mikaku/Fiwix

Kexec implementation

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Fiwix kexec implementation

Kexec is a mechanism that let's you switch into a different kernel without rebooting your machine. It differs from the Linux kexec implementation because it doesn't need a system call nor specific user-space tools, and your current system will completely shutdown before jumping to the new kernel.

The new kernel can be another Fiwix kernel or any other ELF binary kernel. It currently only supports the Multiboot 1 Specification to be passed to the new kernel, but it's open to include support for other boot methods.

How it works

Your system needs to know at boot-time that it might switch to a different kernel. That is, you need to pass some special parameters in your kernel command line during the system boot. The following are the three parameters that you need to specify:

  • kexec_proto=
    This is the boot method of the new kernel. Currently the only supported value is multiboot1.

  • kexec_size=
    Size in KB of the memory space to be reserved to allocate the new kernel.

  • kexec_cmdline=
    Command line to be passed to the new kernel (enclosed in double quotes).

Example of a kernel command line:

/boot/fiwix ro root=/dev/hda2 kexec_proto=multiboot1 kexec_size=500 kexec_cmdline="fiwix ro root=/dev/hda2"

The RAMdisk drives play an important role here. You already know that they can be used to allocate an initrd image specified during the boot, or to allocate one or more all-purpose RAMdisk drives and, of course, they are also used for kexec to allocate the new kernel.

Kexec uses always the first unused RAMdisk drive. You'll know which one is the first unused depending if you specified an initrd image, or if you specified to have all-purpose RAMdisk drives at boot-time.

Fiwix is configured by default to have the following possible RAMdisk drives layout:

  • If an initrd image was specified at boot-time then the first RAMdisk drive is used for it (i.e: /dev/ram0).
  • If ramdisksize= was specified at boot-time then the first unused RAMDISK_DRIVES RAMdisk drives will be used.
  • If kexec_size= was specified at boot-time then the first unused RAMdisk drive will be used.

Let's see some examples to understand better these rules:

Example 1:

  • Kernel cmdline: ... ramdisksize=49152 initrd=/initrd.img
  • Resulting RAMdisk drives layout:
    • /dev/ram0 -> initrd RAMdisk drive of 48MB (/ filesystem).
    • if RAMDISK_DRIVES == 1
      • /dev/ram1 -> all-purpose RAMdisk drive of 48MB.
    • if RAMDISK_DRIVES == 2
      • /dev/ram1 -> all-purpose RAMdisk drive of 48MB.
      • /dev/ram2 -> all-purpose RAMdisk drive of 48MB.
        ...

Example 2:

  • Kernel cmdline: ... ramdisksize=16384
  • Resulting RAMdisk drives layout:
    • if RAMDISK_DRIVES == 1
      • /dev/ram0 -> all-purpose RAMdisk drive of 16MB.
    • if RAMDISK_DRIVES == 2
      • /dev/ram0 -> all-purpose RAMdisk drive of 16MB.
      • /dev/ram1 -> all-purpose RAMdisk drive of 16MB.
        ...

Example 3:

  • Kernel cmdline: ... ramdisksize=16384 kexec_size=500 ...
  • Resulting RAMdisk drives layout:
    • if RAMDISK_DRIVES == 1
      • /dev/ram0 -> all-purpose RAMdisk drive of 16MB.
      • /dev/ram1 -> kexec RAMdisk drive of 500KB.
    • if RAMDISK_DRIVES == 2
      • /dev/ram0 -> all-purpose RAMdisk drive of 16MB.
      • /dev/ram1 -> all-purpose RAMdisk drive of 16MB.
      • /dev/ram2 -> kexec RAMdisk drive of 500KB.
        ...

Once you know what RAMdisk drive will be used to allocate the new kernel, you can proceed to copy the ELF binary kernel into such RAMdisk drive by using any user-space tool like cp or dd.

Finally, when you are ready, just do a normal shutdown to switch automatically to the new kernel.

Example:

# cp fiwix /dev/ram1
# shutdown -h 0