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Thu, 12 Nov 2026 10:33:54 -0800 (PST) Received: from krava ([173.38.220.46]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-c5095b236edsm6328666b.23.2026.11.12.10.33.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 12 Nov 2026 10:33:53 -0800 (PST) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Thu, 12 Nov 2026 19:33:51 +0100 To: Peter Zijlstra Cc: Masami Hiramatsu , Naveen N Rao , "David S. Miller" , Josh Poimboeuf , Jason Baron , Alice Ryhl , Steven Rostedt , Ard Biesheuvel , Thomas Gleixner , Ingo Molnar , Alan Maguire , x86@kernel.org, linux-trace-kernel@vger.kernel.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org Subject: Re: [PATCH 03/13] jump_label/x86: Avoid decoding instructions for jump entry size Message-ID: References: <20261109194916.1089703-1-jolsa@kernel.org> <20261109194916.1089703-4-jolsa@kernel.org> <20261112083945.GF2668804@noisy.programming.kicks-ass.net> X-Mailing-List: bpf@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261112083945.GF2668804@noisy.programming.kicks-ass.net> On Thu, Nov 12, 2026 at 09:39:45AM +0100, Peter Zijlstra wrote: > On Mon, Nov 09, 2026 at 08:49:06PM +0100, Jiri Olsa wrote: > > > int arch_jump_entry_size(struct jump_entry *entry) > > { > > + const u8 *code = (void *)jump_entry_code(entry); > > > > + if (code[0] == JMP8_INSN_OPCODE || > > + !memcmp(code, x86_nops[JMP8_INSN_SIZE], JMP8_INSN_SIZE)) > > + return JMP8_INSN_SIZE; > > > > + if (code[0] == JMP32_INSN_OPCODE || > > + !memcmp(code, x86_nops[JMP32_INSN_SIZE], JMP32_INSN_SIZE)) > > + return JMP32_INSN_SIZE; > > + > > + BUG(); > > } > > Does this actually generate sane code and checks code[0] first for both > values before it goes and does memcmp() ? The naive translation would > suggest it does not. > nope.. you're right, I'll make it explicit thanks, jirka