linux-kernel - Re: [RFC PATCH 24/32] x86/ftrace: Enable HAVE_FUNCTION_GRAPH

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Message-Id: <20231107144328.cc763a2a137391ceb105e9db@kernel.org>
Date:   Tue, 7 Nov 2023 14:43:28 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Florent Revest <revest@...omium.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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Guo Ren <guoren@...nel.org>
Subject: Re: [RFC PATCH 24/32] x86/ftrace: Enable HAVE_FUNCTION_GRAPH_FREGS

On Mon, 6 Nov 2023 22:06:17 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Tue, 7 Nov 2023 09:42:58 +0900
> Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> 
> > Got it. So does ftrace_regs need a placeholder for direct trampoline?
> > (Or, can we use a register to pass it?)
> > I think we don't need to clear it for return_to_handler() but if
> > `ftrace_regs` spec requires it, it is better to do so.
> 
> It's per arch defined. I think I wrote somewhere that it just needs to pass
> back something that can tell if the handler is to return to a direct
> trampoline or not. It could be a unused register, or something else.

Oh, I meant the flag (address) for "return" trampoline. If we have
direct "return" trampoline we may use it, but currently not.

> 
> It's only needed if an architecture supports direct trampolines.

I see, and x86_64 needs it.
OK, maybe better to keep it clear on x86-64 even on the
return handler.

Thank you,

> 
> -- Steve


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