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 X-Spam-Level: X-Spam-Status: No, score=-9.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3CA53C0044C for ; Wed, 7 Nov 2018 19:50:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0DC6E20862 for ; Wed, 7 Nov 2018 19:50:42 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0DC6E20862 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linutronix.de Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726842AbeKHFWa (ORCPT ); Thu, 8 Nov 2018 00:22:30 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:41933 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727668AbeKHFVQ (ORCPT ); Thu, 8 Nov 2018 00:21:16 -0500 Received: from localhost ([127.0.0.1] helo=bazinga.breakpoint.cc) by Galois.linutronix.de with esmtp (Exim 4.80) (envelope-from ) id 1gKTpJ-00070Q-BI; Wed, 07 Nov 2018 20:49:21 +0100 From: Sebastian Andrzej Siewior To: linux-kernel@vger.kernel.org Cc: x86@kernel.org, Andy Lutomirski , Paolo Bonzini , =?UTF-8?q?Radim=20Kr=C4=8Dm=C3=A1=C5=99?= , kvm@vger.kernel.org, "Jason A. Donenfeld" , Rik van Riel , Dave Hansen , Sebastian Andrzej Siewior Subject: [PATCH 15/23] x86/fpu: Only write PKRU if it is different from current Date: Wed, 7 Nov 2018 20:48:50 +0100 Message-Id: <20181107194858.9380-16-bigeasy@linutronix.de> X-Mailer: git-send-email 2.19.1 In-Reply-To: <20181107194858.9380-1-bigeasy@linutronix.de> References: <20181107194858.9380-1-bigeasy@linutronix.de> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Dave Hansen says that the `wrpkru' is more expensive than `rdpkru'. It has a higher cycle cost and it's also practically a (light) speculation barrier. As an optimisation read the current PKRU value and only write the new one if it is different. Signed-off-by: Sebastian Andrzej Siewior --- arch/x86/include/asm/special_insns.h | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h index 43c029cdc3fe8..c2ccf71b22dd6 100644 --- a/arch/x86/include/asm/special_insns.h +++ b/arch/x86/include/asm/special_insns.h @@ -107,7 +107,7 @@ static inline u32 __read_pkru(void) return pkru; } -static inline void __write_pkru(u32 pkru) +static inline void __write_pkru_insn(u32 pkru) { u32 ecx = 0, edx = 0; @@ -118,6 +118,17 @@ static inline void __write_pkru(u32 pkru) asm volatile(".byte 0x0f,0x01,0xef\n\t" : : "a" (pkru), "c"(ecx), "d"(edx)); } + +static inline void __write_pkru(u32 pkru) +{ + /* + * Writting PKRU is expensive. Only write the PKRU value if it is + * different from the current one. + */ + if (pkru == __read_pkru()) + return; + __write_pkru_insn(pkru); +} #else static inline u32 __read_pkru(void) { -- 2.19.1