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

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Message-Id: <20241002233108.9e4c2aaa09b8532870016dc0@kernel.org>
Date: Wed, 2 Oct 2024 23:31:08 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.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 Tue, 1 Oct 2024 19:32:34 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> 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.

Ah, Indeed.

> 
> > 
> > > 
> > > 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.

Yeah, OK, this is good idea.

Thank you,

> 
> -- Steve


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