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

[Igor Mammedov]

On Fri, 24 Jan 2020 11:20:15 +0000
Salil Mehta <address@hidden> 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


> 
> 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
>