Show HN: Smol machines – subsecond coldstart, portable virtual machines

A CLI tool that runs each task in a hardware-isolated VM.

• Works on macOS and Linux, and targets sub-second cold start.
• Packages workloads into a single .smolmachine file, which can be relaunched on other supported platforms.
• Network is off by default, and can be turned on only when needed with --net.
• Also supports egress restriction via --allow-host, which allows only specific hosts.
• SSH agent forwarding lets you use git/ssh without putting host keys directly inside the VM.
• A Smolfile lets you declaratively write image, network, allowed hosts, init commands, volumes, and auth settings.

The implementation uses a libkrun-based VMM along with platform-specific hypervisors.

• Uses Hypervisor.framework on macOS and KVM on Linux.
• Memory is elastically adjusted via virtio balloon, committing only actual usage.
• vCPU threads sleep in the hypervisor when idle, lowering the cost of overcommitment.

The README suggests this tool for the following uses.

• untrusted code sandbox
• Distributing a portable executable with dependencies pre-installed
• A persistent development machine where packages remain
• Flows like smolvm machine run, create, start, exec, stop

It also states its limitations.

• Network is disabled by default, only TCP/UDP is supported, and ICMP is not supported.
• Volume mounts are directory-only.
• On macOS, Hypervisor.framework entitlements are required.