On Mon, 6 Nov 2023 11:37:10 -0500 Steven Rostedt <rost...@goodmis.org> wrote:
> On Mon, 6 Nov 2023 10:05:49 +0900 > Masami Hiramatsu (Google) <mhira...@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) <mhira...@kernel.org>
