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Wed, 29 Jul 2026 04:56:15 -0700 (PDT) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Wed, 29 Jul 2026 13:56:13 +0200 To: Leon Hwang Cc: Catalin Marinas , Will Deacon , Steven Rostedt , Masami Hiramatsu , Mark Rutland , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Eduard Zingerman , Kumar Kartikeya Dwivedi , Martin KaFai Lau , Song Liu , Yonghong Song , Emil Tsalapatis , Shuah Khan , bpf@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH 1/2] arm64: ftrace: enable single ftrace_ops for direct calls Message-ID: References: <20260727142844.21212-1-leon.hwang@linux.dev> <20260727142844.21212-2-leon.hwang@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260727142844.21212-2-leon.hwang@linux.dev> 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