more doc

devices/qemu-aarch64/default.nix

@@ -2,16 +2,29 @@
 # emulator. The default output is a directory containing separate
 # kernel ("Image" format) and root filesystem (squashfs or jffs2)
 # images
-{
+rec {
   system = {
     crossSystem = {
       config = "aarch64-unknown-linux-musl";
     };
   };
 
-  # this device is described by the "qemu" device
-  description = "";
+  description = ''
+    QEMU Aarch64
+    ************
 
+    This target produces an image for
+    the `QEMU "virt" platform <https://www.qemu.org/docs/master/system/arm/virt.html>`_ using a 64 bit CPU type.
+
+    ARM targets differ from MIPS in that the kernel format expected
+    by QEMU is an "Image" (raw binary file) rather than an ELF
+    file, but this is taken care of by :command:`run.sh`. Check the
+    documentation for the :ref:`QEMU` (MIPS) target for more information.
+
+  '';
+
+  # this device is described by the "qemu" device
+  installer = "vmroot";
 
   module = {pkgs, config, ... }: {
     imports = [ ../../modules/arch/aarch64.nix ];
@@ -51,7 +64,7 @@
           klibBuild = config.system.outputs.kernel.modulesupport;
         };
       in {
-        defaultOutput = "vmroot";
+        defaultOutput = installer;
         loadAddress = "0x0";
         entryPoint  = "0x0";
         rootDevice = "/dev/mtdblock0";