From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1753868AbcEIXzO (ORCPT <rfc822;w@1wt.eu>);
	Mon, 9 May 2016 19:55:14 -0400
Received: from mga11.intel.com ([192.55.52.93]:61802 "EHLO mga11.intel.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1753679AbcEIXzM (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Mon, 9 May 2016 19:55:12 -0400
X-ExtLoop1: 1
X-IronPort-AV: E=Sophos;i="5.24,602,1455004800"; 
   d="scan'208";a="976111091"
Date: Mon, 9 May 2016 16:50:42 -0700
From: Yu-cheng Yu <yu-cheng.yu@intel.com>
To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@suse.de>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>
Subject: Re: [PATCH v5 12/13] x86/xsaves: Fix fpstate_init() for XSAVES
Message-ID: <20160509235042.GB28442@test-lenovo>
References: <cover.1462816638.git.yu-cheng.yu@intel.com>
 <9c6c58a0a4a66568b8e887944d2426d59370ae05.1462816638.git.yu-cheng.yu@intel.com>
 <5731202B.2050805@linux.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <5731202B.2050805@linux.intel.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, May 09, 2016 at 04:41:31PM -0700, Dave Hansen wrote:
> 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.

You mean the comments?  I will change it to XRSTORS.

> 
> 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?

Right now if we do fpstate_init(), without this patch, it will 
trigger a warning from copy_kernel_to_fxregs() when the task is
scheduled.