Subject: Re: [PATCH v2 09/13] powerpc/ftrace: Implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS

From: "Naveen N. Rao" <[email-masked]>
Date: Mon, 14 Feb 2022 20:55:33 +0530

Christophe Leroy wrote:
> Implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS. It accelerates the call
> of livepatching.
> 
> Also note that powerpc being the last one to convert to
> CONFIG_DYNAMIC_FTRACE_WITH_ARGS, it will now be possible to remove
> klp_arch_set_pc() on all architectures.
> 
> Signed-off-by: Christophe Leroy <[email-masked]>
> ---
>  arch/powerpc/Kconfig                 |  1 +
>  arch/powerpc/include/asm/ftrace.h    | 17 +++++++++++++++++
>  arch/powerpc/include/asm/livepatch.h |  4 +---
>  3 files changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index cdac2115eb00..e2b1792b2aae 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -210,6 +210,7 @@ config PPC
>  	select HAVE_DEBUG_KMEMLEAK
>  	select HAVE_DEBUG_STACKOVERFLOW
>  	select HAVE_DYNAMIC_FTRACE
> +	select HAVE_DYNAMIC_FTRACE_WITH_ARGS	if MPROFILE_KERNEL || PPC32
>  	select HAVE_DYNAMIC_FTRACE_WITH_REGS	if MPROFILE_KERNEL || PPC32
>  	select HAVE_EBPF_JIT
>  	select HAVE_EFFICIENT_UNALIGNED_ACCESS	if !(CPU_LITTLE_ENDIAN && POWER7_CPU)
> diff --git a/arch/powerpc/include/asm/ftrace.h b/arch/powerpc/include/asm/ftrace.h
> index b3f6184f77ea..45c3d6f11daa 100644
> --- a/arch/powerpc/include/asm/ftrace.h
> +++ b/arch/powerpc/include/asm/ftrace.h
> @@ -22,6 +22,23 @@ static inline unsigned long ftrace_call_adjust(unsigned long addr)
>  struct dyn_arch_ftrace {
>  	struct module *mod;
>  };
> +
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
> +struct ftrace_regs {
> +	struct pt_regs regs;
> +};
> +
> +static __always_inline struct pt_regs *arch_ftrace_get_regs(struct ftrace_regs *fregs)
> +{
> +	return &fregs->regs;
> +}

I think this is wrong. We need to differentiate between ftrace_caller() 
and ftrace_regs_caller() here, and only return pt_regs if coming in 
through ftrace_regs_caller() (i.e., FL_SAVE_REGS is set).

> +
> +static __always_inline void ftrace_instruction_pointer_set(struct ftrace_regs *fregs,
> +							   unsigned long ip)
> +{
> +	regs_set_return_ip(&fregs->regs, ip);

Should we use that helper here? regs_set_return_ip() also updates some 
other state related to taking interrupts and I don't think it makes 
sense for use with ftrace.

- Naveen