From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=47611 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PmhHR-0008OH-UV for qemu-devel@nongnu.org; Tue, 08 Feb 2011 01:42:30 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PmhHQ-000419-Qh for qemu-devel@nongnu.org; Tue, 08 Feb 2011 01:42:29 -0500 Received: from mx1.redhat.com ([209.132.183.28]:29281) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PmhHQ-000413-Eb for qemu-devel@nongnu.org; Tue, 08 Feb 2011 01:42:28 -0500 Date: Tue, 8 Feb 2011 08:42:14 +0200 From: "Michael S. Tsirkin" Message-ID: <20110208064214.GB28096@redhat.com> References: <20110207160751.GC25106@redhat.com> <4D504925.3000807@codemonkey.ws> <20110207195315.GA28096@redhat.com> <4D505C92.5040904@codemonkey.ws> <20110207215203.GA28658@redhat.com> <4D50A90C.1060701@codemonkey.ws> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4D50A90C.1060701@codemonkey.ws> Subject: [Qemu-devel] Re: new->old version migration List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: Amit Shah , Alex Williamson , qemu list , Juan Quintela On Mon, Feb 07, 2011 at 08:23:08PM -0600, Anthony Liguori wrote: > On 02/07/2011 03:52 PM, Michael S. Tsirkin wrote: > >How does it? We need to know we are saving in 0.13 > >format and skip the new subsection, otherwise > >0.13 will see a subsection it does not recognize > >and exit. > > If you used subsections for flow control, presumably you would only > send the new savevm data if you had data buffered. > > If you add a qdev property to enable/disable flow control, then if > it's disabled, you naturally would never send the subsection because > you'd never buffer data. So no explicit code is needed to support > migration. But the result is we get a new property that we can never remove as any qdev property is part of interface. > The difficult case is when you truly need to change the savevm > version. I don't think we have a proper fix for this because > versions are linear so the proposed patch certainly wouldn't be a > good way to do it. if flow_control=0 causes savevm 3 to be used > instead of 4, and then the next_feature=0 causes savevm 4 to be used > instead of 5, the semantics of flow_control=0,next_feature=1 becomes > problematic. > > But as long as the feature has isolated state, we can solve the > problem robustly with subsections. > > Regards, > > Anthony Liguori I see. I'm unhappy with the facts that 1. if (feature) is spread all over the code instead of just in migration 2. it is also obfuscated with if (flow_control) instead of plain if (migrate to qemu < 0.14) so removing it will be much harder 3. this forces anyone who wants a VM compatible with qemu 0.13 to also lose data, even if migration to 0.13 is never attempted. > >We also need API to add subsections without vmstate, > >because virtio serial wasn't yet converted. > >