Re: [Question] Regarding containers "unattached/peripheral/anonymous" - their relation with hot(un)plug of devices

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jan 24, 2020 at 02:54:04PM +0100, Igor Mammedov wrote:
> On Fri, 24 Jan 2020 11:20:15 +0000
> Salil Mehta <salil.mehta@huawei.com> wrote:
> 
> > Hello,
> > I am working on vCPU Hotplug feature for ARM64 and I am in mid of understanding some aspect of device_add/device_del interface of the QEMU.
> > 
> > Observations:
> > 1. Any object initialised by qmp_device_add() gets into /machine/unattached container. I traced the flow to code leg inside  device_set_realized()
> > 2. I could see the reverse qmp_device_del() expects the device to be in  /machine/peripheral container.
> > 3. I could see any object initially added to unattached container did not had their parents until object_add_property_child() was called further in the leg.
> >     which effectively meant a new property was created and property table populated and child was parented.
> > 4. Generally, container  /machine/peripheral was being used wherever DEVICE(dev)->id was present and non-null.
> > 
> > Question:
> > 1. Wanted to confirm my understanding about the use of having separate containers like unattached, peripheral and anonymous.
> 
> > 2. At init time all the vcpus goes under *unattached* container. Now, qmp_device_del() cannot be used to unplug them. I am wondering
> 
> device is put into 'unattached' in case it wasn't assigned a parent.
> Usually it happens when board creates device directly.
> 
> >    if all the hotplug devices need to go under the *peripheral* container while they are hotplugged and during object init time as well?
> 
> theoretically device_del may use QOM path (the later users can get with query-hotpluggable-cpus),
> but I think it's mostly debugging feature.
> 
> users are supposed to specify 'id' during -device/device_add if they are going to manage that device
> afterwards (like unplugging it). Then they could use that 'id' in other commands (including device_del)
> 
> So 'id'-ed devices end up in 'peripheral' container
> 
> > 3. I could not see any device being place under *anonymous* container during init time. What is the use of this container?
> 
> if I recall it right, devices created with help of device_add but without 'id' go to this container

BTW, ATM hw/acpi/cpu.c creates _EJ0 for all CPUs (except the 1st one).
It might be cleaner to skip it for CPUs which don't have an id - what
do you think?


> 
> > 
> > I would be thankful for your valuable insights and answers and help in highlighting any gap in my understanding.
> > 
> > Thanks in anticipation!
> > 
> > Best Regards
> > Salil
> >