OpenPOWER boot chain

flowchart LR
    skiroot(["skiroot (petitboot)"])
    SBEs-->hostboot-->skiboot-->skiroot-->OS

• Self-boot engines (SBE) are split between an on-chip ROM and an external EEPROM
• hostboot is a C++ boot loader that does DRAM initialization provides runtime services to skiboot or a hypervisor
• skiboot is a C boot loader and runtime firmware for OpenPOWER that loads skiroot.

skiroot is a term used to describe the LinuxBoot implementation for OpenPOWER. A skiroot repository or package does not exist. The term is only used in the kernel configuration, skiroot_defconfig.¹

petitboot is a Linux user-space application written in C that calls kexec. The kexec installed in the initramfs is not the mainline kexec-tools implementation, but a smaller implementation named kexec-lite. It is claimed to be roughly 32 KB compared to kexec-tools, which is roughly 200 KB.²

See also

• coreboot and Heads as an alternative firmware for OpenPOWER Talos II

1. https://github.com/open-power/op-build/blob/master/openpower/configs/linux/skiroot_defconfig ↩

2. https://github.com/antonblanchard/kexec-lite/issues/4#issuecomment-314936778 ↩