Add Hyper-V backend for Windows support

Motivation

propolis currently only runs on Linux (KVM) and macOS (Hypervisor.framework) via libkrun. There is no Windows support. This issue tracks adding a Hyper-V backend to enable running OCI container images as microVMs on Windows.

Depends on: #1 (hypervisor abstraction layer)

Background

Why Hyper-V?

• Native Windows hypervisor — ships with Windows 10/11 Pro and Server editions
• HCS (Host Compute Service) — well-maintained Go bindings via hcsshim, used by Docker Desktop and containerd
• LCOW support — HCS natively supports Linux Containers on Windows (running Linux guests on Hyper-V)
• No CGO required — hcsshim is pure Go, unlike the libkrun backend

Alternatives considered

Alternative	Why not
Hyperlight	No OS in guest, no filesystem, no networking, no OCI support, no Go SDK. Fundamentally different execution model (function sandbox, not container VM).
QEMU + WHPX	Too heavyweight. Full device emulation stack adds significant complexity and binary size.
WSL2 + KVM	KVM support in WSL2 is unreliable — works in some WSL versions, regressed in others. Not shippable.
Firecracker	Linux/KVM only, no Windows support.

Proposed Implementation

Backend: hypervisor/hyperv/

Implements the hypervisor.Backend interface from #1:

//go:build windows

type Backend struct {
    // KernelPath is the path to the Linux kernel to boot.
    KernelPath string

    // InitRDPath is the optional path to an initrd image.
    InitRDPath string
}

func (b *Backend) Name() string { return "hyperv" }

func (b *Backend) PrepareRootFS(ctx context.Context, rootfsPath string, initCfg InitConfig) (string, error) {
    // 1. Write /.propolis_init.json to rootfs (generic init config)
    // 2. Convert flat rootfs directory to VHDX disk image
    // 3. Return path to VHDX
}

func (b *Backend) Start(ctx context.Context, cfg VMConfig) (VMHandle, error) {
    // 1. Create LCOW utility VM via hcsshim
    // 2. Attach root VHDX
    // 3. Configure networking (hvsock transport)
    // 4. Add filesystem mounts (Plan 9 shares)
    // 5. Start VM
    // 6. Return hcsHandle wrapping uvm.UtilityVM
}

Key components

Rootfs → VHDX conversion

The OCI image extraction pipeline produces a flat directory. Hyper-V needs a VHDX disk image. Options to investigate:

• virtdisk.dll Windows API (CreateVirtualDisk, AttachVirtualDisk)
• hcsshim's internal LCOW layer utilities
• diskpart / Convert-VHD PowerShell (fragile, last resort)

Networking: gvisor-tap-vsock over hvsock

gvisor-tap-vsock already supports Hyper-V socket transport (AF_HYPERV). The same virtual network stack used by libkrun's in-process networking can be reused with a different transport:

• libkrun: AcceptQemu() over Unix socketpair
• Hyper-V: AcceptHyperV() over hvsock

The NetEndpoint type from #1 carries the transport info:

NetEndpoint{Type: NetEndpointHVSocket, Path: serviceGUID}

Guest init

libkrun reads /.krun_config.json via its built-in init. Hyper-V needs a different mechanism:

• Write /.propolis_init.json with the same schema ({cmd, env, working_dir})
• Extend the guest/boot package to read this config and exec the workload
• Or use a minimal custom init binary in the initrd

Linux kernel

Unlike libkrun (which bundles a kernel in libkrunfw), Hyper-V requires a separate kernel image. Options:

• Bundle a kernel build (like Docker Desktop uses LinuxKit)
• Build a custom minimal kernel with virtio + 9p + ext4
• Let users provide their own via Backend.KernelPath

Platform-specific files needed

File	Purpose
hypervisor/hyperv/backend.go	Backend implementation via hcsshim
hypervisor/hyperv/handle.go	VMHandle wrapping uvm.UtilityVM
hypervisor/hyperv/vhd.go	Rootfs dir → VHDX conversion
hypervisor/default_windows.go	DefaultBackendName = "hyperv"
preflight/hyperv_windows.go	Check Hyper-V is enabled
preflight/resources_windows.go	Disk/CPU/RAM checks
internal/procutil/process_windows.go	Process identification via QueryFullProcessImageName
extract/libname_windows.go	.dll naming convention

VM lifecycle

Unlike libkrun (where krun_start_enter() takes over the process and never returns), Hyper-V manages VM lifecycle via API calls:

type hcsHandle struct {
    vm *uvm.UtilityVM
}

func (h *hcsHandle) Stop(ctx context.Context) error {
    // Graceful: vm.Shutdown()
    // Force:    vm.Terminate()
    // Cleanup:  vm.Close()
}

func (h *hcsHandle) IsAlive() bool { return h.vm.State() == StateRunning }
func (h *hcsHandle) ID() string    { return h.vm.ID() }

No two-process model needed. No signals. No Setsid. Pure API-driven lifecycle.

Open Questions

• Kernel bundling: LinuxKit kernel? Custom minimal build? User-provided? What's the right default for a library?
• VHD creation: Which Windows API is most reliable for creating ext4-formatted VHDX from a directory tree?
• Guest init: Extend guest/boot package, or build a separate init binary for Hyper-V guests?
• CI/CD: How to test? Windows runners with nested virtualization? Mock HCS interfaces? Integration test matrix?
• hvsock vs named pipe: Which transport for gvisor-tap-vsock? hvsock is more natural for Hyper-V, named pipes are simpler.
• Hyper-V editions: Require Pro/Enterprise (full Hyper-V) or also support Home (limited WHP)?
• Filesystem mounts: Plan 9 shares via hcsshim, or alternative mechanism?

Implementation phases

1. #1 must land first — the abstraction layer
2. Windows platform files — preflight, procutil, extract, default backend (can be done in parallel with 3)
3. Hyper-V backend — hcsshim integration, VHD creation, hvsock networking
4. Guest init — extend guest/boot or build custom init for non-libkrun backends
5. Kernel packaging — decide on and implement kernel bundling strategy
6. CI — Windows test infrastructure

Start with the hypervisor.Backend interface from #1 and the existing libkrun backend, then review the proposed files under hypervisor/hyperv/, preflight/, internal/procutil/, and extract/. Resolve the open questions around kernel packaging, VHDX creation, guest init, networking, and Windows CI before implementing. Done means Windows can prepare a rootfs, start and stop a Hyper-V VM, and pass the platform checks and tests.