From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0BEB3FA3728 for ; Wed, 16 Oct 2019 15:59:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DE02021928 for ; Wed, 16 Oct 2019 15:59:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2393052AbfJPP7n (ORCPT ); Wed, 16 Oct 2019 11:59:43 -0400 Received: from mga02.intel.com ([134.134.136.20]:36652 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729646AbfJPP7n (ORCPT ); Wed, 16 Oct 2019 11:59:43 -0400 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga003.jf.intel.com ([10.7.209.27]) by orsmga101.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 16 Oct 2019 08:59:42 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.67,304,1566889200"; d="scan'208";a="198994082" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.41]) by orsmga003.jf.intel.com with ESMTP; 16 Oct 2019 08:59:42 -0700 Date: Wed, 16 Oct 2019 08:59:42 -0700 From: Sean Christopherson To: Thomas Gleixner Cc: Fenghua Yu , Ingo Molnar , Borislav Petkov , H Peter Anvin , Peter Zijlstra , Andrew Morton , Dave Hansen , Paolo Bonzini , Radim Krcmar , Ashok Raj , Tony Luck , Dan Williams , Xiaoyao Li , Sai Praneeth Prakhya , Ravi V Shankar , linux-kernel , x86 , kvm@vger.kernel.org Subject: Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split lock Message-ID: <20191016155942.GB5866@linux.intel.com> References: <1560897679-228028-1-git-send-email-fenghua.yu@intel.com> <1560897679-228028-10-git-send-email-fenghua.yu@intel.com> <20190626203637.GC245468@romley-ivt3.sc.intel.com> <20190925180931.GG31852@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Wed, Oct 16, 2019 at 11:29:00AM +0200, Thomas Gleixner wrote: > > - Modify the #AC handler to test/set the same atomic variable as the > > sysfs knob. This is the "disabled by kernel" flow. > > That's the #AC in kernel handler, right? Yes. > > - Modify the debugfs/sysfs knob to only allow disabling split-lock > > detection. This is the "disabled globally" path, i.e. sends IPIs to > > clear MSR_TEST_CTRL.split_lock on all online CPUs. > > Why only disable? What's wrong with reenabling it? The shiny new driver you > are working on is triggering #AC. So in order to test the fix, you need to > reboot the machine instead of just unloading the module, reenabling #AC and > then loading the fixed one? A re-enabling path adds complexity (though not much) and is undesirable for a production environment as a split-lock issue in the kernel isn't going to magically disappear. And I thought that disable-only was also your preferred implementation based on a previous comment[*], but that comment may have been purely in the scope of userspace applications. Anyways, my personal preference would be to keep things simple and not support a re-enabling path. But then again, I do 99.9% of my development in VMs so my vote probably shouldn't count regarding the module issue. [*] https://lkml.kernel.org/r/alpine.DEB.2.21.1904180832290.3174@nanos.tec.linutronix.de > > - Modify the resume/init flow to clear MSR_TEST_CTRL.split_lock if it's > > been disabled on *any* CPU via #AC or via the knob. > > Fine. > > > - Remove KVM loading of MSR_TEST_CTRL, i.e. KVM *never* writes the CPU's > > actual MSR_TEST_CTRL. KVM still emulates MSR_TEST_CTRL so that the > > guest can do WRMSR and handle its own #AC faults, but KVM doesn't > > change the value in hardware. > > > > * Allowing guest to enable split-lock detection can induce #AC on > > the host after it has been explicitly turned off, e.g. the sibling > > hyperthread hits an #AC in the host kernel, or worse, causes a > > different process in the host to SIGBUS. > > > > * Allowing guest to disable split-lock detection opens up the host > > to DoS attacks. > > Wasn't this discussed before and agreed on that if the host has AC enabled > that the guest should not be able to force disable it? I surely lost track > of this completely so my memory might trick me. Yes, I was restating that point, or at least attempting to. > The real question is what you do when the host has #AC enabled and the > guest 'disabled' it and triggers #AC. Is that going to be silently ignored > or is the intention to kill the guest in the same way as we kill userspace? > > The latter would be the right thing, but given the fact that the current > kernels easily trigger #AC today, that would cause a major wreckage in > hosting scenarios. So I fear we need to bite the bullet and have a knob > which defaults to 'handle silently' and allows to enable the kill mechanics > on purpose. 'Handle silently' needs some logging of course, at least a per > guest counter which can be queried and a tracepoint.