On 07/29/2014 12:12 AM, Markus Armbruster wrote: >> Libvirt probably won't use it for normal guests (we don't want to kill >> qemu just because the monitor disconnects), but does have the notion of >> an autodestroy guest where it might be useful (we WANT the guest to go >> away if libvirtd dies early). In fact, autodestroy guests are used >> during migration - we want to kill qemu on the destination side if >> libvirtd dies before the source side finishes sending the migration >> stream. But in that scenario, once migration succeeds, libvirt has to >> be able to convert an autodestroy guest back into a normal guest that no >> longer disappears when libvirtd does; would this be something that QMP >> can toggle the state of this attribute on the fly? Perhaps through qom-set? > > After migration completes, execution moves from source to target. > Wouldn't you want to switch off target auto-destruct together with that > move, atomically?
Libvirt starts the destination with -S, and migration can't complete until libvirt resumes the destination CPUs with 'cont'. Libvirt's current timing of releasing auto-destruct is based on handshaking between source and destination; it occurs after source claims migration is done but before resuming CPUs on the destination, which satisfies the atomicity that you correctly observed to be necessary. >> However, we also have precedence of actions in QAPI that are very >> unlikely to be used outside of qtest, but which are not marked >> experimental; for example, the 'Abort' action in 'transaction' will >> probably never be used by libvirt > > Arguably not a conscious decision to make it ABI forever, more a case of > nobody thought about *not* making it ABI :) Added in June 2013; and we *did* have a discussion on whether to hide the transaction name at the time... https://lists.gnu.org/archive/html/qemu-devel/2013-06/msg03281.html > I don't really mind this instance, but I'm a bit concerned about rank > ABI growth. And that's a good position to maintain - it's always good to justify new knobs, especially since once they are ABI, it's harder to refactor around them. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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