Given that hotplug exists, couldn't it be used to communicate the amount of available RAM to the guest instead of ballooning? That would leave the OS responsibl

Given that hotplug exists, couldn't it be used to communicate the amount of available RAM to the guest instead of ballooning? That would leave the OS responsible for swapping hungry programs out and resizing kernel structures. I guess the missing piece is that it doesn't leave a good way for the guest to request more RAM (maxing out the balloon?).