diff options
Diffstat (limited to 'Documentation/core-api/kho/concepts.rst')
| -rw-r--r-- | Documentation/core-api/kho/concepts.rst | 74 |
1 files changed, 0 insertions, 74 deletions
diff --git a/Documentation/core-api/kho/concepts.rst b/Documentation/core-api/kho/concepts.rst deleted file mode 100644 index d626d1dbd678..000000000000 --- a/Documentation/core-api/kho/concepts.rst +++ /dev/null @@ -1,74 +0,0 @@ -.. SPDX-License-Identifier: GPL-2.0-or-later -.. _kho-concepts: - -======================= -Kexec Handover Concepts -======================= - -Kexec HandOver (KHO) is a mechanism that allows Linux to preserve memory -regions, which could contain serialized system states, across kexec. - -It introduces multiple concepts: - -KHO FDT -======= - -Every KHO kexec carries a KHO specific flattened device tree (FDT) blob -that describes preserved memory regions. These regions contain either -serialized subsystem states, or in-memory data that shall not be touched -across kexec. After KHO, subsystems can retrieve and restore preserved -memory regions from KHO FDT. - -KHO only uses the FDT container format and libfdt library, but does not -adhere to the same property semantics that normal device trees do: Properties -are passed in native endianness and standardized properties like ``regs`` and -``ranges`` do not exist, hence there are no ``#...-cells`` properties. - -KHO is still under development. The FDT schema is unstable and would change -in the future. - -Scratch Regions -=============== - -To boot into kexec, we need to have a physically contiguous memory range that -contains no handed over memory. Kexec then places the target kernel and initrd -into that region. The new kernel exclusively uses this region for memory -allocations before during boot up to the initialization of the page allocator. - -We guarantee that we always have such regions through the scratch regions: On -first boot KHO allocates several physically contiguous memory regions. Since -after kexec these regions will be used by early memory allocations, there is a -scratch region per NUMA node plus a scratch region to satisfy allocations -requests that do not require particular NUMA node assignment. -By default, size of the scratch region is calculated based on amount of memory -allocated during boot. The ``kho_scratch`` kernel command line option may be -used to explicitly define size of the scratch regions. -The scratch regions are declared as CMA when page allocator is initialized so -that their memory can be used during system lifetime. CMA gives us the -guarantee that no handover pages land in that region, because handover pages -must be at a static physical memory location and CMA enforces that only -movable pages can be located inside. - -After KHO kexec, we ignore the ``kho_scratch`` kernel command line option and -instead reuse the exact same region that was originally allocated. This allows -us to recursively execute any amount of KHO kexecs. Because we used this region -for boot memory allocations and as target memory for kexec blobs, some parts -of that memory region may be reserved. These reservations are irrelevant for -the next KHO, because kexec can overwrite even the original kernel. - -.. _kho-finalization-phase: - -KHO finalization phase -====================== - -To enable user space based kexec file loader, the kernel needs to be able to -provide the FDT that describes the current kernel's state before -performing the actual kexec. The process of generating that FDT is -called serialization. When the FDT is generated, some properties -of the system may become immutable because they are already written down -in the FDT. That state is called the KHO finalization phase. - -Public API -========== -.. kernel-doc:: kernel/liveupdate/kexec_handover.c - :export: |