KGDB is a source level debugger for linux kernel. It is used along with gdb to debug linux kernel. Kernel developers can debug a kernel similar to application programs with use of KGDB. It makes it possible to place breakpoints in kernel code, step through the code and observe variables.

Two machines are required for using KGDB. One of these machines is a development machine and the other is a test machine. The machines are connected through a serial line, a null-modem cable which connects their serial ports.  The kernel to be debugged runs on the test machine. gdb runs on the development machine. The serial line is used by gdb to communicate to the kernel being debugged.

KGDB is available for i386, x86_64, ppc and s390 architectures.

KGDB is a kernel patch. It has to be applied to a linux kernel to enable kernel debugging.  KGDB patch adds following components to a kernel

• gdb stub - The gdb stub is heart of the debugger. It is the part that handles requests coming from gdb on the development machine. It has control of all processors in the target machine when kernel running on it is inside the debugger. 
• modifications to fault handlers - Kernel gives control to debugger when an unexpected fault fault occurs. A kernel which does not contain gdb panics on unexpected faults. Modifications to fault handles allow kernel developers to analyze unexpected faults.
• serial communication - This component uses a serial driver in the kernel and offers an interface to gdb stub in the kernel. It is responsible for sending and receiving data from a serial line. This component is also responsible for handling control break request sent by gdb.