From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753832AbcEIXle (ORCPT ); Mon, 9 May 2016 19:41:34 -0400 Received: from mga01.intel.com ([192.55.52.88]:58057 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753790AbcEIXld (ORCPT ); Mon, 9 May 2016 19:41:33 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.24,602,1455004800"; d="scan'208";a="802612837" Subject: Re: [PATCH v5 12/13] x86/xsaves: Fix fpstate_init() for XSAVES To: Yu-cheng Yu , linux-kernel@vger.kernel.org, x86@kernel.org, "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar References: <9c6c58a0a4a66568b8e887944d2426d59370ae05.1462816638.git.yu-cheng.yu@intel.com> Cc: Andy Lutomirski , Borislav Petkov , Sai Praneeth Prakhya , "Ravi V. Shankar" , Fenghua Yu From: Dave Hansen Message-ID: <5731202B.2050805@linux.intel.com> Date: Mon, 9 May 2016 16:41:31 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 MIME-Version: 1.0 In-Reply-To: <9c6c58a0a4a66568b8e887944d2426d59370ae05.1462816638.git.yu-cheng.yu@intel.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/09/2016 01:46 PM, Yu-cheng Yu wrote: > When XSAVES is used, xsave.header.xcomp_bv[63] must be set. So, that's not strictly correct. XSAVES can write to a completely empty (0'd) memory buffer. It's only XRSTORS that _needs_ bit 63 set. The instruction reference is pretty clear on this point. Oh, and if you decided to do this for some reason, please have mercy and go make a macro for 1<<63. Also, I don't think the kernel ever checks for this bit. So are we really calling XRSTORS on otherwise uninitialized xsave buffers?