[libvirt] clean/simple Q35 support in libvirt+QEMU for guest OSes that don't support virtio-1.0

[Gerd Hoffmann]

  Hi,

> b) Rather than a "legacy-only" model for virtio-0.9, it would be more
> useful to have "transitional". This way the config would work for older
> OSes that don't support virtio-1.0, and when/if the OS was upgraded such
> that it supported virtio-1.0, that would be automatically used without
> needing to change the config.

Having a legacy-only (instead of transitional) model could be useful for
regression-testing (i.e. whenever virtio-0.9 mode still works properly
in guests with virtio-1.0 support).

But that is pretty much the only reason I can think of to prefer the
virtio-0.9 devices being legacy-only instead of transitional.

> A) libosinfo starts telling consumers that the preferred virtio device
> model for the relevant OSes is "virtio-0.9", and leaves the
> recommendation for other OSes as "virtio".
> 
> B) libvirt adds a "virtio-0.9" model for all virtio devices that
> actually have virtio-0.9 support (a couple of devices never existed
> prior to virtio-1.0 (rng and ???) so virtio-0.9 would be nonsensical for
> them).

input, gpu are 1.0 only too.

> C) inside libvirt, the implementation of the "virtio-0.9" model is
> identical to "virtio", except that the VIR_PCI_CONNECT_TYPE flags for
> these devices contain VIR_PCI_CONNECT_TYPE_PCI rather than
> VIR_PCI_CONNECT_TYPE_PCIE, resulting in those devices being assigned to
> a legacy PCI slot, and thus they would be transitional mode by default.

Looks good to me.

cheers,
  Gerd