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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 83C99C3F2CD for ; Fri, 28 Feb 2020 18:12:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6A718246A8 for ; Fri, 28 Feb 2020 18:12:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726151AbgB1SMe (ORCPT ); Fri, 28 Feb 2020 13:12:34 -0500 Received: from mga18.intel.com ([134.134.136.126]:61772 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725805AbgB1SMd (ORCPT ); Fri, 28 Feb 2020 13:12:33 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by orsmga106.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 28 Feb 2020 10:12:33 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,496,1574150400"; d="scan'208";a="437503672" Received: from yyu32-desk.sc.intel.com ([143.183.136.146]) by fmsmga005.fm.intel.com with ESMTP; 28 Feb 2020 10:12:32 -0800 Message-ID: <9a283ad42da140d73de680b1975da142e62e016e.camel@intel.com> Subject: Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for __fpu__restore_sig() From: Yu-cheng Yu To: Borislav Petkov Cc: linux-kernel@vger.kernel.org, x86@kernel.org, "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , Dave Hansen , Tony Luck , Andy Lutomirski , Rik van Riel , "Ravi V. Shankar" , Sebastian Andrzej Siewior , Fenghua Yu , Peter Zijlstra Date: Fri, 28 Feb 2020 10:11:44 -0800 In-Reply-To: <20200228172202.GD25261@zn.tnic> References: <20200121201843.12047-1-yu-cheng.yu@intel.com> <20200121201843.12047-9-yu-cheng.yu@intel.com> <20200221175859.GL25747@zn.tnic> <77f3841a92df5d0c819699ee3612118d566b7445.camel@intel.com> <20200228121724.GA25261@zn.tnic> <89bcab262d6dad4c08c4a21e522796fea2320db3.camel@intel.com> <20200228162359.GC25261@zn.tnic> <6f91699c91f9ea0f527e80ed3ea2999444a8d2d1.camel@intel.com> <20200228172202.GD25261@zn.tnic> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.32.4 (3.32.4-1.fc30) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2020-02-28 at 18:22 +0100, Borislav Petkov wrote: > On Fri, Feb 28, 2020 at 08:20:27AM -0800, Yu-cheng Yu wrote: > > When XSAVES writes to an xsave buffer, xsave->header.xcomp_bv is set to > > include only saved components, effectively changing the buffer's format. > > So you want to *save* the supervisor states and xcomp_bv will be set to > supervisor states only and since we don't care about the user states > there - they will be loaded later - we're good. No! > Or do you have to set xcomp_bv later in order to save the user > components too and also rearrange the buffer to undo the format change > above? That is the case. If we save only supervisor states, the buffer becomes smaller and has only supervisor states. > We have using_compacted_format() and we do conversion from compacted to > standard buffers - I'm looking at copy_xstate_to_kernel() et al - so it > shouldn't be impossible. So to repeat Sebastian's question which you > ignored: > > "How large is this supervisor state at most? I guess saving the AVX512 > state just to get the 2 bytes of the supervisor state at the right spot > is not really optimal." I thought Sebastian was saying XSAVES is not optimal. CET has 16 bytes for ring-3 setting, 24 bytes for ring-0. Saving supervisor states somewhere else and copying back is not better either. > In any case, this performance penalty better be paid only by those > who are actually using some supervisor states. I haven't looked at > the CET patchset but I'm assuming you're setting the CET bit in > xfeatures_mask_all only when the feature is being actually used? We save supervisor states only when xfeatures_mask_supervisor() is not zero. Yu-cheng