In addition to the libkern functionality described above which is intended to facilitate implementing kernels, the library also provides complete, easy-to-use functionality to facilitate debugging kernels. The OSKit does not itself contain a complete kernel debugger (at least, not yet), but it contains extensive support for remote debugging using GDB, the GNU debugger. This remote debugging support allows you to run the debugger on one machine, and run the actual OS kernel being debugged on a different machine. The two machines can be of different architectures. A small ``debugging stub'' is linked into the OS kernel; this piece of code handles debugging-related traps and interrupts and communicates with the remote debugger, acting as a ``slave'' that simply interprets and obeys the debugger's commands.
This section describes remote debugging in general, applicable to any mechanism for communicating with the remote kernel (e.g., serial line or ethernet). The next section (10.18) describes kernel debugging support specific to the serial line mechanism (currently the only one implemented).
XXX diagram
One of the main advantages of remote debugging is that you can use a complete, full-featured source-level debugger, since it can run on a stable, well-established operating system such as Unix; a debugger running on the same machine as the kernel being debugged would necessarily have to be much smaller and simpler because of the lack of a stable underlying OS it can rely on. Another advantage is that remote debugging is less invasive: since most of the debugging code is on a different machine, and the remote debugging stub linked into the OS is much smaller than even a simple stand-alone debugger, there is much less that can ``go wrong'' with the debugging code when Strange Things start to happen due to subtle kernel bugs. The main disadvantage of remote debugging, of course, is that it requires at least two machines with an appropriate connection between them.
The GNU debugger, GDB, supports a variety of remote debugging protocols. The most common and well-supported is the serial-line protocol, which operates over an arbitrary serial line (typically null-modem) connection operating at any speed supported by the two machines involved. The serial-line debugging protocol supports a multitude of features such as multiple threads, signals, and data compression. GDB also supports an Ethernet-based remote debugging protocol and a variety of existing vendor- and OS-specific protocols.
Ths OS kit's GDB support has been tested with GDB versions 4.15 and 4.16; probably a version >= 4.15 is required.