qmemman: use memory hotplug in qmemman

If VM supports memory hotplug, start it with maxmem=memory and then use
memory hotplug to (potentially) increase its memory. Qmemman almost had
all the needed parts - especially mem_set function already adjusts
maxmem too. The missing part was letting it know actual maxmem - do that
via xenstore (as qmemman uses xenstore heavily already).

The use of memory hotplug can be enabled (or disabled) explicitly via
'memory-hotplug' feature, or detected based on
'memory-hotplug-supported' file in dom0-provided kernel dir, or
'supported-feature.memory-hotplug' for VM-provided kernel.

Fixes QubesOS/qubes-issues#7956

qubes/qmemman/__init__.py

@@ -44,6 +44,7 @@ def __init__(self, id):
         self.mem_used = None		# used memory, computed based on meminfo
         self.id = id			    # domain id
         self.last_target = 0		# the last memset target
+        self.use_hoplug = False     # use memory hotplug for mem-set
         self.no_progress = False    # no react to memset
         self.slow_memset_react = False  # slow react to memset (after few
                                         # tries still above target)
@@ -96,7 +97,7 @@ def get_free_xen_memory(self):
         # used - do not count it as "free", because domain is free to use it
         # at any time
         # assumption: self.refresh_memactual was called before
-        # (so domdict[id].memory_actual is up to date)
+        # (so domdict[id].memory_actual is up-to-date)
         assigned_but_unused = functools.reduce(
             lambda acc, dom: acc + max(0, dom.last_target-dom.memory_current),
             self.domdict.values(),
@@ -127,9 +128,16 @@ def refresh_memactual(self):
                     self.domdict[id].memory_current,
                     self.domdict[id].last_target
                 )
-                self.domdict[id].memory_maximum = self.xs.read('', '/local/domain/%s/memory/static-max' % str(id))
-                if self.domdict[id].memory_maximum:
-                    self.domdict[id].memory_maximum = int(self.domdict[id].memory_maximum)*1024
+                hotplug_max = self.xs.read(
+                    '', '/local/domain/%s/memory/hotplug-max' % str(id))
+                static_max = self.xs.read(
+                    '', '/local/domain/%s/memory/static-max' % str(id))
+                if hotplug_max:
+                    self.domdict[id].memory_maximum = int(hotplug_max)*1024
+                    self.domdict[id].use_hotplug = True
+                elif static_max:
+                    self.domdict[id].memory_maximum = int(static_max)*1024
+                    self.domdict[id].use_hotplug = False
                 else:
                     self.domdict[id].memory_maximum = self.ALL_PHYS_MEM
                     # the previous line used to be
@@ -174,6 +182,9 @@ def mem_set(self, id, val):
         #  handle Xen view of memory
         self.xs.write('', '/local/domain/' + id + '/memory/target',
             str(int(val/1024 - 16 * 1024)))
+        if self.domdict[id].use_hotplug:
+            self.xs.write('', '/local/domain/' + id + '/memory/static-max',
+                          str(int(val / 1024)))
 
     # this is called at the end of ballooning, when we have Xen free mem already
     # make sure that past mem_set will not decrease Xen free mem

qubes/vm/qubesvm.py

@@ -23,6 +23,7 @@
 import asyncio
 import base64
 import grp
+import pathlib
 import re
 import os
 import os.path
@@ -1627,6 +1628,28 @@ def is_memory_balancing_possible(self):
                 return False
         return True
 
+    @property
+    def use_memory_hotplug(self):
+        """Use memory hotplug for memory balancing.
+        This is preferred if supported, because it has less initial overhead
+        and reduces Xen's attack surface.
+        The support needs to be enabled in the VM's kernel.
+        """
+        feature = self.features.check_with_template('memory-hotplug', None)
+        if feature is not None:
+            return bool(feature)
+        # if not explicitly set, check if support is advertised
+        # for dom0-provided kernel - check there
+        if self.kernel:
+            return (pathlib.Path(self.storage.kernels_dir) /
+                    'memory-hotplug-supported').exists()
+        # otherwise - check advertised VM's features
+        feature = self.features.check_with_template(
+            'supported-feature.memory-hotplug', None)
+        if feature is not None:
+            return bool(feature)
+        return False
+
     def request_memory(self, mem_required=None):
         if not qmemman_present:
             return None
@@ -2277,6 +2300,10 @@ def create_qdb_entries(self):
             self.app.vmm.xs.set_permissions('',
                                             f"{xs_basedir}/memory/meminfo",
                                             [{'dom': self.xid}])
+            if self.use_memory_hotplug:
+                self.app.vmm.xs.write('',
+                                      '/local/domain/{}/memory/hotplug-max',
+                                      str(self.maxmem))
 
         self.fire_event('domain-qdb-create')
 

templates/libvirt/xen.xml

@@ -3,7 +3,7 @@
         <name>{{ vm.name }}</name>
         <uuid>{{ vm.uuid }}</uuid>
         {% if ((vm.virt_mode == 'hvm' and vm.devices['pci'].persistent() | list)
-            or vm.maxmem == 0) -%}
+            or vm.maxmem == 0 or vm.use_memory_hotplug) -%}
             <memory unit="MiB">{{ vm.memory }}</memory>
         {% else -%}
             <memory unit="MiB">{{ vm.maxmem }}</memory>