On Thu, Feb 16, 2017 at 09=53=22AM +0000, Dr. David Alan Gilbert wrote: > * Markus Armbruster (arm...@redhat.com) wrote: > > Eduardo Otubo <eduardo.ot...@profitbricks.com> writes: > > > > > This first patch extends the command line option `-writeconfig <file>' 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 > > > <file>'. > > > > > > 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.
Certainly this series is part of the QemuOpts refactoring, right? [PATCH 00/24] QemuOpts util/cutils: Fix and clean up number conversions I'm not sure what's the big picture you have in mind, but I can help you out. > > > > 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. > Can't we lock hotplug operations while a live migration is being performed? Not exactly return an error, but perhaps delay the hotplug until the migration is finished. > > 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. > Yes, this looks bad. Does your QemuOpts redesign considers these problems as well? I mean, proper work with devices and objects to actually update QemuOpts as soon as they change, etc. -- Eduardo Otubo ProfitBricks GmbH