[v2,07/17] kexec: Add documentation for KHO

diff --git a/Documentation/kho/concepts.rst b/Documentation/kho/concepts.rst
new file mode 100644
index 000000000000..8e4fe8c57865
--- /dev/null
+++ b/Documentation/kho/concepts.rst
@@ -0,0 +1,88 @@ 
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+=======================
+Kexec Handover Concepts
+=======================
+
+Kexec HandOver (KHO) is a mechanism that allows Linux to preserve state -
+arbitrary properties as well as memory locations - across kexec.
+
+It introduces multiple concepts:
+
+KHO Device Tree
+---------------
+
+Every KHO kexec carries a KHO specific flattened device tree blob that
+describes the state of the system. Device drivers can register to KHO to
+serialize their state before kexec. After KHO, device drivers can read
+the device tree and extract previous state.
+
+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 introduces a new concept to its device tree: ``mem`` properties. A
+``mem`` property can inside any subnode in the device tree. When present,
+it contains an array of physical memory ranges that the new kernel must mark
+as reserved on boot. It is recommended, but not required, to make these ranges
+as physically contiguous as possible to reduce the number of array elements ::
+
+    struct kho_mem {
+            __u64 addr;
+            __u64 len;
+    };
+
+After boot, drivers can call the kho subsystem to transfer ownership of memory
+that was reserved via a ``mem`` property to themselves to continue using memory
+from the previous execution.
+
+The KHO device tree follows the in-Linux schema requirements. Any element in
+the device tree is documented via device tree schema yamls that explain what
+data gets transferred.
+
+Mem cache
+---------
+
+The new kernel needs to know about all memory reservations, but is unable to
+parse the device tree yet in early bootup code because of memory limitations.
+To simplify the initial memory reservation flow, the old kernel passes a
+preprocessed array of physically contiguous reserved ranges to the new kernel.
+
+These reservations have to be separate from architectural memory maps and
+reservations because they differ on every kexec, while the architectural ones
+get passed directly between invocations.
+
+The less entries this cache contains, the faster the new kernel will boot.
+
+Scratch Region
+--------------
+
+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 it ingests the mem cache.
+
+We guarantee that we always have such a region through the scratch region: On
+first boot, you can pass the ``kho_scratch`` kernel command line option. When
+it is set, Linux allocates a CMA region of the given size. 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 irrenevant for
+the next KHO, because kexec can overwrite even the original kernel.
+
+KHO active phase
+----------------
+
+To enable user space based kexec file loader, the kernel needs to be able to
+provide the device tree that describes the previous kernel's state before
+performing the actual kexec. The process of generating that device tree is
+called serialization. When the device tree is generated, some properties
+of the system may become immutable because they are already written down
+in the device tree. That state is called the KHO active phase.
diff --git a/Documentation/kho/index.rst b/Documentation/kho/index.rst
new file mode 100644
index 000000000000..5e7eeeca8520
--- /dev/null
+++ b/Documentation/kho/index.rst
@@ -0,0 +1,19 @@ 
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+========================
+Kexec Handover Subsystem
+========================
+
+.. toctree::
+   :maxdepth: 1
+
+   concepts
+   usage
+
+.. only::  subproject and html
+
+
+   Indices
+   =======
+
+   * :ref:`genindex`
diff --git a/Documentation/kho/usage.rst b/Documentation/kho/usage.rst
new file mode 100644
index 000000000000..5efa2a58f9c3
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+++ b/Documentation/kho/usage.rst
@@ -0,0 +1,57 @@ 
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+====================
+Kexec Handover Usage
+====================
+
+Kexec HandOver (KHO) is a mechanism that allows Linux to preserve state -
+arbitrary properties as well as memory locations - across kexec.
+
+This document expects that you are familiar with the base KHO
+:ref:`Documentation/kho/concepts.rst <concepts>`. If you have not read
+them yet, please do so now.
+
+Prerequisites
+-------------
+
+KHO is available when the ``CONFIG_KEXEC_KHO`` config option is set to y
+at compile team. Every KHO producer has 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_scratch`` command
+line parameter set to allocate a scratch region. For example
+``kho_scratch=512M`` will reserve a 512 MiB scratch region on boot.
+
+Perform a KHO kexec
+-------------------
+
+Before you can perform a KHO kexec, you need to move the system into the
+:ref:`Documentation/kho/concepts.rst <KHO active phase>` ::
+
+  $ echo 1 > /sys/kernel/kho/active
+
+After this command, the KHO device tree is available in ``/sys/kernel/kho/dt``.
+
+Next, 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 Image --initrd=initrd -s
+  # kexec -e
+
+The new kernel will boot up and contain some of the previous kernel's state.
+
+For example, if you enabled ``CONFIG_FTRACE_KHO``, the new kernel will contain
+the old kernel's trace buffers in ``/sys/kernel/debug/tracing/trace``.
+
+Abort a KHO exec
+----------------
+
+You can move the system out of KHO active phase again by calling ::
+
+  $ echo 1 > /sys/kernel/kho/active
+
+After this command, the KHO device tree is no longer available in
+``/sys/kernel/kho/dt``.
diff --git a/Documentation/subsystem-apis.rst b/Documentation/subsystem-apis.rst
index 930dc23998a0..8207b6514d87 100644
--- a/Documentation/subsystem-apis.rst
+++ b/Documentation/subsystem-apis.rst
@@ -86,3 +86,4 @@  Storage interfaces
    misc-devices/index
    peci/index
    wmi/index
+   kho/index