From patchwork Thu Feb 21 11:50:10 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Sebastian Andrzej Siewior X-Patchwork-Id: 10823949 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id CF21513B5 for ; Thu, 21 Feb 2019 11:52:54 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id BFA7F308CB for ; Thu, 21 Feb 2019 11:52:54 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id B3E66308D0; Thu, 21 Feb 2019 11:52:54 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.9 required=2.0 tests=BAYES_00,MAILING_LIST_MULTI, RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 6C5B7308CB for ; Thu, 21 Feb 2019 11:52:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728075AbfBULws (ORCPT ); Thu, 21 Feb 2019 06:52:48 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:45672 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727862AbfBULur (ORCPT ); Thu, 21 Feb 2019 06:50:47 -0500 Received: from localhost ([127.0.0.1] helo=flow.W.breakpoint.cc) by Galois.linutronix.de with esmtp (Exim 4.80) (envelope-from ) id 1gwmsD-0001RC-Ej; Thu, 21 Feb 2019 12:50:41 +0100 From: Sebastian Andrzej Siewior To: linux-kernel@vger.kernel.org Cc: x86@kernel.org, Andy Lutomirski , Paolo Bonzini , =?utf-8?b?UmFkaW0gS3LEjW3DocWZ?= , kvm@vger.kernel.org, "Jason A. Donenfeld" , Rik van Riel , Dave Hansen , Sebastian Andrzej Siewior Subject: [PATCH 12/22] x86/fpu: Only write PKRU if it is different from current Date: Thu, 21 Feb 2019 12:50:10 +0100 Message-Id: <20190221115020.12385-13-bigeasy@linutronix.de> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20190221115020.12385-1-bigeasy@linutronix.de> References: <20190221115020.12385-1-bigeasy@linutronix.de> MIME-Version: 1.0 Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP 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 | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h index 43c029cdc3fe8..2d3adeb268e38 100644 --- a/arch/x86/include/asm/special_insns.h +++ b/arch/x86/include/asm/special_insns.h @@ -111,6 +111,12 @@ static inline void __write_pkru(u32 pkru) { u32 ecx = 0, edx = 0; + /* + * WRPKRU is relatively expensive compared to RDPKRU. + * Avoid WRPKRU when it would not change the value. + */ + if (pkru == __read_pkru()) + return; /* * "wrpkru" instruction. Loads contents in EAX to PKRU, * requires that ecx = edx = 0.