One more problem:
Argument passing by setcontext() is broken -- the fast
syscall path clobbers %rcx, which is needed by the
sysret insn as return PC argument.
So the fourth argument isn't passed correctly to the
new context. (can be verified by regress/lib/libc/context)
I can imagine 2 ways to fix this: change makecontext() to put
a trampoline between the setcontext() syscall and the real
function, or use OSYSCALL.

best regards
Matthias