qemu: pick hostPackages/buildPackages depending on cpu emulation

Follow-up to Github PR #339. Fixes Github issue #369.

Author: astro

lib/default.nix

@@ -29,42 +29,48 @@ rec {
       lib.drop offset lib.strings.lowerChars
     ));
 
-  createVolumesScript = pkgs: pkgs.lib.concatMapStringsSep "\n" (
-    { image
-    , label
-    , size ? throw "Specify a size for volume ${image} or use autoCreate = false"
-    , mkfsExtraArgs
-    , fsType ? defaultFsType
-    , autoCreate ? true
-    , ...
-    }: pkgs.lib.warnIf
-      (label != null && !autoCreate) "Volume is not automatically labeled unless autoCreate is true. Volume has to be labeled manually, otherwise it will not be identified"
-      (let labelOption =
-             if autoCreate then
-               (if builtins.elem fsType ["ext2" "ext3" "ext4" "xfs" "btrfs"] then "-L"
-                else if fsType == "vfat" then "-n"
-                else (pkgs.lib.warnIf (label != null)
-                  "Will not label volume ${label} with filesystem type ${fsType}. Open an issue on the microvm.nix project to request a fix."
-                  null))
-             else null;
-           labelArgument =
-             if (labelOption != null && label != null) then "${labelOption} '${label}'"
-             else "";
-             mkfsExtraArgsString =
-              if mkfsExtraArgs != null
-              then lib.escapeShellArgs mkfsExtraArgs
-              else " ";
-      in (lib.optionalString autoCreate ''
-      PATH=$PATH:${with pkgs; lib.makeBinPath [ coreutils util-linux e2fsprogs xfsprogs dosfstools btrfs-progs ]}
+  createVolumesScript = pkgs: volumes:
+    lib.optionalString (volumes != []) (
+      lib.optionalString (lib.any (v: v.autoCreate) volumes) ''
+        PATH=$PATH:${with pkgs.buildPackages; lib.makeBinPath [ coreutils util-linux e2fsprogs xfsprogs dosfstools btrfs-progs ]}
+      '' +
+      pkgs.lib.concatMapStringsSep "\n" (
+        { image
+        , label
+        , size ? throw "Specify a size for volume ${image} or use autoCreate = false"
+        , mkfsExtraArgs
+        , fsType ? defaultFsType
+        , autoCreate ? true
+        , ...
+        }: pkgs.lib.warnIf
+          (label != null && !autoCreate) "Volume is not automatically labeled unless autoCreate is true. Volume has to be labeled manually, otherwise it will not be identified"
+          (let labelOption =
+                 if autoCreate then
+                   (if builtins.elem fsType ["ext2" "ext3" "ext4" "xfs" "btrfs"] then "-L"
+                    else if fsType == "vfat" then "-n"
+                    else (pkgs.lib.warnIf (label != null)
+                      "Will not label volume ${label} with filesystem type ${fsType}. Open an issue on the microvm.nix project to request a fix."
+                      null))
+                 else null;
+               labelArgument =
+                 if (labelOption != null && label != null) then "${labelOption} '${label}'"
+                 else "";
+               mkfsExtraArgsString =
+                 if mkfsExtraArgs != null
+                 then lib.escapeShellArgs mkfsExtraArgs
+                 else " ";
+           in (lib.optionalString autoCreate ''
 
-      if [ ! -e '${image}' ]; then
-        touch '${image}'
-        # Mark NOCOW
-        chattr +C '${image}' || true
-        truncate -s ${toString size}M '${image}'
-        mkfs.${fsType} ${labelArgument} ${mkfsExtraArgsString} '${image}'
-      fi
-    '')));
+              if [ ! -e '${image}' ]; then
+                touch '${image}'
+                # Mark NOCOW
+                chattr +C '${image}' || true
+                truncate -s ${toString size}M '${image}'
+                mkfs.${fsType} ${labelArgument} ${mkfsExtraArgsString} '${image}'
+              fi
+            ''))
+      ) volumes
+    );
 
   buildRunner = import ./runner.nix;
 

lib/runner.nix

@@ -26,11 +26,22 @@ let
   execArg = lib.optionalString microvmConfig.prettyProcnames
     ''-a "microvm@${hostName}"'';
 
+  vmHostPackages =
+    if microvmConfig.cpu == null
+    then
+      # When cross-compiling for a target host, select packages for
+      # the target:
+      pkgs.hostPackages
+    else
+      # When cross-compiling for CPU emulation in qemu, select
+      # packages for the host:
+      pkgs.buildPackages;
+
   binScripts = microvmConfig.binScripts // {
     microvm-run = ''
       set -eou pipefail
       ${preStart}
-      ${createVolumesScript pkgs microvmConfig.volumes}
+      ${createVolumesScript vmHostPackages microvmConfig.volumes}
       ${lib.optionalString (hypervisorConfig.requiresMacvtapAsFds or false) openMacvtapFds}
 
       exec ${execArg} ${command}

lib/runners/qemu.nix

@@ -36,7 +36,16 @@ let
     ++ lib.optional microvmConfig.optimize.enable minimizeQemuClosureSize
   );
 
-  qemu = overrideQemu pkgs.qemu_kvm;
+  qemuPkg =
+    if microvmConfig.cpu == null
+    then
+      # When cross-compiling for a target host, select qemu for the target:
+      pkgs.hostPackages.qemu_kvm
+    else
+      # When cross-compiling for CPU emulation, select qemu for the host:
+      pkgs.buildPackages.qemu;
+
+  qemu = overrideQemu qemuPkg;
 
   inherit (microvmConfig) hostName vcpu mem balloon initialBalloonMem deflateOnOOM hotplugMem hotpluggedMem user interfaces shares socket forwardPorts devices vsock graphics storeOnDisk kernel initrdPath storeDisk;
   inherit (microvmConfig.qemu) machine extraArgs serialConsole;