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nix module and flake for declarative non-NixOS VMs
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Git Sagar 97fdbce6d0 windows images: stop a non-matching display glob from poisoning the build env 
Image builds have been dying with "Argument list too long" from sed, mktemp
and timeout alike -- commands whose argv is trivial, which is the tell that it
was the environment that had grown, not the arguments.

The X11 forwarding hint is looked up with `ls -t /tmp/.vmix-display-* | head -1`.
Under nullglob a non-matching pattern is removed from the command line rather
than passed through literally, so `ls -t` runs with no arguments at all and
lists the working directory instead. In a nix build that directory is the
build tree, whose newest file is nix's own env-vars dump. The result is that
VMIX_DF becomes "env-vars", the SDL branch is taken on a machine with no X at
all, and DISPLAY is exported with a slice of the env dump inside it. From that
line onward every exec in the build fails with E2BIG.

find does the same lookup without depending on how the shell treats an
unmatched pattern, and -type f keeps a stray directory out of it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117qMyjpuXsjpVAcpJbFD8g
2026-09-10 00:06:07 -03:00
lib windows images: stop a non-matching display glob from poisoning the build env 2026-09-10 00:06:07 -03:00
nixos windows/pci: vIOMMU for nested passthrough, and a data disk built with the image 2026-09-09 22:27:44 -03:00
.env-export-vmix-cli-local vmix CLI, laptop images, SDL display 2026-05-23 21:56:51 -03:00
.gitignore fist commit - images lib in a working condition for debian 2024-05-23 16:33:38 +00:00
cli.nix pass XAUTHORITY to builds for SDL display in X11 sessions 2026-05-31 04:51:53 -03:00
flake.lock vmix CLI, laptop images, SDL display 2026-05-23 21:56:51 -03:00
flake.nix fix: use vmix's own locked nixpkgs for all image building 2026-06-15 10:04:52 -03:00
README.md add README with usage, architecture, and examples 2026-06-07 10:46:32 +05:30

README.md

vmix

Composable QEMU VM image building and orchestration for NixOS.

What it does

vmix provides:

  • Image building — reproducible Linux (Debian) and Windows qcow2 images via Nix derivations
  • NixOS module — declarative VM management with QEMU in network namespaces
  • CLI tool — build, copy-to-disk, and run images from the command line

Usage

As a flake input

# flake.nix
inputs.vmix.url = "git+https://git.sagar.ch/dotfiles/vmix.nix.git";

# In your NixOS configuration (or globally via serverFunctions)
imports = [ inputs.vmix.nixosModules.default ];

# Use vmixLib via overlay
nixpkgs.overlays = [ inputs.vmix.overlays.default ];
# Then: pkgs.vmixLib.linux, pkgs.vmixLib.windows, pkgs.vmixLib.network

As a CLI

# Enter dev shell
nix develop

# Build an image
vmix build --image windows.images.win10.laptop \
  --generalize username=User,password=secret,hostname=PC

# Write to local disk
vmix copy --image windows.images.win10.laptop \
  --generalize username=User,password=secret \
  --to-disk /dev/sda

# Write to remote disk (streamed via SSH + LZ4)
vmix copy --image windows.images.win10.laptop \
  --generalize username=User,password=secret \
  --to-remote-disk root@10.10.10.100:/dev/nvme0n1

# Boot a built image with QEMU
vmix run ./result --mem 8192 --smp 8 --ahci

Flake outputs

Output Description
overlays.default Nix overlay exposing pkgs.vmixLib
nixosModules.default NixOS module for declarative VM management
lib.x86_64-linux Library functions (images + network utilities)
packages.x86_64-linux.default vmix CLI tool
apps.x86_64-linux.default Runnable vmix app

Repository structure

flake.nix          # Flake entry point
module.nix         # NixOS module export
overlay.nix        # Nix overlay (vmixLib pinned to nixpkgs 25-11)
cli.nix            # CLI tool (build, copy, run)

lib/
  default.nix      # Exports: images (linux + windows) + network
  network.nix      # IPv4/CIDR utilities
  images/
    linux/         # Debian image building + customization
    windows/       # Windows image building + customization
      helpers/     # makeImage, customizeImage, makeWinISO, etc.
      templates/   # Registry tweaks, app installers, essentials
      drivers/     # VirtIO, AMD GPU drivers
      win10/       # Windows 10 LTSC images
      win11/       # Windows 11 images

nixos/
  default.nix      # NixOS module entry point
  networks/        # Network namespace management (LAN, WAN, macvtap)
  vms/             # VM lifecycle management (QEMU, tap devices, DHCP)

Image building

Windows pipeline

  1. makeImage — unattended install from upstream ISO via QEMU
  2. customizeImageFold — apply modular templates (registry, apps, drivers)
  3. generalize — sysprep + OOBE for deployment to real hardware

Linux pipeline

  1. Fetch upstream Debian cloud image
  2. customizeImageFold — apply templates via virt-customize

Key patterns

  • customizeImageFold — builtins.foldl' over templates for composable layered images
  • _vmixOsType — all images carry "linux" or "windows" metadata for auto-detection
  • Offline registry — Windows templates use ControlSet001 for offline virt-win-reg --merge

NixOS module

Declare VMs and networks:

vmix.namespaces."lab" = {
  networks.lan1 = {
    subnet = "10.99.1.0/24";
    dhcp.enable = true;
  };
  vms.myvm = {
    image = pkgs.vmixLib.linux.images.debian.v12.upstream;
    memory = 2048;
    cores = 2;
    interfaces.lan1 = { ip = "10.99.1.10"; };
    autostart = true;
  };
};

Features:

  • Network namespaces with WAN (veth), LAN (bridge + dnsmasq), macvtap
  • ACPI graceful shutdown via QMP socket
  • 9p shares with auto-created mount targets
  • Conditional macvtap service (only created when macvtaps are configured)