From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4E520C433EF for ; Mon, 28 Mar 2022 14:58:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244154AbiC1PAH (ORCPT ); Mon, 28 Mar 2022 11:00:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51476 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S244146AbiC1O76 (ORCPT ); Mon, 28 Mar 2022 10:59:58 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2307CB84 for ; Mon, 28 Mar 2022 07:58:16 -0700 (PDT) From: Sebastian Andrzej Siewior DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1648479494; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=SOuuvst+UUNcj8L4Pzp1TF8twONkUaE7vZdYgdMuLVo=; b=HgHOi4bJEj3U+SyFteDlU1LoxRHx0XUwJUo5/Krx3azavcESGbVbGBAE/dBrN+TNhfoi1c hlVxi9KlPTd6r5UZLSIjAl9jy607KP1HeZ7ahddP+Vbr3a2Bj+C8wQJOF9ySxfMzDOav/k fzbLG/NF3wwy6OppYP8RKHR0kQu0A5tqF4AnmIbl80/94GaoHphXUhpZWxWKVqlanDGbmf ihhAfqCGMYO+FvBAblg0vABkV0XyfvcNCx32/qaWeO75kL1Bk+ZD0wsovvYLG6jWnAFZ5H AhL6VR9815NgZDhX1J3feMmJjqT6nshKdroiq/wSnYV6f9jzinJBNz2Up12x2Q== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1648479494; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=SOuuvst+UUNcj8L4Pzp1TF8twONkUaE7vZdYgdMuLVo=; b=2aCABIvcnUJpmvSC8UGLXtseBPVnJlnqfN1q1go6xtgFvzsm+iRVJNNKc6wIkd3IrnvyoO f3f+oZc0RjR82VCQ== To: torvalds@linux-foundation.org Cc: akpm@linux-foundation.org, bigeasy@linutronix.de, boqun.feng@gmail.com, bp@alien8.de, linux-kernel@vger.kernel.org, longman@redhat.com, mingo@kernel.org, peterz@infradead.org, tglx@linutronix.de, will@kernel.org, Dennis Zhou , Tejun Heo , Christoph Lameter Subject: [PATCH v2 1/3] x86/percpu: Remove volatile from arch_raw_cpu_ptr(). Date: Mon, 28 Mar 2022 16:58:08 +0200 Message-Id: <20220328145810.86783-2-bigeasy@linutronix.de> In-Reply-To: <20220328145810.86783-1-bigeasy@linutronix.de> References: <20220328145810.86783-1-bigeasy@linutronix.de> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The volatile attribute in the inline assembly of arch_raw_cpu_ptr() forces the compiler to always generate the code, even if the compiler can decide upfront that its result is not needed. For instance invoking __intel_pmu_disable_all(false) (like intel_pmu_snapshot_arch_branch_stack() does) leads to loading the address of &cpu_hw_events into the register while compiler knows that it has no need for it. This ends up with code like: | movq $cpu_hw_events, %rax #, tcp_ptr__ | add %gs:this_cpu_off(%rip), %rax # this_cpu_off, tcp_ptr__ | xorl %eax, %eax # tmp93 It also creates additional code within local_lock() with !RT && !LOCKDEP which is not desired. By removing the volatile attribute the compiler can place the function freely and avoid it if it is not needed in the end. By using the function twice the compiler properly caches only the variable offset and always loads the CPU-offset. this_cpu_ptr() also remains properly placed within a preempt_disable() sections because - arch_raw_cpu_ptr() assembly has a memory input ("m" (this_cpu_off)) - prempt_{dis,en}able() fundamentally has a 'barrier()' in it Therefore this_cpu_ptr() is already properly serialized and does not rely on the 'volatile' attribute. Remove volatile from arch_raw_cpu_ptr(). [ bigeasy: Added Linus' explanation why this_cpu_ptr() is not moved out of a preempt_disable() section without the 'volatile' attribute. ] Suggested-by: Linus Torvalds Signed-off-by: Sebastian Andrzej Siewior --- arch/x86/include/asm/percpu.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h index a3c33b79fb865..5d572a19a389c 100644 --- a/arch/x86/include/asm/percpu.h +++ b/arch/x86/include/asm/percpu.h @@ -38,7 +38,7 @@ #define arch_raw_cpu_ptr(ptr) \ ({ \ unsigned long tcp_ptr__; \ - asm volatile("add " __percpu_arg(1) ", %0" \ + asm ("add " __percpu_arg(1) ", %0" \ : "=3Dr" (tcp_ptr__) \ : "m" (this_cpu_off), "0" (ptr)); \ (typeof(*(ptr)) __kernel __force *)tcp_ptr__; \ --=20 2.35.1