From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752832AbdEEQDh (ORCPT ); Fri, 5 May 2017 12:03:37 -0400 Received: from aserp1040.oracle.com ([141.146.126.69]:30396 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751951AbdEEQDf (ORCPT ); Fri, 5 May 2017 12:03:35 -0400 Subject: Re: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when running under Xen To: Juergen Gross , linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org, x86@kernel.org References: <20170427050120.6894-1-jgross@suse.com> <9c435a66-f8b1-5250-f3bf-cfada84d3d97@suse.com> Cc: hpa@zytor.com, tglx@linutronix.de, mingo@redhat.com From: Boris Ostrovsky Message-ID: <4e6a3981-97c8-c628-03c2-444c06c559d6@oracle.com> Date: Fri, 5 May 2017 12:02:02 -0400 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: <9c435a66-f8b1-5250-f3bf-cfada84d3d97@suse.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-Source-IP: aserv0021.oracle.com [141.146.126.233] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/05/2017 01:09 AM, Juergen Gross wrote: > 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. FWIW, Reviewed-by: Boris Ostrovsky >> >> 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); >>