NetWatch!

This is NetWatch, a system for remote system-management-mode-based control
of a machine without support from or awareness by the OS.  It works by
taking over a second network card to provide a standard VNC server, such
that a machine elsewhere on the network can see the text or graphics console
of the machine and inject keystrokes as needed.

System management mode, introduced with the 386SL, essentially allows system
driver code to run outside of OS control, caused by a special interrupt pin
on the CPU.  This was originally intended for applications such as laptop
fan control; it is also the mechanism by which USB legacy keyboard emulation
occurs.  When a system management interrupt occurs, the northbridge remaps
portions of memory to expose previously-hidden code, and asserts an SMI#
signal, causing the CPU to save all its state into system management RAM and
vector to a magic entry point.

This is somewhat slow, and so there is a moderate performance impact caused
by running NetWatch, more significant when a VNC session is open.  Because
NetWatch is invisible to the OS, its CPU usage is difficult to monitor; we
do so by comparing the MD5 throughput of the system with NetWatch running
versus without.  The only way that the OS could detect this performance
drain is by spinning tightly and watching for a sudden jump in the CPU's
time stamp counters.

Although it would be possible to start up NetWatch after an OS kernel has
already loaded, it is easier and more useful to load it from GRUB before the
OS boots, such that even the bootloader itself can be controlled over the
network.  We do this by providing a stub loader (grubload/) which can be
invoked from GRUB, and takes care of loading the main NetWatch ELF image. 
Once this is done and NetWatch is up and running, the loader returns to real
mode and reinvokes GRUB via the BIOS.

Our current development platform, the Intel ICH2, does not allow SMM traps
on arbitrary PCI accesses.  This makes stealing the network card from the OS
somewhat difficult, since there is nothing SMM code can do to cleanly block
access.  NetWatch simply chooses its desired network card, and then
repeatedly clobbers the PCI base address registers.  Although Linux resets
the BARs to sane values when it probes the PCI bus, by the time it attempts
to actually load the network driver, the card will no longer be accessible;
fortunately, the driver quickly gives up, and Linux no longer attempts to
access the card.

The northbridge can be configured to invoke a system management interrupt
every 64 milliseconds, and so the bulk of NetWatch's work is done from this
interrupt: checking the network card for incoming packets, invoking lwIP,
and sending any response packets necessary.  SMM entry also occurs when when
the OS reads from the keyboard I/O ports, to inject scan codes as needed.

Much of NetWatch is very hardware-dependent, and although we've tried to
maintain clean interface separation to allow for easy porting, the current
implementation requires:

  * Intel ICH2 system chipset
  * 3C509 Ethernet card to be used by NetWatch, plus another card of
    any type for the OS
  * BIOS which does not set the D_LCK bit. Any system old enough to be
    based on the ICH2 is very likely to have a suitable BIOS.

Current open issues are listed in the TODO file. See GUIDE for an overview
of which source files do what.