From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755274AbdEEFJV (ORCPT ); Fri, 5 May 2017 01:09:21 -0400 Received: from mx2.suse.de ([195.135.220.15]:36757 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751101AbdEEFJU (ORCPT ); Fri, 5 May 2017 01:09:20 -0400 Subject: Re: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when running under Xen To: linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org, x86@kernel.org References: <20170427050120.6894-1-jgross@suse.com> Cc: boris.ostrovsky@oracle.com, hpa@zytor.com, tglx@linutronix.de, mingo@redhat.com From: Juergen Gross Message-ID: <9c435a66-f8b1-5250-f3bf-cfada84d3d97@suse.com> Date: Fri, 5 May 2017 07:09:16 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: <20170427050120.6894-1-jgross@suse.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Any comments? Juergen On 27/04/17 07:01, Juergen Gross wrote: > When running as Xen pv guest X86_BUG_SYSRET_SS_ATTRS must not be set > on AMD cpus. > > This bug/feature bit is kind of special as it will be used very early > when switching threads. Setting the bit and clearing it a little bit > later leaves a critical window where things can go wrong. This time > window has enlarged a little bit by using setup_clear_cpu_cap() instead > of the hypervisor's set_cpu_features callback. It seems this larger > window now makes it rather easy to hit the problem. > > The proper solution is to never set the bit in case of Xen. > > Signed-off-by: Juergen Gross > --- > arch/x86/kernel/cpu/amd.c | 5 +++-- > arch/x86/xen/enlighten_pv.c | 1 - > 2 files changed, 3 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c > index c36140d788fe..b6da6e75e3a8 100644 > --- a/arch/x86/kernel/cpu/amd.c > +++ b/arch/x86/kernel/cpu/amd.c > @@ -799,8 +799,9 @@ static void init_amd(struct cpuinfo_x86 *c) > if (cpu_has(c, X86_FEATURE_3DNOW) || cpu_has(c, X86_FEATURE_LM)) > set_cpu_cap(c, X86_FEATURE_3DNOWPREFETCH); > > - /* AMD CPUs don't reset SS attributes on SYSRET */ > - set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS); > + /* AMD CPUs don't reset SS attributes on SYSRET, Xen does. */ > + if (!cpu_has(c, X86_FEATURE_XENPV)) > + set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS); > } > > #ifdef CONFIG_X86_32 > diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c > index a1af4f68278f..dcfd07faf1c3 100644 > --- a/arch/x86/xen/enlighten_pv.c > +++ b/arch/x86/xen/enlighten_pv.c > @@ -290,7 +290,6 @@ static bool __init xen_check_xsave(void) > > static void __init xen_init_capabilities(void) > { > - setup_clear_cpu_cap(X86_BUG_SYSRET_SS_ATTRS); > setup_force_cpu_cap(X86_FEATURE_XENPV); > setup_clear_cpu_cap(X86_FEATURE_DCA); > setup_clear_cpu_cap(X86_FEATURE_APERFMPERF); >