Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752869AbbD3Tfc (ORCPT ); Thu, 30 Apr 2015 15:35:32 -0400 Received: from mail-la0-f41.google.com ([209.85.215.41]:34582 "EHLO mail-la0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751191AbbD3Tfa (ORCPT ); Thu, 30 Apr 2015 15:35:30 -0400 MIME-Version: 1.0 In-Reply-To: <554282BC.5000103@oracle.com> References: <1430420925-20474-1-git-send-email-boris.ostrovsky@oracle.com> <554282BC.5000103@oracle.com> From: Andy Lutomirski Date: Thu, 30 Apr 2015 12:35:08 -0700 Message-ID: Subject: Re: [PATCH] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests To: Boris Ostrovsky Cc: Konrad Rzeszutek Wilk , David Vrabel , "H. Peter Anvin" , Ingo Molnar , Thomas Gleixner , "linux-kernel@vger.kernel.org" , "xen-devel@lists.xenproject.org" , linux@eikelenboom.it Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1940 Lines: 47 On Thu, Apr 30, 2015 at 12:30 PM, Boris Ostrovsky wrote: > On 04/30/2015 03:17 PM, Andy Lutomirski wrote: >> >> On Thu, Apr 30, 2015 at 12:08 PM, Boris Ostrovsky >> wrote: >>> >>> Commit 61f01dd941ba ("x86_64, asm: Work around AMD SYSRET SS descriptor >>> attribute issue") makes AMD processors set SS to __KERNEL_DS in >>> __switch_to() to deal with cases when SS is NULL. >>> >>> This breaks Xen PV guests who do not want to load SS with__KERNEL_DS. >>> >>> Since the problem that the commit is trying to address would have to be >>> fixed in the hypervisor (if it in fact exists under Xen) there is no >>> reason to set X86_BUG_SYSRET_SS_ATTRS flag for PV VPCUs here. >>> >> Seems reasonable. >> >> Have you run the test case on a Xen PV guest on AMD? It's possible >> that Xen is affected, since the old accidental workaround that we >> deleted was in the vdso and probably would have worked on Xen. > > > Is there a specific test that would trigger this bug? I > booted/suspended/resumed a bunch of various guest types on both AMD and > Intel but not much more than that. What is the commit that you deleted? > > I doubt that a change in the guest could suddenly start triggering this > issue but I will take a look at hypervisor code. http://article.gmane.org/gmane.linux.kernel/1937899 The change was that we stopped reloading ss after 32-bit sysret in the vdso trampoline. Before we always reloaded ss in userspace before doing any stack operations. I have no idea whether this was deliberate. It had done that (with the attendant performance hit) since the very beginning, and there was no comment explaining why. --Andy -- 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/