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Date: Mon, 6 Nov 2023 11:37:10 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>, Alexei Starovoitov
 <alexei.starovoitov@gmail.com>, Florent Revest <revest@chromium.org>,
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 Borkmann <daniel@iogearbox.net>, Alan Maguire <alan.maguire@oracle.com>,
 Mark Rutland <mark.rutland@arm.com>, Thomas Gleixner <tglx@linutronix.de>,
 Guo Ren <guoren@kernel.org>
Subject: Re: [RFC PATCH 24/32] x86/ftrace: Enable HAVE_FUNCTION_GRAPH_FREGS
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On Mon, 6 Nov 2023 10:05:49 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> So for x86-64,
> 
>  - rdi, rsi, rcx, rdx, r8, r9, and rsp
>  - rax and rdx
>  - rbp
> 
> (BTW, why orig_rax is cleared?)

You mean from ftrace_caller?

That's a "hack" to determine if we need to call the direct trampoline or
not. When you have both a direct trampoline and ftrace functions on the
same function, it will call ftrace_ops_list_func() to iterate all the
registered ftrace callbacks. The direct callback helper will set "orig_rax"
to let the return of the ftrace trampoline call the direct callback.

Remember if a direct callback is by itself, the fentry will call that
direct trampoline without going through the ftrace trampoline. This is used
to tell the ftrace trampoline that it's attached to a direct caller and
needs to call that and not return back to the function it is tracing.

See later down in that file we have:

	/*
	 * If ORIG_RAX is anything but zero, make this a call to that.
	 * See arch_ftrace_set_direct_caller().
	 */
	testq	%rax, %rax

-- Steve