Rumpkernels then allow, for example, taking all file system drivers in the kernel and running them in userspace without having to rewrite all of the file system tricky logic. Think of "mtools" if you ever used them, but by reusing existing FAT code. Or making it trivial to create disk images from userspace without having to have special kernel primitives nor root access.

And also, they allow taking a userspace application and packaging it with the minimum set of drivers required to run "bare metal". Which is what the SSH example in the smol page brought to mind. See https://github.com/rumpkernel/wiki/wiki/Repo%3A-rumprun

Edit: NetBSD is 7.3m LoC, FreeBSD is 9m.

I tried out smolBSD - the build and boot speed were impressive - as was the hint at a patch that boot time will be reduced from 100ms to 10ms. That's neat - in my experience adding systemd to a modern Linux Kernel pushes Firecracker up to 1-2s.

This smolBSD idea reminds me of unikernels and also of LinuxKit.

The documentation for smolBSD is a great start and could be so much better - for instance - the SSH example shows no way to configure an authorized SSH key or how to log in. The port-map to the host for the open port is also not mentioned.

I'm sure the author knows how to do these things - but even reading around in the repo, it wasn't clear. So hoping he'll improve on this if he's listening.

If anyone's interested in the Linux equivalent of this - check out my blog post on building a Linux microVM from a container [1] and video talk on Firecracker/Linux with Richard Case that led much of the work on Weave Ignite/Flintlock [2]

[1] https://actuated.com/blog/firecracker-container-lab [2] https://www.youtube.com/watch?v=CYCsa5e2vqg