Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751276AbdHXFrk (ORCPT ); Thu, 24 Aug 2017 01:47:40 -0400 Received: from mail-pg0-f66.google.com ([74.125.83.66]:34328 "EHLO mail-pg0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751019AbdHXFri (ORCPT ); Thu, 24 Aug 2017 01:47:38 -0400 From: Hoeun Ryu To: Kees Cook , Andrew Morton , Nicholas Piggin , Ingo Molnar , Michal Marek , Laura Abbott , Al Viro , Andy Lutomirski , mic@digikod.net, Russell King , Vladimir Murzin , Heiko Carstens , Jonathan Austin , Masahiro Yamada , Mark Rutland , Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, hoeun.ryu@gmail.com, linux-arch@vger.kernel.org Subject: [RFC 1/3] arch: add 64BIT_ATOMIC_ACCESS to support 64bit atomic access on 32bit machines Date: Thu, 24 Aug 2017 14:42:55 +0900 Message-Id: <1503553377-3646-2-git-send-email-hoeun.ryu@gmail.com> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1503553377-3646-1-git-send-email-hoeun.ryu@gmail.com> References: <1503553377-3646-1-git-send-email-hoeun.ryu@gmail.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2345 Lines: 59 On some 32-bit architectures, 64bit accesses are atomic when certain conditions are satisfied. For example, on LPAE (Large Physical Address Extension) enabled ARM architecture, 'ldrd/strd' (load/store doublewords) instructions are 64bit atomic as long as the address is 64-bit aligned. This feature is to guarantee atomic accesses on newly introduced 64bit wide descriptors in the translation tables, and 's/u64' variables can be accessed atomically when they are aligned(8) on LPAE enabled ARM architecture machines. Introducing 64BIT_ATOMIC_ACCESS and 64BIT_ATOMIC_ALIGNED_ACCESS, which can be true for the 32bit architectures as well as 64bit architectures, we can optimize some kernel codes using seqlock (timekeeping) or mimics of it (like in sched/cfq) simply to read or write 64bit variables. The existing codes depend on CONFIG_64BIT to determine whether the 64bit variables can be directly accessed or need additional synchronization primitives like seqlock. CONFIG_64BIT_ATOMIC_ACCESS can be used instead of CONFIG_64BIT in the cases. 64BIT_ATOMIC_ALIGNED_ACCESS can be used in the variable declaration to indicate the alignment requirement to the compiler (__attribute__((aligned(8)))) in the way of #ifdef. Signed-off-by: Hoeun Ryu --- arch/Kconfig | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/arch/Kconfig b/arch/Kconfig index 21d0089..1def331 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -115,6 +115,26 @@ config UPROBES managed by the kernel and kept transparent to the probed application. ) +config 64BIT_ATOMIC_ACCESS + def_bool 64BIT + help + On some 32bit architectures as well as 64bit architectures, + 64bit accesses are atomic when certain conditions are satisfied. + + This symbol should be selected by an architecture if 64 bit + accesses can be atomic. + +config 64BIT_ATOMIC_ALIGNED_ACCESS + def_bool n + depends on 64BIT_ATOMIC_ACCESS + help + On 64BIT_ATOMIC_ACCESS enabled system, the address should be + aligned by 8 to guarantee the accesses are atomic. + + This symbol should be selected by an architecture if 64 bit + accesses are required to be 64 bit aligned to guarantee that + the 64bit accesses are atomic. + config HAVE_64BIT_ALIGNED_ACCESS def_bool 64BIT && !HAVE_EFFICIENT_UNALIGNED_ACCESS help -- 2.7.4