nixos/*qemu*: Add systemd initrd support

nixos/modules/profiles/qemu-guest.nix

@@ -1,13 +1,13 @@
 # Common configuration for virtual machines running under QEMU (using
 # virtio).
 
-{ ... }:
+{ config, lib, ... }:
 
 {
   boot.initrd.availableKernelModules = [ "virtio_net" "virtio_pci" "virtio_mmio" "virtio_blk" "virtio_scsi" "9p" "9pnet_virtio" ];
   boot.initrd.kernelModules = [ "virtio_balloon" "virtio_console" "virtio_rng" ];
 
-  boot.initrd.postDeviceCommands =
+  boot.initrd.postDeviceCommands = lib.mkIf (!config.boot.initrd.systemd.enable)
     ''
       # Set the system time from the hardware clock to work around a
       # bug in qemu-kvm > 1.5.2 (where the VM clock is initialised

nixos/modules/testing/test-instrumentation.nix

@@ -65,33 +65,26 @@ in
       };
     };
 
-    boot.initrd.preDeviceCommands =
-      ''
-        echo 600 > /proc/sys/kernel/hung_task_timeout_secs
-      '';
-
-    boot.initrd.postDeviceCommands =
-      ''
-        # Using acpi_pm as a clock source causes the guest clock to
-        # slow down under high host load.  This is usually a bad
-        # thing, but for VM tests it should provide a bit more
-        # determinism (e.g. if the VM runs at lower speed, then
-        # timeouts in the VM should also be delayed).
-        echo acpi_pm > /sys/devices/system/clocksource/clocksource0/current_clocksource
-      '';
-
-    boot.postBootCommands =
-      ''
-        # Panic on out-of-memory conditions rather than letting the
-        # OOM killer randomly get rid of processes, since this leads
-        # to failures that are hard to diagnose.
-        echo 2 > /proc/sys/vm/panic_on_oom
-      '';
+    boot.kernel.sysctl = {
+      "kernel.hung_task_timeout_secs" = 600;
+      # Panic on out-of-memory conditions rather than letting the
+      # OOM killer randomly get rid of processes, since this leads
+      # to failures that are hard to diagnose.
+      "vm.panic_on_oom" = lib.mkDefault 2;
+    };
 
-    # Panic if an error occurs in stage 1 (rather than waiting for
-    # user intervention).
-    boot.kernelParams =
-      [ "console=${qemu-common.qemuSerialDevice}" "panic=1" "boot.panic_on_fail" ];
+    boot.kernelParams = [
+      "console=${qemu-common.qemuSerialDevice}"
+      # Panic if an error occurs in stage 1 (rather than waiting for
+      # user intervention).
+      "panic=1" "boot.panic_on_fail"
+      # Using acpi_pm as a clock source causes the guest clock to
+      # slow down under high host load.  This is usually a bad
+      # thing, but for VM tests it should provide a bit more
+      # determinism (e.g. if the VM runs at lower speed, then
+      # timeouts in the VM should also be delayed).
+      "clock=acpi_pm"
+    ];
 
     # `xwininfo' is used by the test driver to query open windows.
     environment.systemPackages = [ pkgs.xorg.xwininfo ];

nixos/modules/virtualisation/qemu-vm.nix

@@ -754,13 +754,13 @@ in
     );
     boot.loader.grub.gfxmodeBios = with cfg.resolution; "${toString x}x${toString y}";
 
-    boot.initrd.extraUtilsCommands =
+    boot.initrd.extraUtilsCommands = lib.mkIf (!config.boot.initrd.systemd.enable)
       ''
         # We need mke2fs in the initrd.
         copy_bin_and_libs ${pkgs.e2fsprogs}/bin/mke2fs
       '';
 
-    boot.initrd.postDeviceCommands =
+    boot.initrd.postDeviceCommands = lib.mkIf (!config.boot.initrd.systemd.enable)
       ''
         # If the disk image appears to be empty, run mke2fs to
         # initialise.
@@ -770,7 +770,7 @@ in
         fi
       '';
 
-    boot.initrd.postMountCommands =
+    boot.initrd.postMountCommands = lib.mkIf (!config.boot.initrd.systemd.enable)
       ''
         # Mark this as a NixOS machine.
         mkdir -p $targetRoot/etc
@@ -789,6 +789,11 @@ in
         ''}
       '';
 
+    systemd.tmpfiles.rules = lib.mkIf config.boot.initrd.systemd.enable [
+      "f /etc/NIXOS 0644 root root -"
+      "d /boot 0644 root root -"
+    ];
+
     # After booting, register the closure of the paths in
     # `virtualisation.additionalPaths' in the Nix database in the VM.  This
     # allows Nix operations to work in the VM.  The path to the