making a qdev bus available from a (non-qtree?) device

[Klaus Jensen <its@irrelevant.dk>]

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Hi all,

I need some help with grok'ing qdev busses. Stefan, Michael - David 
suggested on IRC that I CC'ed you guys since you might have solved a 
similar issue with virtio devices. I've tried to study how that works, 
but I'm not exactly sure how to apply it to the issue I'm having.

Currently, to support multiple namespaces on the emulated nvme device, 
one can do something like this:

   -device nvme,id=nvme-ctrl-0,serial=foo,...
   -device nvme-ns,id=nvme-ns-0,bus=nvme-ctrl-0,...
   -device nvme-ns,id-nvme-ns-1,bus=nvme-ctrl-0,...

The nvme device creates an 'nvme-bus' and the nvme-ns devices has 
dc->bus_type = TYPE_NVME_BUS. This all works very well and provides a 
nice overview in `info qtree`:

   bus: main-system-bus
   type System
     ...
     dev: q35-pcihost, id ""
       ..
       bus: pcie.0
	type PCIE
	..
	dev: nvme, id "nvme-ctrl-0"
	  ..
	  bus: nvme-ctrl-0
	    type nvme-bus
	    dev: nvme-ns, id "nvme-ns-0"
	      ..
	    dev: nvme-ns, id "nvme-ns-1"
	      ..


Nice and qdevy.

We have since introduced support for NVM Subsystems through an 
nvme-subsys device. The nvme-subsys device is just a TYPE_DEVICE and 
does not show in `info qtree` (I wonder if this should actually just 
have been an -object?). Anyway. The nvme device has a 'subsys' link 
parameter and we use this to manage the namespaces across the subsystem 
that may contain several nvme devices (controllers). The problem is that 
this doesnt work too well with unplugging since if the nvme device is 
`device_del`'ed, the nvme-ns devices on the nvme-bus are unrealized 
which is not what we want. We really want the namespaces to linger, 
preferably on an nvme-bus of the nvme-subsys device so they can be 
attached to other nvme devices that may show up (or already exist) in 
the subsystem.

The core problem I'm having is that I can't seem to create an nvme-bus 
from the nvme-subsys device and make it available to the nvme-ns device 
on the command line:

   -device nvme-subsys,id=nvme-subsys-0,...
   -device nvme-ns,bus=nvme-subsys-0

The above results in 'No 'nvme-bus' bus found for device 'nvme-ns', even 
though I do `qbus_create_inplace()` just like the nvme device. However, 
I *can* reparent the nvme-ns device in its realize() method, so if I 
instead define it like so:

   -device nvme-subsys,id=nvme-subsys-0,...
   -device nvme,id=nvme-ctrl-0,subsys=nvme-subsys-0
   -device nvme-ns,bus=nvme-ctrl-0

I can then call `qdev_set_parent_bus()` and set the parent bus to the 
bus creates in the nvme-subsys device. This solves the problem since the 
namespaces are not "garbage collected" when the nvme device is removed, 
but it just feels wrong you know? Also, if possible, I'd of course 
really like to retain the nice entries in `info qtree`.


Thanks,
Klaus

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