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

[Steven Rostedt]

On Mon, 6 Nov 2023 10:05:49 +0900
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> 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