From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933675AbYETPCV (ORCPT ); Tue, 20 May 2008 11:02:21 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753903AbYETPCL (ORCPT ); Tue, 20 May 2008 11:02:11 -0400 Received: from terminus.zytor.com ([198.137.202.10]:45170 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752358AbYETPCK (ORCPT ); Tue, 20 May 2008 11:02:10 -0400 Message-ID: <4832E705.2010900@zytor.com> Date: Tue, 20 May 2008 07:58:13 -0700 From: "H. Peter Anvin" User-Agent: Thunderbird 2.0.0.14 (X11/20080501) MIME-Version: 1.0 To: Mikael Pettersson CC: Andi Kleen , Suresh Siddha , mingo@elte.hu, tglx@linutronix.de, torvalds@linux-foundation.org, akpm@linux-foundation.org, roland@redhat.com, drepper@redhat.com, Hongjiu.lu@intel.com, linux-kernel@vger.kernel.org, arjan@linux.intel.com, rmk+lkml@arm.linux.org.uk, dan@debian.org, asit.k.mallick@intel.com Subject: Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions References: <20080513011030.GA31448@linux-os.sc.intel.com> <18477.35703.679574.760417@harpo.it.uu.se> <20080518013416.GB30034@linux-os.sc.intel.com> <18481.37905.297556.288317@harpo.it.uu.se> <20080520015723.GD30034@linux-os.sc.intel.com> <4832A173.6020203@firstfloor.org> <18482.53246.642835.894623@harpo.it.uu.se> In-Reply-To: <18482.53246.642835.894623@harpo.it.uu.se> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Mikael Pettersson wrote: > > > > Are they always zeroed in earlier CPUs though? If not that wouldn't > > work 100% reliably because whatever cookie you put in could have been > > there before by chance. > > I wrote a test program (fill an area with zeroes, fxsave, inspect > reserved fields, then fill it with ones, fxsave, inspect again), > and all processors appear to just not write anything to the reserved > fields after the last xmm register. (Tested on an old Mobile Athlon64, > Opteron 280, P4 Xeon, Pentium-D, and C2 Xeon E5345.) > > So the question now is what if anything has the Linux kernel written > to those reserved fields. (Looking..) Hmm, signal delivery on x86-64 > seems to do fxsave directly to the fxsave area in the user's sigframe, > which would imply that the reserved fields have unpredictable values. > OK, so that's not a usable path unless we can find some area in the existing data set to put a flag. Groan. -hpa