cxl: docs/linux/memory-hotplug

Add documentation on how the CXL driver surfaces memory through the
DAX driver and memory-hotplug.

Signed-off-by: Gregory Price <gourry@gourry.net>
Link: https://patch.msgid.link/20250512162134.3596150-13-gourry@gourry.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Documentation/driver-api/cxl/linux/memory-hotplug.rst

@@ -0,0 +1,78 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============
+Memory Hotplug
+==============
+The final phase of surfacing CXL memory to the kernel page allocator is for
+the `DAX` driver to surface a `Driver Managed` memory region via the
+memory-hotplug component.
+
+There are four major configurations to consider:
+
+1) Default Online Behavior (on/off and zone)
+2) Hotplug Memory Block size
+3) Memory Map Resource location
+4) Driver-Managed Memory Designation
+
+Default Online Behavior
+=======================
+The default-online behavior of hotplug memory is dictated by the following,
+in order of precedence:
+
+- :code:`CONFIG_MHP_DEFAULT_ONLINE_TYPE` Build Configuration
+- :code:`memhp_default_state` Boot parameter
+- :code:`/sys/devices/system/memory/auto_online_blocks` value
+
+These dictate whether hotplugged memory blocks arrive in one of three states:
+
+1) Offline
+2) Online in :code:`ZONE_NORMAL`
+3) Online in :code:`ZONE_MOVABLE`
+
+:code:`ZONE_NORMAL` implies this capacity may be used for almost any allocation,
+while :code:`ZONE_MOVABLE` implies this capacity should only be used for
+migratable allocations.
+
+:code:`ZONE_MOVABLE` attempts to retain the hotplug-ability of a memory block
+so that it the entire region may be hot-unplugged at a later time.  Any capacity
+onlined into :code:`ZONE_NORMAL` should be considered permanently attached to
+the page allocator.
+
+Hotplug Memory Block Size
+=========================
+By default, on most architectures, the Hotplug Memory Block Size is either
+128MB or 256MB.  On x86, the block size increases up to 2GB as total memory
+capacity exceeds 64GB.  As of v6.15, Linux does not take into account the
+size and alignment of the ACPI CEDT CFMWS regions (see Early Boot docs) when
+deciding the Hotplug Memory Block Size.
+
+Memory Map
+==========
+The location of :code:`struct folio` allocations to represent the hotplugged
+memory capacity are dictated by the following system settings:
+
+- :code:`/sys_module/memory_hotplug/parameters/memmap_on_memory`
+- :code:`/sys/bus/dax/devices/daxN.Y/memmap_on_memory`
+
+If both of these parameters are set to true, :code:`struct folio` for this
+capacity will be carved out of the memory block being onlined.  This has
+performance implications if the memory is particularly high-latency and
+its :code:`struct folio` becomes hotly contended.
+
+If either parameter is set to false, :code:`struct folio` for this capacity
+will be allocated from the local node of the processor running the hotplug
+procedure.  This capacity will be allocated from :code:`ZONE_NORMAL` on
+that node, as it is a :code:`GFP_KERNEL` allocation.
+
+Systems with extremely large amounts of :code:`ZONE_MOVABLE` memory (e.g.
+CXL memory pools) must ensure that there is sufficient local
+:code:`ZONE_NORMAL` capacity to host the memory map for the hotplugged capacity.
+
+Driver Managed Memory
+=====================
+The DAX driver surfaces this memory to memory-hotplug as "Driver Managed". This
+is not a configurable setting, but it's important to note that driver managed
+memory is explicitly excluded from use during kexec.  This is required to ensure
+any reset or out-of-band operations that the CXL device may be subject to during
+a functional system-reboot (such as a reset-on-probe) will not cause portions of
+the kexec kernel to be overwritten.