Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754014Ab3JQJv5 (ORCPT ); Thu, 17 Oct 2013 05:51:57 -0400 Received: from nat28.tlf.novell.com ([130.57.49.28]:59153 "EHLO nat28.tlf.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753798Ab3JQJvz convert rfc822-to-8bit (ORCPT ); Thu, 17 Oct 2013 05:51:55 -0400 Message-Id: <525FCF5802000078000FBC04@nat28.tlf.novell.com> X-Mailer: Novell GroupWise Internet Agent 12.0.2 Date: Thu, 17 Oct 2013 10:51:52 +0100 From: "Jan Beulich" To: "Gleb Natapov" Cc: "Ingo Molnar" , "Thomas Gleixner" , "Linus Torvalds" , "KVM list" , "Linux Kernel Mailing List" , "Peter Anvin" Subject: Re: [PATCH, RFC] x86-64: properly handle FPU code/data selectors References: <525E9BFF02000078000FB74E@nat28.tlf.novell.com> <525ECEA102000078000FB906@nat28.tlf.novell.com> <525ED75402000078000FB95B@nat28.tlf.novell.com> <20131017092727.GN15657@redhat.com> <525FCB0D02000078000FBBCF@nat28.tlf.novell.com> <20131017094146.GP15657@redhat.com> In-Reply-To: <20131017094146.GP15657@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8BIT Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1727 Lines: 37 >>> On 17.10.13 at 11:41, Gleb Natapov wrote: > On Thu, Oct 17, 2013 at 10:33:33AM +0100, Jan Beulich wrote: >> >>> On 17.10.13 at 11:27, Gleb Natapov wrote: >> > On Wed, Oct 16, 2013 at 05:13:40PM +0100, Jan Beulich wrote: >> >> > It preserves *less* state, because the upper 32 bits of rip are now >> >> > corrupted. Any 64-bit application that actually looks at the FP >> >> > rip/rdp fields now get the WRONG VALUES. >> >> >> >> But again - this isn't being done for ordinary 64-bit applications, >> >> this is only happening for KVM guests. And there not being a >> >> protocol for telling the caller whether a certain context hold >> >> 64-bit offsets or selector/offset pairs shouldn't be a reason to >> >> think of a solution to the problem. >> >> >> > KVM knows what mode guest vcpu is in. is_long_mode(vcpu) will tell you >> > if it is in long mode or not. No need to guess it. >> >> So what if that 64-bit guest OS is running a 32-bit app? You can >> only positively know the _current_ guest word size when the >> guest is not in long mode. >> > KVM obviously knows the complete state of virtual CPU. It can figure the > situation above by looking at CS descriptor, not need to check > is_long_mode() at all. Here is how emulator does it: And again - no: The last floating point operation may have happened in 32-bit user mode context, while the state saving may happen when the guest is already back in 64-bit kernel mode. Jan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/