ADR-001: Firecracker-Only Execution

Status

Accepted

Context

mvm needs a VM engine for running isolated workloads. Options considered:

1. Docker/OCI containers — Widely adopted, large ecosystem
2. QEMU/KVM — Full hardware virtualization, maximum compatibility
3. Firecracker — Purpose-built microVM monitor, minimal attack surface
4. Cloud Hypervisor — Similar to Firecracker, more features

Decision

Use Firecracker as the sole engine. No container runtime.

Rationale

• Security: Firecracker’s minimalist design (no BIOS, no USB, no PCI) reduces attack surface to <50K LOC
• Performance: ~125ms cold boot, ~5ms snapshot restore, minimal memory overhead
• Snapshot support: Built-in VM snapshotting enables the sleep/wake lifecycle
• Predictable resources: Each microVM gets dedicated vCPUs and memory, no noisy-neighbor
• Multi-tenancy: Hardware-level isolation via KVM, not namespace isolation

Consequences

• Requires Linux with /dev/kvm (macOS uses Lima VM for nested virtualization)
• Guests must use a Linux kernel (no Windows/macOS guests)
• No OCI image compatibility — uses Nix flakes for image building instead
• Limited device model — no GPU passthrough, limited disk types