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
