From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S938338AbdD0FBj (ORCPT ); Thu, 27 Apr 2017 01:01:39 -0400 Received: from mx2.suse.de ([195.135.220.15]:46452 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S933955AbdD0FBa (ORCPT ); Thu, 27 Apr 2017 01:01:30 -0400 From: Juergen Gross To: linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org, x86@kernel.org Cc: boris.ostrovsky@oracle.com, hpa@zytor.com, tglx@linutronix.de, mingo@redhat.com, Juergen Gross Subject: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when running under Xen Date: Thu, 27 Apr 2017 07:01:20 +0200 Message-Id: <20170427050120.6894-1-jgross@suse.com> X-Mailer: git-send-email 2.12.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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); -- 2.12.0