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For starters, assuming the Linux variant, build a statically compiled application, pack it into an initramfs as the only file there, for simplicity name it `/init`, bundle the initramfs with the kernel, boot. At this point, your app should be the PID 1 and the only process running (with the exception of a bunch of kernel threads). At this point you can do whatever you want.

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This is the most realistic comment on this thread (so far).

Realistic, yes, but it's not a unikernel.

There are projects that permit statically linking a traditional kernel with a traditional application into a unikernel. NetBSD pioneered this with their rump kernel build framework, and I believe there's at least one Linux build framework that mimics this. The build frameworks cut out the syscall layer; an application calling read(2) is basically calling the kernel's read syscall implementation directly. Often you don't need to change any application source code. The build frameworks handle configuring and building the kernel image, and statically linking the kernel image with your application binary to produce the unikernel image.

I probably should have mentioned I've built unikernels with some of the tooling you've described here. It just seems very academic and edge case compared to a single static user space Linux binary that while technically isn't a by the book unikernel, all I guess I meant was that it's diminishing returns beyond that.

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