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

[Eduardo Habkost]

On Tue, Nov 22, 2016 at 09:18:21AM +0100, David Hildenbrand wrote:
> 
> > 
> > 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" :) )

Yes. See the "Out of scope: Configurable buses" section on the
cover letter.

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

Yes, a new mapping may be necessary to find out which options
(e.g. "usb=on") or device-types (e.g. "-device isa-fdc", "-device
piix3-usb-uhci") will add new buses and make new device-types
supported by the machine. But this will probably require new QMP
commands, and I still want to keep the default case simple.

> 
> 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

-- 
Eduardo