Kexec - Booting into another kernel
This article describes how to use the kexec tool to boot into another kernel. It does not require a restart of the system.
As a kernel developer you often have to test a new kernel. The kexec command allows you to boot into another kernel. The kexec command utilizes the kexec system call and enables you to load and boot into another kernel. Kexec performans the function of the boot loader. The major difference is that a kexec boot does not perform the hardware initiaiization that is normally performed by the BIOS or firmware.
The first step is to install the kexec tools.
> sudo dnf install kexec-tools
To be able to install a new kernel, it first needs to be installed. Once the kernel is installed, it should be listed in the /boot. The new kernel can then be installed with the following command line:
> sudo kexec -l path-to-kernel-vmlinuz --initrd=path-to-init-rd --command-line="$(cat /proc/cmdline)" > sudo kexec -e
An example would be for instance:
> sudo kexec -l /boot/vmlinuz-5.12.0 \ --initrd=/boot/initramfs-5.12.0-0_6656_gc85768aa64da.img \ --command-line="$(cat /proc/cmdline)" > sudo kexec -e
After executing the kexec -e commmand, the user process gets terminated and the new kernel is booted. Once testing has completed and the new kernel is no longer needed, the same procedure can be used to boot into the original kernel. This makes sure that the same kernel is active as before the test.
For more detailed information about the kexec command you can check the kexec man page.
Sometimes the kexec -l command returns the error “Ramdisks not supported with generic elf arguments”. If that happens, appending the program option
resolves the problem.