Kexec Handover Usage

Kexec HandOver (KHO) is a mechanism that allows Linux to preserve memory regions, which could contain serialized system states, across kexec.

This document expects that you are familiar with the base KHO concepts. If you have not read them yet, please do so now.

Prerequisites

KHO is available when the kernel is compiled with CONFIG_KEXEC_HANDOVER set to y. Every KHO producer may have its own config option that you need to enable if you would like to preserve their respective state across kexec.

To use KHO, please boot the kernel with the kho=on command line parameter. You may use kho_scratch parameter to define size of the scratch regions. For example kho_scratch=16M,512M,256M will reserve a 16 MiB low memory scratch area, a 512 MiB global scratch region, and 256 MiB per NUMA node scratch regions on boot.

Perform a KHO kexec

To perform a KHO kexec, load the target payload and kexec into it. It is important that you use the -s parameter to use the in-kernel kexec file loader, as user space kexec tooling currently has no support for KHO with the user space based file loader

# kexec -l /path/to/bzImage --initrd /path/to/initrd -s
# kexec -e

The new kernel will boot up and contain some of the previous kernel’s state.

For example, if you used reserve_mem command line parameter to create an early memory reservation, the new kernel will have that memory at the same physical address as the old kernel.

debugfs Interfaces

These debugfs interfaces are available when the kernel is compiled with CONFIG_KEXEC_HANDOVER_DEBUGFS enabled.

Currently KHO creates the following debugfs interfaces. Notice that these interfaces may change in the future. They will be moved to sysfs once KHO is stabilized.

/sys/kernel/debug/kho/out/fdt

The kernel exposes the flattened device tree blob that carries its current KHO state in this file. Kexec user space tooling can use this as input file for the KHO payload image.

/sys/kernel/debug/kho/out/scratch_len

Lengths of KHO scratch regions, which are physically contiguous memory regions that will always stay available for future kexec allocations. Kexec user space tools can use this file to determine where it should place its payload images.

/sys/kernel/debug/kho/out/scratch_phys

Physical locations of KHO scratch regions. Kexec user space tools can use this file in conjunction to scratch_phys to determine where it should place its payload images.

/sys/kernel/debug/kho/out/sub_fdts/

KHO producers can register their own FDT or another binary blob under this directory.

/sys/kernel/debug/kho/in/fdt

When the kernel was booted with Kexec HandOver (KHO), the state tree that carries metadata about the previous kernel’s state is in this file in the format of flattened device tree. This file may disappear when all consumers of it finished to interpret their metadata.

/sys/kernel/debug/kho/in/sub_fdts/

Similar to kho/out/sub_fdts/, but contains sub FDT blobs of KHO producers passed from the old kernel.