EXPERIMENTAL Hotplug memory for real

Changing maxmem property of a running domain allows qmemman to read new
information and balance appropriately. For the maxmem property to
accurately reflect current state for clients to query, it is forbidden
to change it for a running non memory balanced domain.

Author: ben-grande

qubes/vm/qubesvm.py

@@ -2898,6 +2898,33 @@ def create_qdb_entries(self):
 
         self.fire_event("domain-qdb-create")
 
+    @qubes.events.handler("property-pre-set:maxmem")
+    def on_property_pre_set_maxmem(self, event, name, newvalue, oldvalue=None):
+        # pylint: disable=unused-argument
+        if newvalue == oldvalue:
+            return
+        if not self.is_halted() and not self.is_memory_balancing_possible():
+            raise qubes.exc.QubesVMNotHaltedError(
+                "Can't change maxmem of VM {!s}, because it's Halted.".format(
+                    self
+                )
+            )
+
+    @qubes.events.handler("property-set:maxmem")
+    def on_property_set_maxmem(self, event, name, newvalue, oldvalue=None):
+        # pylint: disable=unused-argument
+        if not newvalue or newvalue == oldvalue:
+            return
+        if not (
+            qmemman_present and self.use_memory_hotplug and self.is_running()
+        ):
+            return
+        self.app.vmm.xs.write(
+            "",
+            f"/local/domain/{self.xid}/memory/hotplug-max",
+            str(newvalue * 1024),
+        )
+
     # TODO async; update this in constructor
     def _update_libvirt_domain(self):
         """Re-initialise :py:attr:`libvirt_domain`."""