Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757711Ab0GNUzA (ORCPT ); Wed, 14 Jul 2010 16:55:00 -0400 Received: from mx1.redhat.com ([209.132.183.28]:15610 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757626Ab0GNUy5 (ORCPT ); Wed, 14 Jul 2010 16:54:57 -0400 Message-ID: <4C3E2406.8060907@redhat.com> Date: Wed, 14 Jul 2010 10:54:30 -1000 From: Zachary Amsden User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.10) Gecko/20100621 Fedora/3.0.5-1.fc13 Thunderbird/3.0.5 MIME-Version: 1.0 To: Jeremy Fitzhardinge CC: Avi Kivity , "H. Peter Anvin" , Linus Torvalds , Peter Palfrader , Greg KH , linux-kernel@vger.kernel.org, stable@kernel.org, stable-review@kernel.org, akpm@linux-foundation.org, alan@lxorguk.ukuu.org.uk, Glauber Costa , Marcelo Tosatti Subject: Re: [patch 134/149] x86, paravirt: Add a global synchronization point for pvclock References: <20100707124731.GJ15122@anguilla.noreply.org> <4C359D5A.1050906@redhat.com> <20100713102350.GW15122@anguilla.noreply.org> <4C3C68C8.4060409@redhat.com> <20100713141902.GB15122@anguilla.noreply.org> <4C3C8CE5.1080705@redhat.com> <20100713162207.GC15122@anguilla.noreply.org> <4C3C9589.4090602@redhat.com> <4C3C96EC.8060901@redhat.com> <4C3C9839.4090404@redhat.com> <20100713172526.GE15122@anguilla.noreply.org> <4C3CAE8F.10900@goop.org> <4C3CE560.5050701@zytor.com> <4C3CFB8B.1090804@goop.org> <4C3DF1BE.2070404@goop.org> <4C3DF447.1000801@zytor.com> <4C3DF519.6030406@goop.org> <4C3DF7AF.7010402@zytor.com> <4C3DFA88.5020007@goop.org> <4C3E1B13.5030304@redhat.com> <4C3E20B3.6020007@goop.org> <4C3E21D1.3010207@redhat.com> In-Reply-To: <4C3E21D1.3010207@redhat.com> 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 Content-Length: 2004 Lines: 48 On 07/14/2010 10:45 AM, Zachary Amsden wrote: > On 07/14/2010 10:40 AM, Jeremy Fitzhardinge wrote: >> On 07/14/2010 01:16 PM, Avi Kivity wrote: >>> On 07/14/2010 08:57 PM, Jeremy Fitzhardinge wrote: >>>> Anything else? >>> 1. set up a mapping >>> 2. invlpg or set cr3 >>> 3. use the mapping >>> >>> Moving the invlpg will break your code. >> invlpg uses memory clobbers. All the crX ops seem to use a >> __force_order variable to sequence them - but it looks like it's done >> precisely backwards and it's barking mad to do allow write_crX to be >> reordered with respect to memory ops. >> >> Hm, looks like glommer added it surreptitiously while unifying >> system_32.h and system_64.h (system_32.h relied on asm volatile not >> being reordered; system_64.h used memory clobbers). >> J > > clts() has no memory clobber; it is used to serialize execution of > code within kernel_fpu_begin() / kernel_fpu_end() blocks. > > If the code within is reordered before the clts(), we've corrupted > guest FPU state. > > That's the kind of bug I think Linus is talking about. We've been > expecting volatile to work that way for over a decade, by my > recollection, and if it doesn't, there is going to be a lot of broken > code. > > Shouldn't we at least get a compiler switch to force the volatile > behavior? I'd suggest it default to conservative. Hmm, well, despite that not being quite correct (if guest has used FPU, we save it, which has a memory clobber), it seems to be the case that a reordering of the clts() among the other volatile asm statements would be a very bad thing - you'd get kernel FPU exceptions. And among asm volatiles, clts() is fairly unique in not having any clobbers or dependencies at all, so it could happen. Zach -- 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/