VMware balloon driver over virtio

Hi,

VMware would like to get its Linux balloon driver merged into mainline
and is considering doing it within virtio framework.

The following is the initial implementation of our ballooning interface
as a virtio device. It allows our hypervisor to work in tandem with
virtio_balloon driver and inflate and deflate balloon on demand.

Due to the fact that our monitor works a bit differently from KVM some
changes are needed in virtio_balloon itself, namely:

- our hypervisor may refuse locking certain pages. This may happen,
  for example, when target balloon size is changed since guest polled
  it last time. We expect guest to try locking different page and
  release refused pages once target balloon size is reached.

- our hypervisor may, in certain situations, reset the balloon. In this
  case it expects guest to quickly release all memory allocated by
  by the balloon but to not bother telling the host about pages being
  released.

There are more differences (adaptive allocation rates, sleeping
allocations) which we intend to address at a later date if it is
decided that virtio is the proper subsystem to be used for our
driver.

Thanks!

-- 
Dmitry
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