Re: [PATCH 1/2] arm64: ftrace: enable single ftrace_ops for direct calls

On Mon, Jul 27, 2026 at 10:28:43PM +0800, Leon Hwang wrote:
> The BPF tracing multi link updates several direct-call sites through one
> ftrace_ops. Its implementation is therefore gated by
> HAVE_SINGLE_FTRACE_DIRECT_OPS in addition to
> DYNAMIC_FTRACE_WITH_DIRECT_CALLS.
> 
> Select HAVE_SINGLE_FTRACE_DIRECT_OPS whenever arm64 enables dynamic ftrace
> direct calls. This enables BPF tracing multi links on arm64. Also
> generalize the unreachable-trampoline comment because the single-ops path
> does not use ops->direct_call.

hi,
iirc arm needs ops->direct_call for trampolines to work properly,
that's the reason we enabled single ftrace_ops on x86 only:

  424f6a361096 bpf,x86: Use single ftrace_ops for direct calls

I don't understand the arm usage of op->direct_call, but it looks like
(codex thinks) that the fast path won't always work without op->direct_call
being set

from arch/arm64/kernel/entry-ftrace.S:

	#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
		/*
		 * If the op has a direct call, handle it immediately without
		 * saving/restoring registers.
		 */
		ldr     x17, [x11, #FTRACE_OPS_DIRECT_CALL]             // op->direct_call
		cbnz    x17, ftrace_caller_direct
	#endif
	#endif

jirka