On Wed, Apr 12, 2017 at 05:18:51PM +0200, Markus Armbruster wrote: > Cc'ing a few more people who might have a reasoned opinion. > > Roman Kagan <rka...@virtuozzo.com> writes: > > > While hammering out the VMBus / storage series, we've been struggling to > > figure out the best practices solution to the following problem: > > > > VMBus is provided by a vmbus bridge; it appears the most natural to have > > it subclassed from SysBusDevice. There can only be one VMBus in the > > VM. > > TYPE_DEVICE unless you actually need something TYPE_SYS_BUS_DEVICE > provides. > > > Now the question is how to add it to the system: > > > > 1) with a boolean machine property "vmbus" that would trigger the > > creation of the VMBus bridge; its class would have > > ->cannot_instantiate_with_device_add_yet = true > > This makes it an optional onboard device. Similar ones exist already, > e.g. various optional onboard USB host controllers controlled by machine > property "usb". > > > 2) with a regular -device option; this would require setting > > ->has_dynamic_sysbus = true for i440fx machines (q35 already have it) > > This makes it a pluggable sysbus device. > > I'd be tempted to leave old i400FX rot in peace, but your use case may > not allow that.
I have sent a RFC some time ago that replaces the all-or-nothing has_dynamic_sysbus flag with an explicit sysbus device whitelist, so i440fx wouldn't be a big problem. But as you noted above, if you don't need TYPE_SYS_BUS_DEVIC, you can just use TYPE_DEVICE. > > > > > 3) anything else > > > > > > So far we went with 1) but since it's essentially the API the management > > layer would have to use we'd like to get it right from the beginning. > > Asking for advice here is a good idea. > > Anyone? > I would go with (2) instead of (1): it allows more flexibility in case the device needs additional arguments, and will automatically benefit from (present and future) mechanisms for reporting available device-types and buses. Asking QEMU if "-device FOO" is supported is easy and reliable; the mechanisms for asking QEMU about supported "-machine" options are obscure and probably not well-tested. -- Eduardo
