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 S1751546AbdBJBIY (ORCPT <rfc822;w@1wt.eu>);
        Thu, 9 Feb 2017 20:08:24 -0500
Received: from mx1.redhat.com ([209.132.183.28]:38262 "EHLO mx1.redhat.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1751464AbdBJBIW (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 9 Feb 2017 20:08:22 -0500
Message-ID: <1486688411.2096.31.camel@redhat.com>
Subject: Re: [PATCH v2] x86/fpu: copy MXCSR & MXCSR_FLAGS with SSE/YMM state
From: Rik van Riel <riel@redhat.com>
To: Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, luto@kernel.org,
        dave.hansen@linux.intel.com, bp@suse.de, hpa@zytor.com
Date: Thu, 09 Feb 2017 20:00:11 -0500
In-Reply-To: <20170210004541.GA20800@test-lenovo>
References: <20170209184347.2ef977b9@annuminas.surriel.com>
         <20170210004541.GA20800@test-lenovo>
Organization: Red Hat, Inc
Content-Type: text/plain; charset="UTF-8"
Mime-Version: 1.0
Content-Transfer-Encoding: 8bit
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Fri, 10 Feb 2017 01:00:14 +0000 (UTC)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, 2017-02-09 at 16:45 -0800, Yu-cheng Yu wrote:
> On Thu, Feb 09, 2017 at 06:43:47PM -0500, Rik van Riel wrote:
> >  /*
> > + * Weird legacy quirk: SSE and YMM states store information in the
> > + * MXCSR and MXCSR_FLAGS fields of the FP area. That means if the
> > FP
> > + * area is marked as unused in the xfeatures header, we need to
> > copy
> > + * MXCSR and MXCSR_FLAGS if either SSE or YMM are in use.
> > + */
> > +static inline bool xfeatures_need_mxcsr_copy(u64 xfeatures)
> > +{
> > +	if (!(xfeatures & (XFEATURE_MASK_SSE|XFEATURE_MASK_YMM)))
> > +		return 0;
> > +
> > +	if (xfeatures & XFEATURE_MASK_FP)
> > +		return 0;
> > +
> > +	return 1;
> > +}
> 
> Would you consider separating MXCSR & MXCSR_FLAGS from
> XFEATURE_MASK_FP.  Here we assume if both xstate[0] and xstate[1]
> are being copied, then there is no need to copy MXCSR/MXCSR_FLAGS
> again.  What if only xstate[0] is copied and MXCSR/MXCSR_FLAGS
> is invalid?

Surely then the CPU would ignore the contents of
MXCSR/MXCSR_FLAGS because the SSE and YMM bits in
the xfeatures header are clear?

What am I missing?