KDB FAQ

What is different about the old KDB vs the merged KDB?

In April 2009 KDB 4.4 (found at: ftp://oss.sgi.com/projects/kdb/download/v4.4/) had significant chunks of the code base removed and hooked it up to the same debug core and polled I/O drivers used by kgdb.

At a really high level, the only difference is how you configure and connect. The kdb disassembler was the main causality of the merger. Architecturally the merged kdb is 99% platform independent and actually gained new features via the debug core API.

Here is a list of the key differences between the out of tree KDB 4.4 and the new merged kdb & kgdb.

The kdb command shell differences

• There are fewer commands in the merged kdb & kgdb.
  • The kdump / kexec analysis modules tools were removed in the process of merging the code bases.
• The bt command does uses the kernel's backtracer and not a disassembly engine which shows you function arguments
• In the merged kdb & kgdb, presently there is no disassembler (known as the "id" command)
• In the merged kdb & kgdb, when using a non-vga style connection you can transition into kgdb mode and attach gdb, by using the kgdb command.
• The ability to reference elements of a structures was removed. This may get implemented a different way in the future.

Configuration

• Activating kdb is different. NOTE that kbd is short for keyboard.
  • If you want the kernel to stop as soon as possible such that you can install a breakpoint or execute a memory write, you need to add the kgdbwait argument at the end of your kernel boot arguments.
  • Configure kdb for early debugging: earlyprintk=vga ekgdboc=kbd
  • Configure kdb for normal keyboard debugging:
    • Kernel boot arg: kgdboc=kbd
    • At run time: echo kbd > /sys/module/kgboc/parameters/kgdboc
  • Configure kdb for a serial port at 115200 baud:
    • Kernel boot arg: kgdboc=ttyS0,115200
    • At run time: echo ttyS0,115200 > /sys/module/kgboc/parameters/kgdboc
  • Configure kdb for a keyboard and the serial port at the same time:
    • Kernel boot arg: kgdboc=kbd,ttyS0,115200
    • At run time: echo kbd,ttyS0,115200 > /sys/module/kgboc/parameters/kgdboc

• In order to break into the kdb you must use sysrq-g
  • Press and hold the Alt key, and the press and release the SysRq. While still holding alt, press the g key, then release all the keys.
  • OR, use "echo g > /proc/sysrq-trigger"
  • OR, If you are using a serial line send the serial break via your terminal emulator and press g

Number of architectures supported

• KDB 4.4 supported only ia64, x86_64 and i386
• The merged kdb & kgdb supports arm, blackfin, mips, x86_64, i386, sh, powerpc, and sparc.

Internal differences

• The capability to debug before the kernel memory allocator required an 2 stage initialization because the dynamically added commands require the kernel memory allocator.
• The kdb pager was redesigned so as not to rely upon in kernel versions of setjmp() longjmp().
• The kernel's printk can be redirected to a kdb command result.
• kdb command results can be passed to gdb with by using gdb's monitor command. You can try "monitor help" in gdb while connected to kgdb.
• The kdb keyboard driver was refactored to conform to the kgdb I/O driver model and to allow for sending key up events on kernel resume for any keys that were depressed at the time the kernel entered the debugger.
• The kdb breakpoint code was completely overhauled to connect to work in conjunction with the debug core API.
• The debug core gained the ability to change CPU cores without exiting the kernel debug context
• Hardware breakpoint work correctly and are system wide (all cores).