... one question that's never been definitively answered is how the
kernel can block and then call out to a user level application.
You may not mean this literally, but just in case...
The kernel does not "call out" to user space. If you think of the
issue this way you are likely to make some poor design choices and
have a rough time of it.
The correct approach is to have a thread belonging to your background
userland app call into your KEXT and block there waiting for
something to do. Then, when some other event requires the utility app
to do something it can (after arranging for some state or data to be
passed to it) unblock that blocked thread. When it's done, it can
call into the KEXT and block again.
The key difference is thread ownership. If a kernel thread were to
execute userland code and that code were to fault it would be very
hard to kill the process and clean up the thread without causing a
kernel panic.
By doing this using a blocking user thread this problem is avoided.
Good luck!
-Mike
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