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

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Message-ID: <20231106113710.3bf69211@gandalf.local.home>
Date:   Mon, 6 Nov 2023 11:37:10 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.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 10:05:49 +0900
Masami Hiramatsu (Google) <mhiramat@...nel.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