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

[Leon Hwang <leon.hwang@linux.dev>]

On 29/7/26 19:56, Jiri Olsa wrote:
> 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


In theory, op->direct_call is the fallback for the case:
HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS=y, out-of-range target, and non-BPF
direct ops. This is what Codex thinks about.

However, after searching the call sites of
register_ftrace_direct()/__modify_ftrace_direct() in kernel, which would
set ops->direct_call, the actual callers are in bpf/trampoline.c, which
have been gated with HAVE_SINGLE_FTRACE_DIRECT_OPS. That said, for
normal usage of HAVE_SINGLE_FTRACE_DIRECT_OPS=y, op->direct_call is unused.

If someone uses register_ftrace_direct() or
modify_ftrace_direct/_nolock() outside BPF in the future,
op->direct_call will be used.

So, I'd like to drop the comment change in the next revision.

Thanks,
Leon

> 
> 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