[PATCH v3 04/17] cxl: docs/platform/bios-and-efi documentation

Gregory Price
2025-05-12T16:21:49Z

Add some docs on CXL configurations done in bios/efi that affect
linux configuration - information vendors may care to consider.

Signed-off-by: Gregory Price <gourry@gourry.net>
---
Documentation/driver-api/cxl/index.rst | 6 +
.../driver-api/cxl/platform/bios-and-efi.rst | 262 ++++++++++++++++++
2 files changed, 268 insertions(+)
create mode 100644 Documentation/driver-api/cxl/platform/bios-and-efi.rst

diff --git a/Documentation/driver-api/cxl/index.rst b/Documentation/driver-api/cxl/index.rst
index a2d1c5b18a8a..ffa0462ad950 100644
--- a/Documentation/driver-api/cxl/index.rst
+++ b/Documentation/driver-api/cxl/index.rst
@@ -21,6 +21,12 @@ that have impacts on each other. The docs here break up configurations steps.

devices/device-types

+.. toctree::
+ :maxdepth: 2
+ :caption: Platform Configuration
+
+ platform/bios-and-efi
+
.. toctree::
:maxdepth: 1
:caption: Linux Kernel Configuration
diff --git a/Documentation/driver-api/cxl/platform/bios-and-efi.rst b/Documentation/driver-api/cxl/platform/bios-and-efi.rst
new file mode 100644
index 000000000000..552a83992bcc
--- /dev/null
+++ b/Documentation/driver-api/cxl/platform/bios-and-efi.rst
@@ -0,0 +1,262 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======================
+BIOS/EFI Configuration
+======================
+
+BIOS and EFI are largely responsible for configuring static information about
+devices (or potential future devices) such that Linux can build the appropriate
+logical representations of these devices.
+
+At a high level, this is what occurs during this phase of configuration.
+
+* The bootloader starts the BIOS/EFI.
+
+* BIOS/EFI do early device probe to determine static configuration
+
+* BIOS/EFI creates ACPI Tables that describe static config for the OS
+
+* BIOS/EFI create the system memory map (EFI Memory Map, E820, etc)
+
+* BIOS/EFI calls :code:`start_kernel` and begins the Linux Early Boot process.
+
+Much of what this section is concerned with is ACPI Table production and
+static memory map configuration. More detail on these tables can be found
+under Platform Configuration -> ACPI Table Reference.
+
+.. note::
+ Platform Vendors should read carefully, as this sections has recommendations
+ on physical memory region size and alignment, memory holes, HDM interleave,
+ and what linux expects of HDM decoders trying to work with these features.
+
+UEFI Settings
+=============
+If your platform supports it, the :code:`uefisettings` command can be used to
+read/write EFI settings. Changes will be reflected on the next reboot. Kexec
+is not a sufficient reboot.
+
+One notable configuration here is the EFI_MEMORY_SP (Specific Purpose) bit.
+When this is enabled, this bit tells linux to defer management of a memory
+region to a driver (in this case, the CXL driver). Otherwise, the memory is
+treated as "normal memory", and is exposed to the page allocator during
+:code:`__init`.
+
+uefisettings examples
+---------------------
+
+:code:`uefisettings identify` ::
+
+ uefisettings identify
+
+ bios_vendor: xxx
+ bios_version: xxx
+ bios_release: xxx
+ bios_date: xxx
+ product_name: xxx
+ product_family: xxx
+ product_version: xxx
+
+On some AMD platforms, the :code:`EFI_MEMORY_SP` bit is set via the :code:`CXL
+Memory Attribute` field. This may be called something else on your platform.
+
+:code:`uefisettings get "CXL Memory Attribute"` ::
+
+ selector: xxx
+ ...
+ question: Question {
+ name: "CXL Memory Attribute",
+ answer: "Enabled",
+ ...
+ }
+
+Physical Memory Map
+===================
+
+Physical Address Region Alignment
+---------------------------------
+
+As of Linux v6.14, the hotplug memory system requires memory regions to be
+uniform in size and alignment. While the CXL specification allows for memory
+regions as small as 256MB, the supported memory block size and alignment for
+hotplugged memory is architecture-defined.
+
+A Linux memory blocks may be as small as 128MB and increase in powers of two.
+
+* On ARM, the default block size and alignment is either 128MB or 256MB.
+
+* On x86, the default block size is 256MB, and increases to 2GB as the
+ capacity of the system increases up to 64GB.
+
+For best support across versions, platform vendors should place CXL memory at
+a 2GB aligned base address, and regions should be 2GB aligned. This also helps
+prevent the creating thousands of memory devices (one per block).
+
+Memory Holes
+------------
+
+Holes in the memory map are tricky. Consider a 4GB device located at base
+address 0x100000000, but with the following memory map ::
+
+ ---------------------
+ | 0x100000000 |
+ | CXL |
+ | 0x1BFFFFFFF |
+ ---------------------
+ | 0x1C0000000 |
+ | MEMORY HOLE |
+ | 0x1FFFFFFFF |
+ ---------------------
+ | 0x200000000 |
+ | CXL CONT. |
+ | 0x23FFFFFFF |
+ ---------------------
+
+There are two issues to consider:
+
+* decoder programming, and
+* memory block alignment.
+
+If your architecture requires 2GB uniform size and aligned memory blocks, the
+only capacity Linux is capable of mapping (as of v6.14) would be the capacity
+from `0x100000000-0x180000000`. The remaining capacity will be stranded, as
+they are not of 2GB aligned length.
+
+Assuming your architecture and memory configuration allows 1GB memory blocks,
+this memory map is supported and this should be presented as multiple CFMWS
+in the CEDT that describe each side of the memory hole separately - along with
+matching decoders.
+
