From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1033150AbeE0Pts (ORCPT ); Sun, 27 May 2018 11:49:48 -0400 Received: from mga02.intel.com ([134.134.136.20]:28383 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1032739AbeE0PqS (ORCPT ); Sun, 27 May 2018 11:46:18 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.49,448,1520924400"; d="scan'208";a="227741076" From: Fenghua Yu To: "Thomas Gleixner" , "Ingo Molnar" , "H. Peter Anvin" Cc: "Ashok Raj" , "Dave Hansen" , "Rafael Wysocki" , "Tony Luck" , "Alan Cox" , "Ravi V Shankar" , "Arjan van de Ven" , "linux-kernel" , "x86" , Fenghua Yu Subject: [RFC PATCH 05/16] x86/split_lock: Use non atomic set and clear bit instructions in clear_cpufeature() Date: Sun, 27 May 2018 08:45:54 -0700 Message-Id: <1527435965-202085-6-git-send-email-fenghua.yu@intel.com> X-Mailer: git-send-email 2.5.0 In-Reply-To: <1527435965-202085-1-git-send-email-fenghua.yu@intel.com> References: <1527435965-202085-1-git-send-email-fenghua.yu@intel.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org x86_capability can span two cache lines depending on kernel configuration and building environment. When #AC exception is enabled for split locked accesses, clear_cpufeature() may generate #AC exception because of atomic setting or clearing bits in x86_capability. But kernel clears cpufeature only during a CPU is booting up. Therefore, there is no racing condition when clear_cpufeature() is called and no need to atomically clear or set bits in x86_capability. To avoid #AC exception caused by split lock, call non atomic __set_bit() and __clear_bit(). They are faster than atomic set_bit() and clear_bit() as well. Signed-off-by: Fenghua Yu --- arch/x86/kernel/cpu/cpuid-deps.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c index 2c0bd38a44ab..b2c2a004c769 100644 --- a/arch/x86/kernel/cpu/cpuid-deps.c +++ b/arch/x86/kernel/cpu/cpuid-deps.c @@ -65,15 +65,15 @@ static const struct cpuid_dep cpuid_deps[] = { static inline void clear_feature(struct cpuinfo_x86 *c, unsigned int feature) { /* - * Note: This could use the non atomic __*_bit() variants, but the - * rest of the cpufeature code uses atomics as well, so keep it for - * consistency. Cleanup all of it separately. + * Because this code is only called during boot time and there + * is no need to be atomic, use non atomic __*_bit() for better + * performance and to avoid #AC exception for split locked access. */ if (!c) { clear_cpu_cap(&boot_cpu_data, feature); - set_bit(feature, (unsigned long *)cpu_caps_cleared); + __set_bit(feature, (unsigned long *)cpu_caps_cleared); } else { - clear_bit(feature, (unsigned long *)c->x86_capability); + __clear_bit(feature, (unsigned long *)c->x86_capability); } } -- 2.5.0