Really cool! Will definitely take a closer look in my spare time.

>I just let userspace do i/o ports [...] and do memory mapped i/o by letting it mmap anything (more or less). The kernel manages timers/timekeeping and interrupts [...]

This is how QNX does it too, allowing privileged processes to use MAP_PHYS and port I/O instructions on x86, and handle interrupts like they're POSIX signals. It all boils down to how you structure your design, but personally, I think that's not a bad approach at all. The cool thing about it is that, after the initial setup, you can drop the privileges for creating further mappings and handlers, reducing the attack surface.

Unless you're trying to absolutely minimize the cost and amount of context switches, I think moving BEAM into the kernel would be a downgrade, but again, I'm a big proponent of microkernels :)

Looking forward to the UEFI and AArch64 ports!