Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754521Ab3JQKh4 (ORCPT ); Thu, 17 Oct 2013 06:37:56 -0400 Received: from nat28.tlf.novell.com ([130.57.49.28]:50023 "EHLO nat28.tlf.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752314Ab3JQKhy convert rfc822-to-8bit (ORCPT ); Thu, 17 Oct 2013 06:37:54 -0400 Message-Id: <525FDA1C02000078000FBCC0@nat28.tlf.novell.com> X-Mailer: Novell GroupWise Internet Agent 12.0.2 Date: Thu, 17 Oct 2013 11:37:48 +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> <525FCF5802000078000FBC04@nat28.tlf.novell.com> <20131017102354.GR15657@redhat.com> In-Reply-To: <20131017102354.GR15657@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: 1754 Lines: 39 >>> On 17.10.13 at 12:23, Gleb Natapov wrote: > On Thu, Oct 17, 2013 at 10:51:52AM +0100, Jan Beulich wrote: >> >>> On 17.10.13 at 11:41, Gleb Natapov wrote: >> > 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. >> > Hmm, OK so the scenarios you are talking about is: > 1. Guest's 32bit process uses FPU > 2. Guest switch to 64bit kernel. > 3. Before guest's kernel saves 32bit process's FPU state VMEXIT happens > 4. KVM need to save FPU but it does not know what mode it is in > Correct? Yes. > KVM gives FPU to a guest lazily, meaning that on a first FPU use #NM > (intercepted by KVM) happens at which point FPU is granted to a guest. > KVM can check what mode CPU was in at this point and use this info > while saving FPU. But there is additional optimization that will prevent > this from working for all cases: when FPU is granted to a guest KVM > disabled CR0.TS/#NM intercepts, so guest is free to switch FPU from > 32bit to 64bit mode without KVM knowing. Disabling this optimization > will make FP intensive workload slow in a guest. Not sure what you're trying to tell me with this explanation. 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/