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Developing Built-in Application VMs

Spectrum comes with some built-in application VMs, to demonstrate certain aspects of the system.

These VMs are defined inside the vm/app directory. The built-in VMs all use a common template, img/app, and supply a script of their own for the template to run that starts the appropriate application. Unlike user-defined VMs, Nix store paths inside built-in VMs are deduplicated against the template’s store. The template provides the root filesystem. It mounts a filesystem from a second block device containing the application-specific parts, overlays that filesystem’s Nix store onto its own, then runs the provided script.

It’s possible to run these VMs directly from your development system, without needing to start a whole Spectrum system, and then start the VM inside that system. This is done using the development shell for the template. For example, to run the built-in mg VM, run the following in the img/app directory:

nix-shell \
  -I nixpkgs= \
  --arg run ../../vm/app/mg.nix \
  --run 'make clean && make -j$NIX_BUILD_CORES run'

If you’re working on the template itself, you can keep a single development shell session open, and just make run every time you want to test a new version. Make will only track changes to the template, though, so if you change any Nix files, you’ll have to re-enter the development shell, and run make clean if they affected the template.

By default, make run runs the VM in QEMU, because it can do user mode networking. For a more Spectrum-like environment, you can instead run VMs in cloud-hypervisor by running Make with VMM=cloud-hypervisor.