linux-kernel - Re: [RFC PATCH v2 2/6] tracing: Expose ftrace_regs regardless of CONFIG_FUNCTION

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Message-Id: <20230809231607.0c5c75e7c3b69fcc96d82cb4@kernel.org>
Date:   Wed, 9 Aug 2023 23:16:07 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Florent Revest <revest@...omium.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-trace-kernel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        bpf <bpf@...r.kernel.org>, Sven Schnelle <svens@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alan Maguire <alan.maguire@...cle.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH v2 2/6] tracing: Expose ftrace_regs regardless of
 CONFIG_FUNCTION_TRACER

On Wed, 9 Aug 2023 12:29:13 +0200
Florent Revest <revest@...omium.org> wrote:

> On Mon, Aug 7, 2023 at 8:48 AM Masami Hiramatsu (Google)
> <mhiramat@...nel.org> wrote:
> > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> > index ce156c7704ee..3fb94a1a2461 100644
> > --- a/include/linux/ftrace.h
> > +++ b/include/linux/ftrace.h
> > @@ -112,11 +112,11 @@ static inline int ftrace_mod_get_kallsym(unsigned int symnum, unsigned long *val
> >  }
> >  #endif
> >
> > -#ifdef CONFIG_FUNCTION_TRACER
> > -
> > -extern int ftrace_enabled;
> > -
> > -#ifndef CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS
> > +/*
> > + * If the architecture doesn't support FTRACE_WITH_ARGS or disable function
> 
> nit: disables*

Thanks!

> 
> > + * tracer, define the default(pt_regs compatible) ftrace_regs.
> > + */
> > +#if !defined(CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS) || !defined(CONFIG_FUNCTION_TRACER)
> 
> I wonder if we should make things simpler with:
> 
> #if defined(HAVE_PT_REGS_COMPAT_FTRACE_REGS) || !defined(CONFIG_FUNCTION_TRACER)
> 
> And remove the ftrace_regs definitions that are copy-pastes of this
> block in arch specific headers. Then we can enforce in a single point
> that HAVE_PT_REGS_COMPAT_FTRACE_REGS holds.

Here, the "HAVE_PT_REGS_COMPAT_FTRACE_REGS" does not mean that the
ftrace_regs is completely compatible with pt_regs, but on the memory
it wraps struct pt_regs (thus we can just cast the type).

- CONFIG_DYNAMIC_FTRACE_WITH_REGS
  ftrace_regs is completely compatible with pt_regs

- CONFIG_DYNAMIC_FTRACE_WITH_ARGS
  |   ftrace_regs may not compatible with pt_regs
  |
  +- CONFIG_HAVE_PT_REGS_COMPAT_FTRACE_REGS
     but on memory image, ftrace_regs includes pt_regs.

Thank you,

> 
> Maybe that's a question for Steven ?


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>