[RFC] Host-to-guest file-watch (inotify) event relay over vsock for instant HMR / dev loops

[Vishv07]

Problem & Context

When using smol up with [dev] volumes (e.g. volumes = ["./src:/app"]), developers rely on file watchers inside the guest VM (such as Vite HMR, Next.js Fast Refresh, cargo watch, or Python hot-reloaders) to detect code edits.

However, standard VirtioFS / 9p mounts do not natively forward host OS file events (macOS FSEvents or Windows ReadDirectoryChangesW) into Linux guest inotify / fanotify queues. Watchers inside the guest either miss updates or are forced to enable polling (CHOKIDAR_USEPOLLING=1 / WATCHPACK_POLLING=true), which causes high CPU utilization and adds noticeable latency.

Proposed Architecture

Implement a lightweight host-to-guest file event relay:

1. Host-side watcher: When bind mounts are active in smol up, smolvm watches the mounted host directory paths on the host side using the cross-platform notify crate.
2. VSOCK notification channel: On file events (Create, Modify, Delete), smolvm sends a lightweight event message over the vsock connection to smolvm-agent.
3. Guest event trigger: smolvm-agent receives the event and triggers an inotify ping or touch on the corresponding path inside the guest mount point, allowing guest watchers to react instantaneously with zero polling overhead.

Benefits

• Instant (<10ms) hot-module replacement for local web/backend development inside SmolVM.
• Eliminates CPU spikes and battery drain from continuous polling.
• Significantly improves inner-loop developer experience when running dev servers inside microVMs.

Would love to hear maintainers' thoughts on whether this direction fits the smol up roadmap!

[BinSquare]

It's coming in the next release! 🙂‍↕️

[Vishv07]

Hii @BinSquare! , checking in on this , is this currently actively in development on your end, or is there a design / branch i can help take off your plate and build out?

I'd love to implement anything i can help with in this

[BinSquare]

This is actually already live since #547 @Vishv07

[BinSquare]

it's on by default for every -v mount

[Vishv07]

Ah my bad, I had tested a dev server earlier and didn't see the reload trigger on my setup, so I assumed it was still on the list , great to know it's already live , I am gonna give it a try @BinSquare , thanks