Re: qom device lifecycle interaction with hotplug/hotunplug ?

[Eduardo Habkost <ehabkost@redhat.com>]

+jfreimann, +mst

On Sat, Nov 30, 2019 at 11:10:19AM +0000, Peter Maydell wrote:
> On Fri, 29 Nov 2019 at 20:05, Eduardo Habkost <ehabkost@redhat.com> wrote:
> > So, to summarize the current issues:
> >
> > 1) realize triggers a plug operation implicitly.
> > 2) unplug triggers unrealize implicitly.
> >
> > Do you expect to see use cases that will require us to implement
> > realize-without-plug?
> 
> I don't think so, but only because of the oddity that
> we put lots of devices on the 'sysbus' and claim that
> that's plugging them into the bus. The common case of
> 'realize' is where one device (say an SoC) has a bunch of child
> devices (like UARTs); the SoC's realize method realizes its child
> devices. Those devices all end up plugged into the 'sysbus'
> but there's no actual bus there, it's fictional and about
> the only thing it matters for is reset propagation (which
> we don't model right either). A few devices don't live on
> buses at all.

That's my impression as well.

> 
> > Similarly, do you expect use cases that will require us to
> > implement unplug-without-unrealize?
> 
> I don't know enough about hotplug to answer this one:
> it's essentially what I'm hoping you'd be able to answer.
> I vaguely had in mind that eg the user might be able to
> create a 'disk' object, plug it into a SCSI bus, then
> unplug it from the bus without the disk and all its data
> evaporating, and maybe plug it back into the SCSI
> bus (or some other SCSI bus) later ? But I don't know
> anything about how we expose that kind of thing to the
> user via QMP/HMP.

This ability isn't exposed to the user at all.  Our existing
interfaces are -device, device_add and device_del.

We do have something new that sounds suspiciously similar to
"unplugged but not unrealized", though: the new hidden device
API, added by commit f3a850565693 ("qdev/qbus: add hidden device
support").

Jens, Michael, what exactly is the difference between a "hidden"
device and a "unplugged" device?

-- 
Eduardo