DragonFlyBSD

When testing with qemu:
host-side:
- add "-balloon virtio" to the qemu command-line, to provide a PCI Balloon interface to the guest.
- make sure you have access to the monitor interface (qemu [...] -qmp tcp:localhost:4444,server,nowait)

guest-side:
Load driver
- kldload virtio_balloon
Increase debug level
- sysctl dev.vtballoon.0.debug_level=0x7

Inflating the balloon to 96 Mb on my 4096Mb Guest (4096->4000) (ie:reverse)
host side (monitor interface):

telnet localhost 4444 (to connect to the monitor)

{"QMP": {"version": {"qemu": {"micro": 0, "minor": 6, "major": 1}, "package": ""}, "capabilities": []}}

{ "execute": "qmp_capabilities" } { "execute": "balloon","arguments":{"value":"4194304000"} } # 4000 * 1024 * 1024 { "execute": "query-balloon" } { "execute": "qom-list", "arguments": { "path": "/machine/peripheral-anon/" }} # to get the balloon interface device name { "execute": "qom-get", "arguments": { "path": "/machine/peripheral-anon/device⁰", "property": "guest-stats" } } # Collect quest stats { "execute": "qom-set", "arguments": { "path": "/machine/peripheral-anon/device⁰", "property": "guest-stats-polling-interval", "value": 5}} # stat collection every 5 seconds

or host side (using virsh):

qemu-monitor-command --domain xxxxx --hmp 'balloon 4000'
qemu-monitor-command --domain xxxxx --hmp 'info balloon'
qemu-monitor-command --domain xxxxx '{ "execute": "qom-get", "arguments": { "path": "/machine/peripheral/balloon0", "property": "guest-stats" } }'