1.. SPDX-License-Identifier: GPL-2.0-or-later 2 3==================== 4Kexec Handover Usage 5==================== 6 7Kexec HandOver (KHO) is a mechanism that allows Linux to preserve memory 8regions, which could contain serialized system states, across kexec. 9 10This document expects that you are familiar with the base KHO 11:ref:`concepts <kho-concepts>`. If you have not read 12them yet, please do so now. 13 14Prerequisites 15============= 16 17KHO is available when the kernel is compiled with ``CONFIG_KEXEC_HANDOVER`` 18set to y. Every KHO producer may have its own config option that you 19need to enable if you would like to preserve their respective state across 20kexec. 21 22To use KHO, please boot the kernel with the ``kho=on`` command line 23parameter. You may use ``kho_scratch`` parameter to define size of the 24scratch regions. For example ``kho_scratch=16M,512M,256M`` will reserve a 2516 MiB low memory scratch area, a 512 MiB global scratch region, and 256 MiB 26per NUMA node scratch regions on boot. 27 28Perform a KHO kexec 29=================== 30 31To perform a KHO kexec, load the target payload and kexec into it. It 32is important that you use the ``-s`` parameter to use the in-kernel 33kexec file loader, as user space kexec tooling currently has no 34support for KHO with the user space based file loader :: 35 36 # kexec -l /path/to/bzImage --initrd /path/to/initrd -s 37 # kexec -e 38 39The new kernel will boot up and contain some of the previous kernel's state. 40 41For example, if you used ``reserve_mem`` command line parameter to create 42an early memory reservation, the new kernel will have that memory at the 43same physical address as the old kernel. 44 45Kexec Metadata 46============== 47 48KHO automatically tracks metadata about the kexec chain, passing information 49about the previous kernel to the next kernel. This feature helps diagnose 50bugs that only reproduce when kexecing from specific kernel versions. 51 52On each KHO kexec, the kernel logs the previous kernel's version and the 53number of kexec reboots since the last cold boot:: 54 55 [ 0.000000] KHO: exec from: 6.19.0-rc4-next-20260107 (count 1) 56 57The metadata includes: 58 59``previous_release`` 60 The kernel version string (from ``uname -r``) of the kernel that 61 initiated the kexec. 62 63``kexec_count`` 64 The number of kexec boots since the last cold boot. On cold boot, 65 this counter starts at 0 and increments with each kexec. This helps 66 identify issues that only manifest after multiple consecutive kexec 67 reboots. 68 69Use Cases 70--------- 71 72This metadata is particularly useful for debugging kexec transition bugs, 73where a buggy kernel kexecs into a new kernel and the bug manifests only 74in the second kernel. Examples of such bugs include: 75 76- Memory corruption from the previous kernel affecting the new kernel 77- Incorrect hardware state left by the previous kernel 78- Firmware/ACPI state issues that only appear in kexec scenarios 79 80At scale, correlating crashes to the previous kernel version enables 81faster root cause analysis when issues only occur in specific kernel 82transition scenarios. 83 84debugfs Interfaces 85================== 86 87These debugfs interfaces are available when the kernel is compiled with 88``CONFIG_KEXEC_HANDOVER_DEBUGFS`` enabled. 89 90Currently KHO creates the following debugfs interfaces. Notice that these 91interfaces may change in the future. They will be moved to sysfs once KHO is 92stabilized. 93 94``/sys/kernel/debug/kho/out/fdt`` 95 The kernel exposes the flattened device tree blob that carries its 96 current KHO state in this file. Kexec user space tooling can use this 97 as input file for the KHO payload image. 98 99``/sys/kernel/debug/kho/out/scratch_len`` 100 Lengths of KHO scratch regions, which are physically contiguous 101 memory regions that will always stay available for future kexec 102 allocations. Kexec user space tools can use this file to determine 103 where it should place its payload images. 104 105``/sys/kernel/debug/kho/out/scratch_phys`` 106 Physical locations of KHO scratch regions. Kexec user space tools 107 can use this file in conjunction to scratch_phys to determine where 108 it should place its payload images. 109 110``/sys/kernel/debug/kho/out/sub_fdts/`` 111 KHO producers can register their own FDT or another binary blob under 112 this directory. 113 114``/sys/kernel/debug/kho/in/fdt`` 115 When the kernel was booted with Kexec HandOver (KHO), 116 the state tree that carries metadata about the previous 117 kernel's state is in this file in the format of flattened 118 device tree. This file may disappear when all consumers of 119 it finished to interpret their metadata. 120 121``/sys/kernel/debug/kho/in/sub_fdts/`` 122 Similar to ``kho/out/sub_fdts/``, but contains sub blobs 123 of KHO producers passed from the old kernel. 124