On Wed, Dec 04, 2019 at 10:18:24AM +0100, Jens Freimann wrote: > On Tue, Dec 03, 2019 at 06:40:04PM -0300, Eduardo Habkost wrote: > > +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 <ehabk...@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? > > "hidden" the way we use it for virtio-net failover is actually unplugged. But > it > doesn't have to be that way. You can register a function that decides > if the device should be hidden, i.e. plugged now, or do something else > with it (in the virtio-net failover case we just save everything we > need to plug the device later). > > We did introduce a "unplugged but not unrealized" function too as part > of the failover feature. See "a99c4da9fc pci: mark devices partially > unplugged" > > This was needed so we would be able to re-plug the device in case a > migration failed and we need to hotplug the primary device back to the > guest. To avoid the risk of not getting the resources the device needs > we don't unrealize but just trigger the unplug from the guest OS.
Thanks for the explanation. Let me confirm if I understand the purpose of the new mechanisms: should_be_hidden is a mechanism for implementing realize-without-plug. partially_hotplugged is a mechanism for implementing unplug-without-unrealize. Is that correct? -- Eduardo