Matthew had a hard time finding sources for Mach and the Linux port done by Apple, and doesn't really like Mach anyway, so he decided to do a native port.
Some early servers are a bit more problematic to support.
One problem is cache coherency, and with some 2.4 code this was achieved on some machines, but some older, more hesoteric ones, are quite hard to support. Matthew gave us more technical details on how the port was made. A few of the things he mentionned where that Because there is no equivalent of xchg on the HP platform, Matthew explained the problems they had implementing spinlocks with the ldcw instruction. HPUX is one of the few architectures where the stack grows upwards, which is something else fun to work with.