My UML/s390 now runs on 2.6.12-rc4 + incremental patches.
Here are three additional patches for "s390-preparation":
Insert PTRACE_PEEKUSR_AREA and PTRACE_POKEUSR_AREA
actions into arch/um/kernel/ptrace.c (and remove two
conditionally add subarch-specific action to execve1
remove prototype for boot_timer_handler from
kern_util.h, as this header now indirectly is included
Because of s390's ARCH_SIGHDLR_PARAM not being "int"
compilation of UML/s390 is broken with the latest
Unfortunately it's still unclear, what the necessary change
in s390-host will look like. So I don't want to "release"
implemetation of UML/s390 yet.