From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751952AbeBTOrE (ORCPT ); Tue, 20 Feb 2018 09:47:04 -0500 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:56946 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751359AbeBTOrB (ORCPT ); Tue, 20 Feb 2018 09:47:01 -0500 Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs To: "Van De Ven, Arjan" , "valdis.kletnieks@vt.edu" Cc: Jon Masters , David Woodhouse , "tglx@linutronix.de" , "x86@kernel.org" , "kvm@vger.kernel.org" , "torvalds@linux-foundation.org" , "linux-kernel@vger.kernel.org" , "Hansen, Dave" , Ingo Molnar References: <1518449255-2182-1-git-send-email-dwmw@amazon.co.uk> <1518449255-2182-2-git-send-email-dwmw@amazon.co.uk> <7e2e5ad1-49b6-1fdb-4a62-8ad6aefc30a0@redhat.com> <1518509708.12890.33.camel@infradead.org> <27c85759-e662-d281-f8a0-0a80ca8ee18f@redhat.com> <1518517262.12890.43.camel@infradead.org> <1518518198.12890.48.camel@infradead.org> <02bd3fdd-1b73-6cab-fb09-38ba933396bd@redhat.com> <1518775136.7876.13.camel@infradead.org> <75287047-77a0-e0ff-c2e8-61c81641251e@redhat.com> <1518776517.7876.21.camel@infradead.org> <1518783021.7876.34.camel@infradead.org> <4882860e-23c5-75b3-ac02-c700f615156e@jonmasters.org> <0575AF4FD06DD142AD198903C74E1CC87A61923C@ORSMSX103.amr.corp.intel.com> <10373.1519084385@turing-police.cc.vt.edu> <0575AF4FD06DD142AD198903C74E1CC87A619255@ORSMSX103.amr.corp.intel.com> <0575AF4FD06DD142AD198903C74E1CC87A619465@ORSMSX103.amr.corp.intel.com> From: Paolo Bonzini Message-ID: <2159cdc0-c30d-3bf8-1c25-74bff46a1e91@redhat.com> Date: Tue, 20 Feb 2018 15:46:57 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: <0575AF4FD06DD142AD198903C74E1CC87A619465@ORSMSX103.amr.corp.intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 20/02/2018 15:08, Van De Ven, Arjan wrote: >>>> For bonus points: What should happen to a VM that is live migrated >>>> from one hypervisor to another, and the hypervisors have different >>>> IBRS support? >>> >>> Doctor Doctor it hurts when I do this.... >>> >>> Migration tends to only work between HV's that are relatively >>> homogeneous, that's nothing new... >> >> No Arjan, this is just wrong. Well, I suppose it's right in the present >> tense with the IBRS mess on Skylake, but it's _not_ been true until last >> year. > > I meant software wise. You're not going to live migrate from xen to > kvm or backwards. or between very radically different versions of the > kvm stack. Forwards migration to a radically newer version certainly happens. So when the source hypervisor was too old to tell the VM about IBRS_ALL, for example, migration should work properly and the VM should perform well on the destination hypervisor. Backwards migration to older hypervisors also happens sometimes, but in general it creates more userspace than kernel issues. Paolo