+Multiple decoders can (and should) be used to manage such a memory hole (see
+below), but each chunk of a memory hole should be aligned to a reasonable block
+size (larger alignment is always better). If you intend to have memory holes
+in the memory map, expect to use one decoder per contiguous chunk of host
+physical memory.
+
+As of v6.14, Linux does provide support for memory hotplug of multiple
+physical memory regions separated by a memory hole described by a single
+HDM decoder.
+
+
+Decoder Programming
+===================
+If BIOS/EFI intends to program the decoders to be statically configured,
+there are a few things to consider to avoid major pitfalls that will
+prevent Linux compatibility. Some of these recommendations are not
+required "per the specification", but Linux makes no guarantees of support
+otherwise.
+
+
+Translation Point
+-----------------
+Per the specification, the only decoders which **TRANSLATE** Host Physical
+Address (HPA) to Device Physical Address (DPA) are the **Endpoint Decoders**.
+All other decoders in the fabric are intended to route accesses without
+translating the addresses.
+
+This is heavily implied by the specification, see: ::
+
+ CXL Specification 3.1
+ 8.2.4.20: CXL HDM Decoder Capability Structure
+ - Implementation Note: CXL Host Bridge and Upstream Switch Port Decoder Flow
+ - Implementation Note: Device Decoder Logic
+
+Given this, Linux makes a strong assumption that decoders between CPU and
+endpoint will all be programmed with addresses ranges that are subsets of
+their parent decoder.
+
+Due to some ambiguity in how Architecture, ACPI, PCI, and CXL specifications
+"hand off" responsibility between domains, some early adopting platforms
+attempted to do translation at the originating memory controller or host
+bridge. This configuration requires a platform specific extension to the
+driver and is not officially endorsed - despite being supported.
+
+It is *highly recommended* **NOT** to do this; otherwise, you are on your own
+to implement driver support for your platform.
+
+Interleave and Configuration Flexibility
+----------------------------------------
+If providing cross-host-bridge interleave, a CFMWS entry in the CEDT must be
+presented with target host-bridges for the interleaved device sets (there may
+be multiple behind each host bridge).
+
+If providing intra-host-bridge interleaving, only 1 CFMWS entry in the CEDT is
+required for that host bridge - if it covers the entire capacity of the devices
+behind the host bridge.
+
+If intending to provide users flexibility in programming decoders beyond the
+root, you may want to provide multiple CFMWS entries in the CEDT intended for
+different purposes. For example, you may want to consider adding:
+
+1) A CFMWS entry to cover all interleavable host bridges.
+2) A CFMWS entry to cover all devices on a single host bridge.
+3) A CFMWS entry to cover each device.
+
+A platform may choose to add all of these, or change the mode based on a BIOS
+setting. For each CFMWS entry, Linux expects descriptions of the described
+memory regions in the SRAT to determine the number of NUMA nodes it should
+reserve during early boot / init.
+
+As of v6.14, Linux will create a NUMA node for each CEDT CFMWS entry, even if
+a matching SRAT entry does not exist; however, this is not guaranteed in the
+future and such a configuration should be avoided.
+
+Memory Holes
+------------
+If your platform includes memory holes intersparsed between your CXL memory, it
+is recommended to utilize multiple decoders to cover these regions of memory,
+rather than try to program the decoders to accept the entire range and expect
+Linux to manage the overlap.
+
+For example, consider the Memory Hole described above ::
+
+ ---------------------
+ | 0x100000000 |
+ | CXL |
+ | 0x1BFFFFFFF |
+ ---------------------
+ | 0x1C0000000 |
+ | MEMORY HOLE |
+ | 0x1FFFFFFFF |
+ ---------------------
+ | 0x200000000 |
+ | CXL CONT. |
+ | 0x23FFFFFFF |
+ ---------------------
+
+Assuming this is provided by a single device attached directly to a host bridge,
+Linux would expect the following decoder programming ::
+
+ ----------------------- -----------------------
+ | root-decoder-0 | | root-decoder-1 |
+ | base: 0x100000000 | | base: 0x200000000 |
+ | size: 0xC0000000 | | size: 0x40000000 |
+ ----------------------- -----------------------
+ | |
+ ----------------------- -----------------------
+ | HB-decoder-0 | | HB-decoder-1 |
+ | base: 0x100000000 | | base: 0x200000000 |
+ | size: 0xC0000000 | | size: 0x40000000 |
+ ----------------------- -----------------------
+ | |
+ ----------------------- -----------------------
+ | ep-decoder-0 | | ep-decoder-1 |
+ | base: 0x100000000 | | base: 0x200000000 |
+ | size: 0xC0000000 | | size: 0x40000000 |
+ ----------------------- -----------------------
+
+With a CEDT configuration with two CFMWS describing the above root decoders.
+
+Linux makes no guarantee of support for strange memory hole situations.
+
+Multi-Media Devices
+-------------------
+The CFMWS field of the CEDT has special restriction bits which describe whether
+the described memory region allows volatile or persistent memory (or both). If
+the platform intends to support either:
+
+1) A device with multiple medias, or
+2) Using a persistent memory device as normal memory
+
+A platform may wish to create multiple CEDT CFMWS entries to describe the same
+memory, with the intent of allowing the end user flexibility in how that memory
+is configured. Linux does not presently have strong requirements in this area.
--
2.49.0