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=-7.8 required=3.0 tests=DATE_IN_PAST_06_12, 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 AF0BEC04A6B for ; Wed, 8 May 2019 17:03:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 835B821530 for ; Wed, 8 May 2019 17:03:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729013AbfEHRDH (ORCPT ); Wed, 8 May 2019 13:03:07 -0400 Received: from mga17.intel.com ([192.55.52.151]:26495 "EHLO mga17.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728961AbfEHRDE (ORCPT ); Wed, 8 May 2019 13:03:04 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by fmsmga107.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 08 May 2019 10:03:03 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.60,446,1549958400"; d="scan'208";a="169697570" Received: from chang-linux-3.sc.intel.com ([172.25.66.171]) by fmsmga002.fm.intel.com with ESMTP; 08 May 2019 10:03:03 -0700 From: "Chang S. Bae" To: Andy Lutomirski , Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , Andi Kleen Cc: Ravi Shankar , "Chang S . Bae" , LKML Subject: [PATCH v7 07/18] x86/fsgsbase/64: Preserve FS/GS state in __switch_to() if FSGSBASE is on Date: Wed, 8 May 2019 03:02:22 -0700 Message-Id: <1557309753-24073-8-git-send-email-chang.seok.bae@intel.com> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1557309753-24073-1-git-send-email-chang.seok.bae@intel.com> References: <1557309753-24073-1-git-send-email-chang.seok.bae@intel.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Andy Lutomirski With the new FSGSBASE instructions, we can efficiently read and write the FSBASE and GSBASE in __switch_to(). Use that capability to preserve the full state. This will enable user code to do whatever it wants with the new instructions without any kernel-induced gotchas. (There can still be architectural gotchas: movl %gs,%eax; movl %eax,%gs may change GSBASE if WRGSBASE was used, but users are expected to read the CPU manual before doing things like that.) This is a considerable speedup. It seems to save about 100 cycles per context switch compared to the baseline 4.6-rc1 behavior on my Skylake laptop. [ chang: 5~10% performance improvements were seen by a context switch benchmark that ran threads with different FS/GSBASE values (to the baseline 4.16). Minor edit on the changelog. ] Signed-off-by: Andy Lutomirski Signed-off-by: Chang S. Bae Reviewed-by: Andi Kleen Cc: H. Peter Anvin Cc: Thomas Gleixner Cc: Ingo Molnar --- arch/x86/kernel/process_64.c | 34 ++++++++++++++++++++++++++++------ 1 file changed, 28 insertions(+), 6 deletions(-) diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index 17421c3..e2089c9 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -239,8 +239,18 @@ static __always_inline void save_fsgs(struct task_struct *task) { savesegment(fs, task->thread.fsindex); savesegment(gs, task->thread.gsindex); - save_base_legacy(task, task->thread.fsindex, FS); - save_base_legacy(task, task->thread.gsindex, GS); + if (static_cpu_has(X86_FEATURE_FSGSBASE)) { + /* + * If FSGSBASE is enabled, we can't make any useful guesses + * about the base, and user code expects us to save the current + * value. Fortunately, reading the base directly is efficient. + */ + task->thread.fsbase = rdfsbase(); + task->thread.gsbase = __rdgsbase_inactive(); + } else { + save_base_legacy(task, task->thread.fsindex, FS); + save_base_legacy(task, task->thread.gsindex, GS); + } } #if IS_ENABLED(CONFIG_KVM) @@ -319,10 +329,22 @@ static __always_inline void load_seg_legacy(unsigned short prev_index, static __always_inline void x86_fsgsbase_load(struct thread_struct *prev, struct thread_struct *next) { - load_seg_legacy(prev->fsindex, prev->fsbase, - next->fsindex, next->fsbase, FS); - load_seg_legacy(prev->gsindex, prev->gsbase, - next->gsindex, next->gsbase, GS); + if (static_cpu_has(X86_FEATURE_FSGSBASE)) { + /* Update the FS and GS selectors if they could have changed. */ + if (unlikely(prev->fsindex || next->fsindex)) + loadseg(FS, next->fsindex); + if (unlikely(prev->gsindex || next->gsindex)) + loadseg(GS, next->gsindex); + + /* Update the bases. */ + wrfsbase(next->fsbase); + __wrgsbase_inactive(next->gsbase); + } else { + load_seg_legacy(prev->fsindex, prev->fsbase, + next->fsindex, next->fsbase, FS); + load_seg_legacy(prev->gsindex, prev->gsbase, + next->gsindex, next->gsbase, GS); + } } static unsigned long x86_fsgsbase_read_task(struct task_struct *task, -- 2.7.4