From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [Qemu-devel] Re: [PATCH 26/35] kvm: Eliminate KVMState arguments Date: Tue, 11 Jan 2011 09:12:19 -0600 Message-ID: <4D2C7353.2000008@linux.vnet.ibm.com> References: <4D2616D6.4080309@linux.vnet.ibm.com> <4D26D6CF.5070405@web.de> <4D27A16F.9030809@linux.vnet.ibm.com> <4D282489.90506@web.de> <4D2B6506.6070907@linux.vnet.ibm.com> <4D2B6845.7050809@web.de> <4D2B6ADD.4090505@codemonkey.ws> <4D2C1C5D.2050504@redhat.com> <4D2C6290.1060607@linux.vnet.ibm.com> <1EA102F5-B6C2-43BC-9493-0271B287FC18@suse.de> <4D2C649F.6080508@linux.vnet.ibm.com> <4D2C67C2.5080000@redhat.com> <4D2C6AFA.4040104@linux.vnet.ibm.com> <4D2C6FAB.3050209@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Alexander Graf , Jan Kiszka , Marcelo Tosatti , qemu-devel@nongnu.org, kvm@vger.kernel.org To: Avi Kivity Return-path: Received: from e31.co.us.ibm.com ([32.97.110.149]:59754 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755817Ab1AKPNc (ORCPT ); Tue, 11 Jan 2011 10:13:32 -0500 Received: from d03relay03.boulder.ibm.com (d03relay03.boulder.ibm.com [9.17.195.228]) by e31.co.us.ibm.com (8.14.4/8.13.1) with ESMTP id p0BExEST002848 for ; Tue, 11 Jan 2011 07:59:14 -0700 Received: from d03av01.boulder.ibm.com (d03av01.boulder.ibm.com [9.17.195.167]) by d03relay03.boulder.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id p0BFDC7F036972 for ; Tue, 11 Jan 2011 08:13:16 -0700 Received: from d03av01.boulder.ibm.com (loopback [127.0.0.1]) by d03av01.boulder.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id p0BFDBow021904 for ; Tue, 11 Jan 2011 08:13:12 -0700 In-Reply-To: <4D2C6FAB.3050209@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 01/11/2011 08:56 AM, Avi Kivity wrote: > On 01/11/2011 04:36 PM, Anthony Liguori wrote: >>> They need to use the same device id then. And if they share code, >>> that indicates that they need to be the same device even more. >> >> >> No, it really doesn't :-) Cirrus VGA and std VGA share a lot of >> code. But that doesn't mean that we treat them as one device. > > Cirrus and VGA really are separate devices. They share code because > on evolved from the other, and is backwards compatible with the > other. i8254 and i8254-kvm did not evolve from each other, Actually, they did, but that's besides the point. > both are implementations of the i8254 spec, and both are 100% > compatible with each other (modulu bugs). > >> >> And BTW, there are guest visible differences between the KVM >> IOAPIC/PIC/PIT than the QEMU versions. The only reason PIT live >> migration works today is because usually delivers all interrupts >> quickly. But it actually does maintain state in the work queue that >> isn't saved. If PIT tried to implement gradual catchup, there would >> be no way not to expose that state to userspace. > > Why not? Whatever state the kernel keeps, we expose to userspace and > allow sending it over the wire. What exactly is the scenario you're concerned about? Migration between userspace HPET and in-kernel HPET? One thing I've been considering is essentially migration filters. It would be a set of rules that essentially were "hpet-kvm.* = hpet.*" which would allow migration from hpet to hpet-kvm given a translation of state. I think this sort of higher level ruleset would make it easier to support migration between versions of the device model. Of course, that only gives you a forward path. It doesn't give you a backwards path. Regards, Anthony Liguori