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Design discussion for interested parties. As discussed during the nova
team meeting today, we're looking at providing support for virtio-based
input devices in nova, given benefits w.r.t. performance and security
(no emulated USB bus!). The current proposal I have for this is to
introduce a new image metadata property, 'hw_input_bus', and extend the
existing 'hw_pointer_model' image metadata property, which currently
only accepts a 'usbtablet' value, to accept 'mouse' or 'tablet' values.
This means you'd end up with a matrix something like so:

  +------------------+--------------+------------------------+
  | hw_pointer_model | hw_input_bus | Result                 |
  +------------------|--------------+------------------------+
  | -                | -            | PS2-based mouse [*]    |
  | usbtablet        | -            | USB-based tablet       |
  | usbtablet        | usb          | USB-based tablet       |
  | usbtablet        | virtio       | **ERROR**              |
  | mouse            | -            | USB-based mouse        |
  | mouse            | usb          | USB-based mouse        |
  | mouse            | virtio       | virtio-based mouse     |
  | tablet           | -            | USB-based tablet       |
  | tablet           | usb          | USB-based tablet       |
  | tablet           | virtio       | virtio-based tablet    |
  +------------------+--------------+------------------------+

  [*] libvirt adds these by default on x86 hosts

dansmith noted that the only reason to select the 'mouse' pointer model
nowadays is for compatibility, and something like a virtio-based mouse
didn't really make sense. That being the case, I agree that this change
is likely more complex that it needs to be. We do, however, disagree on
the remedy. dansmith's idea is to drop the 'hw_input_bus' image
metadata property entirely and simply add a new 'virtiotablet' value to
'hw_pointer_model' instead. This still leaves the question of what bus
we should use for keyboards, and his suggestion there is to extrapolate
out and use virtio for keyboards if 'hw_pointer_model=virtiotablet' is
specified and presumably USB if 'hw_pointer_model=usbtablet'.

Needless to say, I don't like this idea and prefer we took another tack
and kept 'hw_input_bus'  but didn't build on 'hw_pointer_model' and
instead "deprecated" it. We can't really ever remove the an image
metadata property, since that would be a breaking upgrade, which means
we'll eventually be left with effectively dead code to maintain
forever. However, I don't think that's a big deal.
'hw_pointer_model=usbtablet' is already on a path to obsolescence as
the Q35 machine type slowly becomes the default on x86 hosts and the
use of non-x86 hosts grows since neither support PS2 and must use a
USB-based input device. In addition, I think the extrapolation of
'virtiotablet' to mean also virtio-based keyboard is unclear and leaves
a gaping hole w.r.t. requesting USB-based keyboards on non-AArch64
hosts (where it's currently added by default), since we don't currently
do this extrapolation and introducing it would be breaking change on
x86 hosts (instances would suddenly switch from PS2-based keyboards to
USB-based ones).

We need to decide what approach to go for before I rework this. If
anyone has input, particularly operators that think they'd use this
feature, I'd love to hear it so that I can t̶e̶l̶l̶ ̶d̶a̶n̶s̶m̶i̶t̶h̶
̶t̶o̶ ̶s̶h̶o̶v̶e̶ ̶i̶t̶ come to the best possible solution ;-) Feel
free to either reply here or on the review [1].

Cheers,
Stephen

[1] https://review.opendev.org/#/c/756552/