From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752330AbeBTPJn (ORCPT ); Tue, 20 Feb 2018 10:09:43 -0500 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:51598 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752265AbeBTPJm (ORCPT ); Tue, 20 Feb 2018 10:09:42 -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> <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> <2159cdc0-c30d-3bf8-1c25-74bff46a1e91@redhat.com> <0575AF4FD06DD142AD198903C74E1CC87A6194AB@ORSMSX103.amr.corp.intel.com> From: Paolo Bonzini Message-ID: <5b39cfbb-a04f-46a8-87cb-3a52fc8e9700@redhat.com> Date: Tue, 20 Feb 2018 16:09:38 +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: <0575AF4FD06DD142AD198903C74E1CC87A6194AB@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 15:59, Van De Ven, Arjan wrote: > >>> 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. > > that makes sense, compatibility in this direction can be done > and is useful as you move a fleet of servers forward > >> Backwards migration to older hypervisors also happens sometimes, but in >> general it creates more userspace than kernel issues. > > that direction is obviously harder >>From KVM's point of view it's mostly a matter of having a complete ioctl API right. I'm not that much worried by it. Paolo