Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932246AbbFDU0t (ORCPT ); Thu, 4 Jun 2015 16:26:49 -0400 Received: from ozlabs.org ([103.22.144.67]:41818 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752609AbbFDU0R (ORCPT ); Thu, 4 Jun 2015 16:26:17 -0400 From: Rusty Russell To: Andrew Cooper , "H. Peter Anvin" , Xen-devel Cc: David Vrabel , Thomas Gleixner , Ingo Molnar , x86@kernel.org, linux-kernel@vger.kernel.org, Konrad Rzeszutek Wilk , Boris Ostrovsky , lguest@lists.ozlabs.org, stable@vger.kernel.org Subject: Re: [PATCH] x86/cpu: Fix SMAP check in PVOPS environments In-Reply-To: <5570131C.1000704@citrix.com> References: <1433323874-6927-1-git-send-email-andrew.cooper3@citrix.com> <556FF270.5060306@zytor.com> <5570131C.1000704@citrix.com> User-Agent: Notmuch/0.17 (http://notmuchmail.org) Emacs/24.4.1 (x86_64-pc-linux-gnu) Date: Fri, 05 Jun 2015 05:25:49 +0930 Message-ID: <873827uu2i.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2051 Lines: 47 Andrew Cooper writes: > On 04/06/15 07:38, H. Peter Anvin wrote: >> On 06/03/2015 02:31 AM, Andrew Cooper wrote: >>> There appears to be no formal statement of what pv_irq_ops.save_fl() is >>> supposed to return precisely. Native returns the full flags, while lguest and >>> Xen only return the Interrupt Flag, and both have comments by the >>> implementations stating that only the Interrupt Flag is looked at. This may >>> have been true when initially implemented, but no longer is. >>> >>> To make matters worse, the Xen PVOP leaves the upper bits undefined, making >>> the BUG_ON() undefined behaviour. Experimentally, this now trips for 32bit PV >>> guests on Broadwell hardware. The BUG_ON() is consistent for an individual >>> build, but not consistent for all builds. It has also been a sitting timebomb >>> since SMAP support was introduced. >>> >>> Use native_save_fl() instead, which will obtain an accurate view of the AC >>> flag. >> Could we fix the Xen pvops wrapper instead to not do things like this? >> >> -hpa >> >> > > We could, and I have a patch for that, but the check would still then be > broken in lguest, and it makes a hotpath rather longer. > > Either pv_irq_ops.save_fl() gets defined to handle all flags, and Xen & > lguest need correcting in this regard, or save_fl() gets defined to > handle the interrupt flag only, and this becomes the single problematic > caller in the codebase. > > The problem with expanding save_fl() to handle all flags is that > restore_fl() should follow suit, and there are a number of system flags > are inapplicable in such a situation. Yeah, hard cases make bad law. I'm not too unhappy with this fix; ideally we'd rename save_fl and restore_fl to save_eflags_if and restore_eflags_if too. Cheers, Rusty. -- 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/