winvm

A Windows 11 VM under QEMU/KVM for building and testing Cygwin programs (such as mintty) from Linux. Everything runs as your user. You need x86-64 Linux with hardware virtualization and read-write access to /dev/kvm (on NixOS, membership in the kvm group), Nix with flakes, an internet connection for the install, and about 25 GB of disk space. QEMU, OVMF, swtpm, and the virtio-win drivers come from the nixpkgs pinned in flake.lock.

nix build -o ~/code/system/winvm/result ~/code/system/winvm
alias winvm=~/code/system/winvm/result/bin/winvm   # or: nix profile add ~/code/system/winvm

winvm install   # about 15 minutes, unattended (plus the 8.2 GB ISO download, if needed)
winvm start
winvm ssh

winvm install installs Windows into a base image that it then never changes. The VM runs on a copy-on-write overlay; winvm reset throws the overlay away, which gives a clean Windows (with Cygwin) again in seconds.

Commands

Command	What it does
install [--replace]	Install Windows and Cygwin into a new base image. --replace deletes the existing VM, once the files for the new one are ready.
start	Start the VM in the background (unless it runs already), and wait until SSH works (about 20 seconds after a cold start).
stop [--force]	Shut Windows down, closing programs without asking them (if SSH fails, it presses the ACPI power button instead). --force powers off at once.
reset	Put the stopped VM back to the clean install.
status	Tell whether the VM runs and whether SSH into it works.
ssh [ARGS...]	Run ssh to the guest; the arguments go to ssh.
gui COMMAND [ARGS...]	Start a program on the guest's desktop, detached from SSH.
view	Show the screen in a VNC viewer (TigerVNC).
screenshot [FILE]	Save the screen as a PNG (default: ~/vm/winvm/screen.png).
type TEXT...	Type the arguments, joined with spaces, on the VM's keyboard (US layout, ASCII only). Text that starts with - works too.
key COMBO...	Press keys, e.g. ret, esc, ctrl-c, alt-f4, ctrl-alt-delete (QEMU key codes joined by -).
click X Y [--button left|right|middle] [--double]	Click at pixel (X, Y) of the screen, as in the screenshot.

type, key, and click go through the VM's keyboard and mouse, so the program in the foreground gets real Windows input events.

Testing mintty

rsync -a --delete --exclude=/.git -e "ssh -F $HOME/vm/winvm/ssh_config" ~/code/system/mintty/ winvm:mintty/
winvm ssh 'cd mintty/src && make -j8'
winvm gui mintty/bin/mintty.exe
winvm type 'echo hello'; winvm key ret
winvm screenshot /tmp/mintty.png

The build does not need .git (without it, the version stamp has the build time instead of the commit), and in a linked worktree, .git is a file that points to a path on the host. With Include ~/vm/winvm/ssh_config in ~/.ssh/config, plain ssh winvm and rsync ... winvm: work too.

The guest

• Windows 11 Enterprise Evaluation, version 26H2. The evaluation ends 90 days after winvm install; after that, Windows shuts down every hour. winvm reset does not help then, but winvm install --replace does (the ISO stays in ~/vm/iso). When Microsoft replaces the ISO on the Evaluation Center, the download fails; then update ISO_URL and ISO_SHA256 in winvm.py.
• Account: the same name as your Linux login, password winvm, an administrator, logged on automatically. UAC still runs programs without elevation, but elevates without a prompt (e.g. powershell Start-Process -Verb RunAs ...).
• Cygwin in C:\cygwin64, with gcc-core, make, git, openssh, rsync, run, procps-ng, psmisc, and python3. Defender does not scan C:\cygwin64.
• sshd is Cygwin's, started at logon from the HKCU\...\Run key, in the desktop session and without elevation. That is why programs started over SSH appear on the screen. It only accepts the key ~/vm/winvm/id_ed25519.
• No automatic Windows updates, no sleep, no lock screen. Time zone: UTC.
• 8 CPUs, 8 GB of memory, a 128 GB disk (allocated as it fills), and a 1920x1080 screen; the constants at the top of winvm.py change these.

QEMU forwards only 127.0.0.1:10022 on the host to the guest's SSH port. The VNC and QMP sockets have no password; they are in ~/vm/winvm, which only you can enter. Note that the guest can reach the host's localhost services through 10.0.2.2, as with any QEMU user-mode network.

When something goes wrong

• During winvm install, ~/vm/winvm/install-screen.png shows the screen, updated about every 30 seconds; winvm view shows it live.
• In the guest, C:\winvm\setup.log is the log of the provisioning script (guest/setup.ps1), and ~/.winvm-sshd/sshd.log is sshd's log.

Files

• winvm.py: the command; its docstring lists the main files in ~/vm/winvm.
• guest/autounattend.xml: answers for Windows Setup.
• guest/setup.ps1: provisioning at the first logon (Cygwin, sshd, settings).
• guest/sshd.sh: runs sshd in the desktop session, at each logon.