Re: [RFC PATCH 24/32] x86/ftrace: Enable HAVE_FUNCTION_GRAPH_FREGS

Masami Hiramatsu
2023-11-07T05:43:45Z

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

> On Tue, 7 Nov 2023 09:42:58 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.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@kernel.org>