On Wed, Jul 01, 2026 at 03:37:02PM +0200, Markus Armbruster wrote: > Daniel P. Berrangé <[email protected]> writes: > > > On Wed, Jul 01, 2026 at 08:32:06AM +0200, Markus Armbruster wrote: > >> [email protected] writes: > >> > >> >> The default monitor is usually a long lived object that will exist for > >> >> the entire lifetime of the VM. A monitor can only service a single > >> >> client at a time though, and so it might be desirable to hotplug > >> >> additional monitors at runtime for specific tasks. If doing that, > >> >> however, there is a need to remove the monitor when it is no longer > >> >> needed. > >> > >> Whatever adds the additional monitor can also delete it. The fact that > >> you propose other means suggests you believe this would be cumbersome in > >> practice. Why? > > > > The use is is that someone/something wants to spawn a script that > > does some job with the QEMU monitor. The thing that spawns the > > script adds the new monitor and launches the script. Having > > auto-delete means that you do not need to then keep track of that > > script to perform cleanup of the dynamically added monitor. It > > gives you "do the right thing" behaviour automatically when the > > script exits, closing its monitor connection. > > > > The initial series proposed by Christian supported the ability > > to run "object-del' on the monitor itself - a "self delete" > > essentially. That is very awkward from the code POV, as it > > required special case hanlding to ensure the QMP response to > > the delete action got sent on the socket before the delete > > action took place. It also made it impossible to then delete > > the character device. > > > > Auto-delete gives us a better solution with less code complexity. > > I agree "self delete" is problematic. > > I wonder how important the ability to fire and forget a script with a > dedicated monitor is. I'd (naively?) expect whatever spawns the script > to reap its exit status. > > If it is important, what about fire and forget a script with a dedicated > character device? Can't come up with a use case for that within ten > seconds. However, we use character devices for all kinds of crap. Food > for thought, not a demand.
In a hypothetical libvirt usage of this functionality, libvirt would not be spawning the script or even know what its PID is. Libvirt would just create a auto-delete monitor, open an FD to it, and pass the FD back to the libvirt client & forget about it. > >> >> Allowing a client to run "object-del" against its own monitor adds > >> >> complex edge cases, as it would be desirable to send the QMP response > >> >> despite the monitor sending it being deleted. Doing "object-del" alone > >> >> will also result in orphaning a character device backend instance, as > >> >> there is no opportunity to run the companion "chardev-del" command. > >> >> > >> >> A simpler way to ensure cleanup is to add the concept of auto-deleting > >> >> monitor objects. Specifically when the "CHR_EVENT_CLOSED" event is > >> >> emitted, the equivalent of "object-del" + "chardev-del" can be run > >> >> internally. Since the transient client has already droppped its > >> >> monitor connection, there is no synchronization to be concerned about. > >> > >> If object-del or chardev-del fail, there's no way to report the error. > >> Can they fail? > > > > object-del can fail if > > > > * An object with the specified "id" does not exist. That shouldn't > > happen in this case but harmless if it odes. > > * object_del command tries to delete the monitor that > > is servicing the object_del command. Cannot happen with > > auto-delete > > * the monitor has not finished initializing it BH with > > chardev handlers. Cannot happen if we know we have a > > live connection already. > > > > chardev-del can fail if > > > > * The chardev with "id" does not exist. SHouldn't happen > > but is harmless if it does > > * The chardev reports it is "busy" - aka the frontend > > is still connected - we just deleted it so cannot happen > > * Record/replay is in use - a niche use case > > > > So I don't think errors are a problem. > > > >> Do we always want to delete both monitor and character device? > > > > IMHO yes they are a pair whose lifetime should be tied together > > for normal use. > > Would we make monitor auto-delete delete its character device if > character devices also had an auto-delete feature? That's a possibility. We would have to accept that it allows users to create a scenario in which auto-delete will frequenty fail. ie configuring the chardev for auto-delete, but not having the frontend auto-delete, which would trigger the "busy" check. Not convinced that's nicer ? With regards, Daniel -- |: https://berrange.com ~~ https://hachyderm.io/@berrange :| |: https://libvirt.org ~~ https://entangle-photo.org :| |: https://pixelfed.art/berrange ~~ https://fstop138.berrange.com :|
