* A module for building KVM/QEMU virtual machines from a NixOS

configuration.  No virtual disk image is generated; the VM shares a
  Nix store with the host.  This makes it very fast to build new VMs.

svn path=/nixos/branches/modular-nixos/; revision=15999

modules/virtualisation/qemu-vm.nix

@@ -0,0 +1,61 @@
+# This module creates a virtual machine from the NixOS configuration.
+# Building the `config.system.build.vm' attribute gives you a command
+# that starts a KVM/QEMU VM running the NixOS configuration defined in
+# `config'.  The Nix store is shared read-only with the host, which
+# makes (re)building VMs very efficient.  However, it also means you
+# can't reconfigure the guest inside the guest - you need to rebuild
+# the VM in the host.  On the other hand, the root filesystem is a
+# read/writable disk image persistent across VM reboots.
+
+{config, pkgs, ...}:
+
+{
+  # All the modules the initrd needs to mount the host filesystem via
+  # CIFS.  Also use paravirtualised network and block devices for
+  # performance.
+  boot.initrd.extraKernelModules =
+    ["cifs" "virtio_net" "virtio_pci" "virtio_blk" "virtio_balloon" "nls_utf8"];
+
+  fileSystems =
+    [ { mountPoint = "/";
+        device = "/dev/vda";
+      }
+    ];
+
+  # Mount the host filesystem and bind-mount its Nix store into our
+  # own root FS.
+  boot.initrd.postMountCommands =
+    ''
+      ipconfig 10.0.2.15:::::eth0:none
+      
+      mkdir /hostfs
+      ${pkgs.vmTools.mountCifs}/bin/mount.cifs //10.0.2.4/qemu /hostfs -o guest,username=nobody
+
+      mkdir -p $targetRoot/nix/store
+      mount --bind /hostfs/nix/store $targetRoot/nix/store
+    '';
+
+  # Starting DHCP brings down eth0, which kills the connection to the
+  # host filesystem and thus deadlocks the system.
+  networking.useDHCP = false;
+
+  system.build.vm = pkgs.runCommand "nixos-vm" {}
+    ''
+      ensureDir $out
+      ln -s ${config.system.build.system} $out/system
+
+      ensureDir $out/bin
+      cat > $out/bin/run-nixos-vm <<EOF
+      #! ${pkgs.stdenv.shell}
+      export PATH=${pkgs.samba}/sbin:\$PATH
+      ${pkgs.kvm}/bin/qemu-system-x86_64 \
+        -net nic,model=virtio -net user -smb / \
+        -drive file=\$diskImage,if=virtio,boot=on \
+        -kernel ${config.system.build.system}/kernel \
+        -initrd ${config.system.build.system}/initrd \
+        -append "\$(cat ${config.system.build.system}/kernel-params) init=${config.system.build.bootStage2} systemConfig=${config.system.build.system}"
+      EOF
+      chmod u+x $out/bin/run-nixos-vm
+    '';
+
+}