this patch changes the way pagefaults are delivered to VM. It adopts
the same model as the out-of-quantum messages sent by kernel to a
scheduler.
- everytime a userspace pagefault occurs, kernel creates a message
which is sent to VM on behalf of the faulting process
- the process is blocked on delivery to VM in the standard IPC code
instead of waiting in a spacial in-kernel queue (stack) and is not
runnable until VM tell kernel that the pagefault is resolved and is
free to clear the RTS_PAGEFAULT flag.
- VM does not need call kernel and poll the pagefault information
which saves many (1/2?) calls and kernel calls that return "no more
data"
- VM notification by kernel does not need to use signals
- each entry in proc table is by 12 bytes smaller (~3k save)