linux-kernel - Re: [PATCH v14 04/19] function_graph: Replace fgraph_ret_regs with ftrace

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Message-ID: <20241001193234.2acb6147@gandalf.local.home>
Date: Tue, 1 Oct 2024 19:32:34 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Will Deacon <will@...nel.org>, Catalin Marinas
 <catalin.marinas@....com>, linux-arm-kernel@...ts.infradead.org, Florent
 Revest <revest@...omium.org>, linux-trace-kernel@...r.kernel.org, LKML
 <linux-kernel@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>, Jiri
 Olsa <jolsa@...nel.org>, Arnaldo Carvalho de Melo <acme@...nel.org>, Daniel
 Borkmann <daniel@...earbox.net>, Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v14 04/19] function_graph: Replace fgraph_ret_regs with
 ftrace_regs

On Wed, 2 Oct 2024 08:10:37 +0900
Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> > 
> > I may add some compiler hacks to enforce this. Something like:
> > 
> > struct ftrace_regs {
> > 	void *nothing_to_see_here;
> > };  
> 
> Yeah, OK. But sizeof(fregs) may be changed. (Shouldn't we do too?)

Honestly, I don't think anything should be doing a sizeof(struct ftrace_regs)

Heck, perhaps we should make it totally zero!

  struct ftrace_regs {
	long nothing_here[];
  };

If someone needs to allocate, then we could provide a:

	ftrace_regs_size()

helper function.

> 
> > 
> > And then change the arch code to be something like:
> > 
> > // in arch/arm64/include/asm/ftrace.h:
> > 
> > struct arch_ftrace_regs {
> >         /* x0 - x8 */
> >         unsigned long regs[9];
> > 
> > #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
> >         unsigned long direct_tramp;
> > #else
> >         unsigned long __unused;
> > #endif
> > 
> >         unsigned long fp;
> >         unsigned long lr;
> > 
> >         unsigned long sp;
> >         unsigned long pc;
> > };  
> 
> And if it is pt_regs compatible, 
> 
> #define arch_ftrace_regs pt_regs
> 
> ?
> 

Only if it is fully pt_regs compatible.

-- Steve