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

[Philippe Mathieu-Daudé]

On 5/11/21 8:17 PM, Klaus Jensen wrote:
> 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.

The experts on this topic are Peter/Markus/Eduardo.

> 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.

IIUC, while we can have unattached drives, we can't (by design) have
qdev unattached to qbus.

Not sure this is a good suggestion (bad design IMO) but you could add
a fake nvme qbus to hold the lingering nvme devices...

> 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