Debian – kexec and USE_GRUB_CONFIG

Normally, when you reboot, the machine will return to grub and either allow you to select a kernel via the keyboard, or boot the default configured kernel. However if you have kexec-tools installed, the reboot command will short circuit this behaviour and directly kexec into a kernel. You can disable this behaviour, and return to grub in reboot, by uninstalling kexec tools or editing the file

/etc/default/kexec 

and setting:

  LOAD_KEXEC=false 

Alternatively, to keep kexec active and have it reboot into the kernel of your choice, try a command line like this to load your desired kernel:

 kexec -l /boot/vmlinux --append=root=/dev/hda1 --initrd=/boot/initrd

then when 'kexec -e' is later run, the configured kernel in the kexec line as well will be run. As I believe the reboot script eventually just calls 'kexec-e' I believe the kernel change should take effect then.

Grub has its own filesystem drivers. It doesn't use Linux's. Grub loads the kernel and initrd into memory according to the Linux kernel boot protocol. A part of the kernel is loaded at a predefined address, and Grub sets some parameters so that the kernel knows where to find the rest.

When the kernel boots, it mounts a root partition using its own drivers (which cannot be modules since there are no modules at that point). The root partition can be an initrd, an initramfs, or (in the absence of an initramfs or initrd) the block device indicated by the root command line argument (or a compiled-in default if there is no root argument).