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=-3.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 55C73C4321D for ; Tue, 21 Aug 2018 15:38:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 24833217BB for ; Tue, 21 Aug 2018 15:38:12 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 24833217BB Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com 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 S1728032AbeHUS6s (ORCPT ); Tue, 21 Aug 2018 14:58:48 -0400 Received: from mx2.suse.de ([195.135.220.15]:50336 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727965AbeHUS6n (ORCPT ); Tue, 21 Aug 2018 14:58:43 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 70B82ADBD; Tue, 21 Aug 2018 15:38:04 +0000 (UTC) From: Juergen Gross To: linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org, x86@kernel.org Cc: boris.ostrovsky@oracle.com, hpa@zytor.com, tglx@linutronix.de, mingo@redhat.com, Juergen Gross Subject: [PATCH v2 2/2] x86/pae: use 64 bit atomic xchg function in native_ptep_get_and_clear Date: Tue, 21 Aug 2018 17:37:55 +0200 Message-Id: <20180821153755.30462-3-jgross@suse.com> X-Mailer: git-send-email 2.13.7 In-Reply-To: <20180821153755.30462-1-jgross@suse.com> References: <20180821153755.30462-1-jgross@suse.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Using only 32-bit writes for the pte will result in an intermediate L1TF vulnerable PTE. When running as a Xen PV guest this will at once switch the guest to shadow mode resulting in a loss of performance. Use arch_atomic64_xchg() instead which will perform the requested operation atomically with all 64 bits. Some performance considerations according to: https://software.intel.com/sites/default/files/managed/ad/dc/Intel-Xeon-Scalable-Processor-throughput-latency.pdf The main number should be the latency, as there is no tight loop around native_ptep_get_and_clear(). "lock cmpxchg8b" has a latency of 20 cycles, while "lock xchg" (with a memory operand) isn't mentioned in that document. "lock xadd" (with xadd having 3 cycles less latency than xchg) has a latency of 11, so we can assume a latency of 14 for "lock xchg". Signed-off-by: Juergen Gross --- In case adding about 6 cycles for native_ptep_get_and_clear() is believed to be too bad I can modify the patch to add a paravirt function for that purpose in order to add the overhead for Xen guests only (in fact the overhead for Xen guests will be less, as only one instruction writing to the PTE has to be emulated by the hypervisor). --- arch/x86/include/asm/pgtable-3level.h | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) diff --git a/arch/x86/include/asm/pgtable-3level.h b/arch/x86/include/asm/pgtable-3level.h index a564084c6141..f8b1ad2c3828 100644 --- a/arch/x86/include/asm/pgtable-3level.h +++ b/arch/x86/include/asm/pgtable-3level.h @@ -2,6 +2,8 @@ #ifndef _ASM_X86_PGTABLE_3LEVEL_H #define _ASM_X86_PGTABLE_3LEVEL_H +#include + /* * Intel Physical Address Extension (PAE) Mode - three-level page * tables on PPro+ CPUs. @@ -150,10 +152,7 @@ static inline pte_t native_ptep_get_and_clear(pte_t *ptep) { pte_t res; - /* xchg acts as a barrier before the setting of the high bits */ - res.pte_low = xchg(&ptep->pte_low, 0); - res.pte_high = ptep->pte_high; - ptep->pte_high = 0; + res.pte = (pteval_t)arch_atomic64_xchg((atomic64_t *)ptep, 0); return res; } -- 2.13.7