virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

The new feature enables the virtio-balloon device to receive hints of
guest free pages from the free page vq.

A notifier is registered to the migration precopy notifier chain. The
notifier calls free_page_start after the migration thread syncs the dirty
bitmap, so that the free page optimization starts to clear bits of free
pages from the bitmap. It calls the free_page_stop before the migration
thread syncs the bitmap, which is the end of the current round of ram
save. The free_page_stop is also called to stop the optimization in the
case when there is an error occurred in the process of ram saving.

Note: balloon will report pages which were free at the time of this call.
As the reporting happens asynchronously, dirty bit logging must be
enabled before this free_page_start call is made. Guest reporting must be
disabled before the migration dirty bitmap is synchronized.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
CC: Michael S. Tsirkin <mst@redhat.com>
CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
CC: Juan Quintela <quintela@redhat.com>
CC: Peter Xu <peterx@redhat.com>
Message-Id: <1544516693-5395-8-git-send-email-wei.w.wang@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  dgilbert: Dropped kernel header update, fixed up CMD_ID_* name change