_ops (r5) to the global variable function_trace_op */
> > +       /* Set pt_regs (r6) to NULL */
> > +
> >  .globl ftrace_call
> >  ftrace_call:
> >         bl      ftrace_stub
> > diff --git a/arch/powerpc/kernel/trace/ftrace_64_pg.S b/arch/powerpc/kernel/trace/ftrace_64_pg.S
> > index 6708e24db0ab..a741448b1df9 100644
> > --- a/arch/powerpc/kernel/trace/ftrace_64_pg.S
> > +++ b/arch/powerpc/kernel/trace/ftrace_64_pg.S
> > @@ -22,6 +22,10 @@ _GLOBAL_TOC(ftrace_caller)
> >         std     r3, 128(r1)
> >         ld      r4, 16(r11)
> >         subi    r3, r3, MCOUNT_INSN_SIZE
> > +
> > +       /* Set ftrace_ops (r5) to the global variable function_trace_op */
> > +       /* Set pt_regs (r6) to NULL */
>
> I'm guessing you are going to do the above here. If so, this looks correct.
>
> > +
> >  .globl ftrace_call
> >  ftrace_call:
> >         bl      ftrace_stub
> >
> > To add support for ftrace_ops to the powerpc architecture is only necessary
> > to fill the r5 and r6 registers before the call to ftrace_stub in all the
> > cases. The register r5 is a pointer to ftrace_ops struct and the register
> > r6 is a pointer to pt_regs struct. These registers are the third and fourth
> > parameters of a function with the following prototype. The first and second
> > ones are yet set.
>
> I guess you mean that the first and second ones are already set. But,
> yeah, you are on the correct path here!
>
> >
> > void func(unsigned long ip, unsigned long parent_ip,
> > 	  struct ftrace_ops *ops, struct pt_regs *regs);
> >
> > Am I in the right direction? or am I totally wrong?
>
> No, you don't look wrong. But the true test is to try it out :-)

Of course. I will do the commented work, I will test it and then I will send
a new patch.

> Don't forget to update ftrace_32.S as well.
>
>
> >
> > Thanks for your time and patience.
>
> My pleasure. Thanks for doing this. The more people that understand all
> this, the better!

Thanks for your guidance and advices.

> -- Steve

Regards,
Oscar Carter