ext/services: add automatic migration meminfo-writer=False -> maxmem=0

Migrate meminfo-writer=False service setting to maxmem=0 as a method to
disable dynamic memory management. Remove the service from vm.features
dict in the process.

Additionally, translate any attempt to set the service.meminfo-writer
feature to either setting maxmem=0 or resetting it to the default (which
is memory balancing enabled if supported by given domain). This is to at
least partially not break existing tools using service.meminfo-writer as
a way to control dynamic memory management. This code does _not_ support
reading service.meminfo-writer feature state to get the current state of
dynamic memory management, as it would require synchronizing with all
the factors affecting its value. One of main reasons for migrating to
maxmem=0 approach is to avoid the need of such synchronization.

QubesOS/qubes-issues#4480

qubes/ext/services.py

@@ -47,6 +47,20 @@ class ServicesExtension(qubes.ext.Extension):
     def on_domain_feature_set(self, vm, event, feature, value, oldvalue=None):
         '''Update /qubes-service/ QubesDB tree in runtime'''
         # pylint: disable=unused-argument
+
+        # TODO: remove this compatibility hack in Qubes 4.1
+        if feature == 'service.meminfo-writer':
+            # if someone try to enable meminfo-writer ...
+            if value:
+                # ... reset maxmem to default
+                vm.maxmem = qubes.property.DEFAULT
+            else:
+                # otherwise, set to 0
+                vm.maxmem = 0
+            # in any case, remove the entry, as it does not indicate memory
+            # balancing state anymore
+            del vm.features['service.meminfo-writer']
+
         if not vm.is_running():
             return
         if not feature.startswith('service.'):
@@ -65,8 +79,22 @@ class ServicesExtension(qubes.ext.Extension):
         if not feature.startswith('service.'):
             return
         service = feature[len('service.'):]
+        # this one is excluded from user control
+        if service == 'meminfo-writer':
+            return
         vm.untrusted_qdb.rm('/qubes-service/{}'.format(service))
 
+    @qubes.ext.handler('domain-load')
+    def on_domain_load(self, vm, event):
+        '''Migrate meminfo-writer service into maxmem'''
+        # pylint: disable=no-self-use,unused-argument
+        if 'service.meminfo-writer' in vm.features:
+            # if was set to false, force maxmem=0
+            # otherwise, simply ignore as the default is fine
+            if not vm.features['service.meminfo-writer']:
+                vm.maxmem = 0
+            del vm.features['service.meminfo-writer']
+
     @qubes.ext.handler('features-request')
     def supported_services(self, vm, event, untrusted_features):
         '''Handle advertisement of supported services'''