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
