Thursday, March 12, 2009

Running Linux kexec

The generic form of kexec command looks like
kexec -l kernel_image --initrd=kernel_initrd --append=command_line_options
The command has available many other options but the presented ones are the most important. To start kernel reset, run
kexec -e
How does it work? Linux kernel is placed in memory at defined address offset. On x86 architecture, it begins at 0x100000. Kexec is capable to call and run another kernel in the context of current kernel. It copies the new kernel somewhere into memory, moves it into kernel dynamic memory and finally copies it to the final destination which is the offset and runs it - kernel is exchanged and the reset is performed. An example how to reset running SLES 10.x kernel follows
kversion=`uname -r`
kexec -l /boot/vmlinuz-$kversion --initrd=/boot/initrd-$kversion --append="`cat /proc/cmdline`"
kexec -e
The example for RHEL 5.x is slightly different:
kexec -l /boot/vmlinuz-$kversion --initrd=/boot/initrd-${kversion}.img --append="`cat /proc/cmdline`"

Does it have any drawbacks? As I said, there may be some buggy devices which won't work after kernel reset. Typically, there are troubles with VGAs and their video memory initialization which results in garbled console after reset. The recommendation is to use normal video mode for console. You can change it with vga parameter set to zero and passed as kernel options (e.g. SLES 10 uses video framebuffer by default)
Next, the earlier version of kexec had stability issues on any other platform than x86. Today, kexec is uspported on x86, x86_64, ppc64 or ia64.

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