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

Masami Hiramatsu
2023-11-07T00:43:05Z

On Mon, 6 Nov 2023 11:37:10 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> 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.

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.

Thank you,

>
> 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

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