Re: [Qemu-devel] [RFC 00/15] qmp: Report supported device types on 'query-machines'

[David Hildenbrand <david@redhat.com>]

>
> Considered alternatives
> =======================
>
> Indirect mapping (machine => bus => device)
> -------------------------------------------
>
> This RFC implements a mechanism to implement ax
>   machine-type => supported-device-types
> mapping. An alternative solution I considered was to expose an
> indirect mapping:
>   machine-type => default-bus-types
> followed by
>   bus-type => supported-device-types.
>
> But exposing only the resulting supported device-types list
> imposes less restrictions on how the device and bus type
> hierarchy is implemented inside QEMU. There's still a
>   machine-type => bus-type => device-type
> mapping implemented internally, but it is an implementation
> detail on the current version, and not part of the
> externally-visible interface.
>
> The Implementation
> ==================
>
> This add a new field to MachineClass: default_bus_types, and a
> new field to BusClass: supported_device_type.

Is it possible to modify a machine (setting some properties e.g. on the
command line), that suddenly more devices are supported? Something like
enabling an additional bus? (I assume so, because it is called "default
bus types" :) )

If so, the indirect mapping could be of more benefit in the long run.

Thinking about my machine at home:

I just care about the available buses. If my machine doesn't have USB,
but PCI, I can buy a USB PCI card and make it support USB. Then I can
happily plug in USB devices. So the "default" state might at least
no longer be sufficient when wanting to plug in a USB fan on a hot
summer day ;) .

But, with the indirect mapping, I guess we would need yet another qmp
command ...

-- 

David