cxl: docs/linux - early boot configuration

Document __init time configurations that affect CXL driver probe
process and memory region configuration.

Signed-off-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20250512162134.3596150-9-gourry@gourry.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Documentation/driver-api/cxl/linux/early-boot.rst

@@ -0,0 +1,131 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=======================
+Linux Init (Early Boot)
+=======================
+
+Linux configuration is split into two major steps: Early-Boot and everything else.
+
+During early boot, Linux sets up immutable resources (such as numa nodes), while
+later operations include things like driver probe and memory hotplug.  Linux may
+read EFI and ACPI information throughout this process to configure logical
+representations of the devices.
+
+During Linux Early Boot stage (functions in the kernel that have the __init
+decorator), the system takes the resources created by EFI/BIOS (ACPI tables)
+and turns them into resources that the kernel can consume.
+
+
+BIOS, Build and Boot Options
+============================
+
+There are 4 pre-boot options that need to be considered during kernel build
+which dictate how memory will be managed by Linux during early boot.
+
+* EFI_MEMORY_SP
+
+  * BIOS/EFI Option that dictates whether memory is SystemRAM or
+    Specific Purpose.  Specific Purpose memory will be deferred to
+    drivers to manage - and not immediately exposed as system RAM.
+
+* CONFIG_EFI_SOFT_RESERVE
+
+  * Linux Build config option that dictates whether the kernel supports
+    Specific Purpose memory.
+
+* CONFIG_MHP_DEFAULT_ONLINE_TYPE
+
+  * Linux Build config that dictates whether and how Specific Purpose memory
+    converted to a dax device should be managed (left as DAX or onlined as
+    SystemRAM in ZONE_NORMAL or ZONE_MOVABLE).
+
+* nosoftreserve
+
+  * Linux kernel boot option that dictates whether Soft Reserve should be
+    supported.  Similar to CONFIG_EFI_SOFT_RESERVE.
+
+Memory Map Creation
+===================
+
+While the kernel parses the EFI memory map, if :code:`Specific Purpose` memory
+is supported and detected, it will set this region aside as
+:code:`SOFT_RESERVED`.
+
+If :code:`EFI_MEMORY_SP=0`, :code:`CONFIG_EFI_SOFT_RESERVE=n`, or
+:code:`nosoftreserve=y` - Linux will default a CXL device memory region to
+SystemRAM.  This will expose the memory to the kernel page allocator in
+:code:`ZONE_NORMAL`, making it available for use for most allocations (including
+:code:`struct page` and page tables).
+
+If `Specific Purpose` is set and supported, :code:`CONFIG_MHP_DEFAULT_ONLINE_TYPE_*`
+dictates whether the memory is onlined by default (:code:`_OFFLINE` or
+:code:`_ONLINE_*`), and if online which zone to online this memory to by default
+(:code:`_NORMAL` or :code:`_MOVABLE`).
+
+If placed in :code:`ZONE_MOVABLE`, the memory will not be available for most
+kernel allocations (such as :code:`struct page` or page tables).  This may
+significant impact performance depending on the memory capacity of the system.
+
+
+NUMA Node Reservation
+=====================
+
+Linux refers to the proximity domains (:code:`PXM`) defined in the SRAT to
+create NUMA nodes in :code:`acpi_numa_init`. Typically, there is a 1:1 relation
+between :code:`PXM` and NUMA node IDs.
+
+SRAT is the only ACPI defined way of defining Proximity Domains. Linux chooses
+to, at most, map those 1:1 with NUMA nodes. CEDT adds a description of SPA
+ranges which Linux may wish to map to one or more NUMA nodes.
+
+If there are CXL ranges in the CFMWS but not in SRAT, then a fake :code:`PXM`
+is created (as of v6.15). In the future, Linux may reject CFMWS not described
+by SRAT due to the ambiguity of proximity domain association.
+
+It is important to note that NUMA node creation cannot be done at runtime. All
+possible NUMA nodes are identified at :code:`__init` time, more specifically
+during :code:`mm_init`. The CEDT and SRAT must contain sufficient :code:`PXM`
+data for Linux to identify NUMA nodes their associated memory regions.
+
+The relevant code exists in: :code:`linux/drivers/acpi/numa/srat.c`.
+
+See the Example Platform Configurations section for more information.
+
+Memory Tiers Creation
+=====================
+Memory tiers are a collection of NUMA nodes grouped by performance characteristics.
+During :code:`__init`, Linux initializes the system with a default memory tier that
+contains all nodes marked :code:`N_MEMORY`.
+
+:code:`memory_tier_init` is called at boot for all nodes with memory online by
+default. :code:`memory_tier_late_init` is called during late-init for nodes setup
+during driver configuration.
+
+Nodes are only marked :code:`N_MEMORY` if they have *online* memory.
+
+Tier membership can be inspected in ::
+
+  /sys/devices/virtual/memory_tiering/memory_tierN/nodelist
+  0-1
+
+If nodes are grouped which have clear difference in performance, check the HMAT
+and CDAT information for the CXL nodes.  All nodes default to the DRAM tier,
+unless HMAT/CDAT information is reported to the memory_tier component via
+`access_coordinates`.
+
+Contiguous Memory Allocation
+============================
+The contiguous memory allocator (CMA) enables reservation of contiguous memory
+regions on NUMA nodes during early boot.  However, CMA cannot reserve memory
+on NUMA nodes that are not online during early boot. ::
+
+  void __init hugetlb_cma_reserve(int order) {
+    if (!node_online(nid))
+      /* do not allow reservations */
+  }
+
+This means if users intend to defer management of CXL memory to the driver, CMA
+cannot be used to guarantee huge page allocations.  If enabling CXL memory as
+SystemRAM in `ZONE_NORMAL` during early boot, CMA reservations per-node can be
+made with the :code:`cma_pernuma` or :code:`numa_cma` kernel command line
+parameters.