Options that come to mind are:
- build your application as a linux kernel module, load it into a normal kernel, and generally ignore the userspace that runs anyway
- take Linux and hack it down pretty aggressively plus splice your code into it
- find some github unikernel effort and go from there (which I think the OP does)
- take some other OS - freebsd? - and similarly hack out parts
Other?
I like the idea of a x64 machine running a VM connected to a network card as a generic compute resource that does whatever tasks are assigned by sending it data over the network. It's not been worth the hassle relative to a userspace daemon, but one day I may find the time and would be interested in the HN perspective on where best to start the OS level hackery.