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https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
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Pasha TatashinandMike Rapoport (Microsoft)
0349ff2887
Introduce a linked-block serialization mechanism for state handover. Previously, LUO used contiguous memory blocks for serializing sessions and files, which imposed limits on the total number of items that could be preserved across a live update. This commit adds the infrastructure for a more flexible, block-based approach where serialized data is stored in a chain of linked blocks. This is a generic KHO serialization block infrastructure that can be used by multiple subsystems. Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com> Link: https://patch.msgid.link/20260603154402.468928-8-pasha.tatashin@soleen.com Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
101 lines
3.5 KiB
ReStructuredText
101 lines
3.5 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0-or-later
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.. _kho-concepts:
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========================
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Kexec Handover Subsystem
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========================
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Overview
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========
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Kexec HandOver (KHO) is a mechanism that allows Linux to preserve memory
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regions, which could contain serialized system states, across kexec.
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KHO uses :ref:`flattened device tree (FDT) <kho_fdt>` to pass information about
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the preserved state from pre-exec kernel to post-kexec kernel and :ref:`scratch
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memory regions <kho_scratch>` to ensure integrity of the preserved memory.
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.. _kho_fdt:
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KHO FDT
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=======
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Every KHO kexec carries a KHO specific flattened device tree (FDT) blob that
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describes the preserved state. The FDT includes properties describing preserved
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memory regions and nodes that hold subsystem specific state.
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The preserved memory regions contain either serialized subsystem states, or
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in-memory data that shall not be touched across kexec. After KHO, subsystems
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can retrieve and restore the preserved state from KHO FDT.
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Subsystems participating in KHO can define their own format for state
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serialization and preservation.
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KHO FDT and structures defined by the subsystems form an ABI between pre-kexec
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and post-kexec kernels. This ABI is defined by header files in
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``include/linux/kho/abi`` directory.
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.. toctree::
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:maxdepth: 1
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abi.rst
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.. _kho_scratch:
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Scratch Regions
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===============
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To boot into kexec, we need to have a physically contiguous memory range that
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contains no handed over memory. Kexec then places the target kernel and initrd
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into that region. The new kernel exclusively uses this region for memory
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allocations before during boot up to the initialization of the page allocator.
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We guarantee that we always have such regions through the scratch regions: On
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first boot KHO allocates several physically contiguous memory regions. Since
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after kexec these regions will be used by early memory allocations, there is a
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scratch region per NUMA node plus a scratch region to satisfy allocations
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requests that do not require particular NUMA node assignment.
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By default, size of the scratch region is calculated based on amount of memory
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allocated during boot. The ``kho_scratch`` kernel command line option may be
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used to explicitly define size of the scratch regions.
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The scratch regions are declared as CMA when page allocator is initialized so
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that their memory can be used during system lifetime. CMA gives us the
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guarantee that no handover pages land in that region, because handover pages
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must be at a static physical memory location and CMA enforces that only
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movable pages can be located inside.
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After KHO kexec, we ignore the ``kho_scratch`` kernel command line option and
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instead reuse the exact same region that was originally allocated. This allows
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us to recursively execute any amount of KHO kexecs. Because we used this region
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for boot memory allocations and as target memory for kexec blobs, some parts
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of that memory region may be reserved. These reservations are irrelevant for
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the next KHO, because kexec can overwrite even the original kernel.
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Kexec Handover Radix Tree
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=========================
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.. kernel-doc:: include/linux/kho_radix_tree.h
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:doc: Kexec Handover Radix Tree
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Public API
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==========
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.. kernel-doc:: kernel/liveupdate/kexec_handover.c
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:export:
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KHO Serialization Blocks API
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============================
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.. kernel-doc:: kernel/liveupdate/kho_block.c
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:doc: KHO Serialization Blocks
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.. kernel-doc:: include/linux/kho_block.h
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.. kernel-doc:: kernel/liveupdate/kho_block.c
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:internal:
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See Also
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========
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- :doc:`/admin-guide/mm/kho`
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