From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37476) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ceIkl-00050R-Mc for qemu-devel@nongnu.org; Thu, 16 Feb 2017 04:53:32 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ceIkh-0004C3-R6 for qemu-devel@nongnu.org; Thu, 16 Feb 2017 04:53:31 -0500 Received: from mx1.redhat.com ([209.132.183.28]:36792) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ceIkh-0004Ba-Ic for qemu-devel@nongnu.org; Thu, 16 Feb 2017 04:53:27 -0500 Date: Thu, 16 Feb 2017 09:53:22 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20170216095322.GB2525@work-vm> References: <20170215101427.23736-1-eduardo.otubo@profitbricks.com> <87k28qlca9.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87k28qlca9.fsf@dusky.pond.sub.org> Subject: Re: [Qemu-devel] [PATCH 0/2] add writeconfig command on monitor List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Markus Armbruster Cc: Eduardo Otubo , qemu-devel@nongnu.org * Markus Armbruster (armbru@redhat.com) wrote: > Eduardo Otubo writes: > > > This first patch extends the command line option `-writeconfig ' to a > > command on HMP and QMP monitors. This is useful when live migrating after a > > series of device hot plug events. One can just generate an updated config file > > for the vm, transport it to the target host and start the vm with `-readconfig > > '. > > > > The second patch re-includes the reference of the memory object on the config > > file generated. > > The high-level idea of having QEMU regurgitate its configuration for the > migration target sounds nice, but there are several issues with > regurgitating QemuOpts state with writeconfig: > > 1. Our needs have outgrown QemuOpts' design. We have accumulated > various hacks and work-arounds to make do, and it's still not enough. > Instead of adding more, I want to revise its design. The work has > started, but it'll take some time. Adding creative new uses of > QemuOpts while this work is in progress can only make it harder. > > If this issue was the only one, I'd take the hit for the team. > > 2. Transmitting configuration at the beginning of migration doesn't > fully solve the problem. What about configuration changes during > migration? Think of hot plug. Doesn't mean transmitting > configuration is a bad idea, only means there's more to the problem > than a naive observer might think. > > In my opinion, the proper solution is to transmit configuration > information in the migration stream, complete with updates as it > changes. Hard to do, which is why it hasn't been done. > > If we can't have the proper solution now, a less-than-ideal partial > solution may still be better than nothing. That's a separate problem from the one Eduardo is trying to solve; I wouldn't trust migration to survive a device hotplugged during the migration as it is. So I wouldn't worry about it as a reason against this series. > 3. The accuracy of QemuOpts information is doubtful. > > Completeness: only certain kinds of configuration are done with > QemuOpts. Incompleteness makes -writeconfig less useful than it > could be, but it's still useful. Monitor command writeconfig could > be similarly useful. > > Correctness: configuration gets stored in QemuOpts when we parse > KEY=VALUE,... strings. It can also be constructed and updated > manually. At certain points in time, bits from QemuOpts are used to > actually configure stuff. > > Example: -device creates an entry in the "device" configuration > group, which is later used to actually create and configure a device > object. > > My point is: whenever we manipulate the actual objects, we may > invalidate information stored in QemuOpts. We can try to keep it in > sync, and we do at least sometimes. But this is a game we can only > lose, except for the period(s) of time where QemuOpts is all there > is, i.e. before actual objects get created. Note that -writeconfig > runs before objects get created, so it's not affected by this issue. > > Out-of-sync QemuOpts is harmless unless something relies on it being > accurate. I know we currently rely on QemuOpts IDs to catch > duplicate IDs for some of the configuration groups. I doubt there's > much else. > > If we add your monitor command, out-of-sync QemuOpts goes from > harmless to serious bug. In other words, we'd create a new class of > bugs, with an unknown number of existing instances that are probably > hard to find and fix. Probably a perpetual source of new instances, > too. > > Feels like a show stopper to me. Hmm this does seem a bigger problem. Dave -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK