Opencomm: proplib-based syscall

[Elad Efrat <elad%NetBSD.org@localhost>]

Hi,

For a long time I've been thinking about implementing a proplib-based
syscall -- I call it "opencomm" -- and only now got around to it.

It's like prop_dictionary_sendrecv_ioctl(), only that:
   - it doesn't require a file-descriptor/command
   - it takes a syscall slot

There are really tons of stuff we can do with it, and I think it opens
the door to a lot of experimenting (and perhaps even lowers the bar).
Some uses that come to mind:

  - We can basically implement any syscall on top of it (with a
    performance impact, yes)

  - We can offer past and future ABI compatibility (think of
    internalizing syscall arguments for mount() or whatever, so you
    don't even have to rebuild the program)

  - Subsystems can register themselves with opencomm() instead of
    adding a new syscall

  - Data passed to the kernel is not just a "struct" that might be
    internal to NetBSD, and can be [sent on the wire to and] inspected
    by other systems

  - We can invoke arbitrary kernel functions, maybe even sequences of
    them, aggregate results, and return to userspace

I'm sure others can think of much more exotic uses. :)

The changes required to implement it are very small (copy/paste of
prop_dictionary_sendrecv_ioctl in prop_kern.c, one line change to
syscalls.master, and (for now) a stub in kern_xxx.c).

Are there any objections for it being added?

Thanks,

-e.