Due to sloppy scheduler/threading behavior on Serenity, threaded IO is disabled in this port for now. Otherwise U-Boot randomly fails to read data from NVMe, or fails to initialize NVMe altogether, along with other IO issues in guests - all due to threaded tasks being randomly delayed for very long.
I am not an expert on how scheduler works in Serenity, so I am unable to fix it yet. This problem was also visible in previous v0.5 version of this port, but back then I thought it's some kind of a temporary problem. Couldn't reproduce this on any other host OS.