Add ballooning monitor for VMs (#945)

* Add ballooning monitor

Add ghaf-memory-monitor and start it for every VM that has a balloon size defined.

Signed-off-by: Santtu Lakkala <santtu.lakkala@unikie.com>

* Increase guest balloon size

Signed-off-by: Santtu Lakkala <santtu.lakkala@unikie.com>

* Make appvm balloon size configurable (default to 2 * ramMb)

Signed-off-by: Santtu Lakkala <santtu.lakkala@unikie.com>

* Only run mem-manager on VMs with balloon

Signed-off-by: Santtu Lakkala <santtu.lakkala@unikie.com>

* Review fixes

Signed-off-by: Santtu Lakkala <santtu.lakkala@unikie.com>

---------

Signed-off-by: Santtu Lakkala <santtu.lakkala@unikie.com>

Author: slakkala

modules/microvm/appvm.nix

@@ -171,6 +171,8 @@ let
               microvm = {
                 optimize.enable = false;
                 mem = vm.ramMb;
+                balloonMem = builtins.ceil (vm.ramMb * vm.balloonRatio);
+                deflateOnOOM = false;
                 vcpu = vm.cores;
                 hypervisor = "qemu";
                 shares = [
@@ -320,10 +322,17 @@ in
             };
             ramMb = mkOption {
               description = ''
-                Amount of RAM for this AppVM
+                Minimum amount of RAM for this AppVM
               '';
               type = types.int;
             };
+            balloonRatio = mkOption {
+              description = ''
+                Amount of dynamic RAM for this AppVM as a multiple of ramMb
+              '';
+              type = types.number;
+              default = 2;
+            };
             cores = mkOption {
               description = ''
                 Amount of processor cores for this AppVM

modules/microvm/flake-module.nix

@@ -18,5 +18,9 @@
       ./power-control.nix
       ../hardware/common/shared-mem.nix
     ];
+
+    mem-manager.imports = [
+      ./mem-manager.nix
+    ];
   };
 }

modules/microvm/mem-manager.nix

@@ -0,0 +1,56 @@
+# Copyright 2025 TII (SSRC) and the Ghaf contributors
+# SPDX-License-Identifier: Apache-2.0
+#
+# VM memory usage manager on host
+#
+{
+  pkgs,
+  config,
+  ...
+}:
+let
+  balloonvms = builtins.filter (
+    name: (config.microvm.vms.${name}.config.config.microvm.balloonMem or 0) > 0
+  ) (builtins.attrNames (config.microvm.vms or { }));
+in
+{
+  systemd.services =
+    builtins.foldl'
+      (
+        result: name:
+        result
+        // (
+          let
+            microvmConfig = config.microvm.vms.${name}.config.config.microvm;
+          in
+          {
+            "ghaf-mem-manager-${name}" = {
+              description = "Manage MicroVM '${name}' memory levels";
+              after = [ "microvm@${name}.service" ];
+              requires = [ "microvm@${name}.service" ];
+              serviceConfig = {
+                Type = "simple";
+                WorkingDirectory = "${config.microvm.stateDir}/${name}";
+                ExecStart = "${pkgs.ghaf-mem-manager}/bin/ghaf-mem-manager -s ${name}.sock -m ${
+                  builtins.toString (microvmConfig.mem * 1024 * 1024)
+                } -M ${builtins.toString ((microvmConfig.mem + microvmConfig.balloonMem) * 1024 * 1024)}";
+              };
+            };
+          }
+        )
+      )
+      {
+        balloon-manager =
+          let
+            balloonvmnames = builtins.map (name: "ghaf-mem-manager-" + name + ".service") balloonvms;
+          in
+          {
+            description = "Manage MicroVM balloons";
+            after = balloonvmnames;
+            requires = balloonvmnames;
+            wantedBy = [ "microvms.target" ];
+            script = ":";
+          };
+      }
+      balloonvms;
+}

targets/laptop/laptop-configuration-builder.nix

@@ -22,6 +22,7 @@ let
           self.nixosModules.profiles-laptop
           self.nixosModules.laptop
           self.nixosModules.microvm
+          self.nixosModules.mem-manager
 
           {
             ghaf = {