Just a nitpick: yes, don't use syscalls, but the syscalls numbers
you're getting gugtest a poorly tuned emulation.  Brian Bershad's
numbers were more like an 8-fold difference between kernel emulation
and restartable sequences.  The numbers you got are more like 20-fold.

At Stanford, the V-kernel mutex emulation was handled in a special
`fast path' in the trap hanlder. I'm not sure, but I think Mach did so
too.

It does lead to an interesting point: how to switch between software
schemes on old MIPS cpus, and using ll/sc on newer cpus?  (cpu
model-specific shared libs?, what about statically-linked code)?