Message-ID: <20251218170747.1278327-1-gourry@gourry.net>
Date: Thu, 18 Dec 2025 12:07:47 -0500
From: Gregory Price <gourry@...rry.net>
To: linux-cxl@...r.kernel.org
Cc: linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
	dave@...olabs.net,
	jonathan.cameron@...wei.com,
	dave.jiang@...el.com,
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	vishal.l.verma@...el.com,
	ira.weiny@...el.com,
	dan.j.williams@...el.com,
	corbet@....net,
	gourry@...rry.net
Subject: [PATCH v2] Documentation/driver-api/cxl: device hotplug section

Describe cxl memory device hotplug implications, in particular how the
platform CEDT CFMWS must be described to support successful hot-add of
memory devices.

Signed-off-by: Gregory Price <gourry@...rry.net>
---
v2: Jonathan's clarifications and diagrams.

 Documentation/driver-api/cxl/index.rst        |   1 +
 .../cxl/platform/device-hotplug.rst           | 112 ++++++++++++++++++
 2 files changed, 113 insertions(+)
 create mode 100644 Documentation/driver-api/cxl/platform/device-hotplug.rst

diff --git a/Documentation/driver-api/cxl/index.rst b/Documentation/driver-api/cxl/index.rst
index c1106a68b67c..5a734988a5af 100644
--- a/Documentation/driver-api/cxl/index.rst
+++ b/Documentation/driver-api/cxl/index.rst
@@ -30,6 +30,7 @@ that have impacts on each other.  The docs here break up configurations steps.
    platform/acpi
    platform/cdat
    platform/example-configs
+   platform/device-hotplug
 
 .. toctree::
    :maxdepth: 2
diff --git a/Documentation/driver-api/cxl/platform/device-hotplug.rst b/Documentation/driver-api/cxl/platform/device-hotplug.rst
new file mode 100644
index 000000000000..617e340bd556
--- /dev/null
+++ b/Documentation/driver-api/cxl/platform/device-hotplug.rst
@@ -0,0 +1,112 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================
+CXL Device Hotplug
+==================
+
+Device hotplug refers to *physical* hotplug of a device (addition or removal
+of a physical device from the machine).
+
+Hot-Remove
+==========
+Hot removal of a device typically requires careful removal of software
+constructs (memory regions, associated drivers) which manage these devices.
+
+Hard-removing a CXL.mem device without carefully tearing down driver stacks
+is likely to cause the system to machine-check (or at least SIGBUS if memory
+access is limited to user space).
+
+Memory Device Hot-Add
+=====================
+A device present at boot will be associated with a CXL Fixed Memory Window
+reported in :doc:`CEDT<acpi/cedt>`.  That CFMWS may match the size of the
+device, but the construction of the CEDT CFMWS is platform-defined.
+
+Hot-adding a memory device requires this pre-defined (*static*) CFMWS has
+sufficient space to describe that device.
+
+There are a few common scenarios to consider.
+
+Single-Endpoint Memory Device Present at Boot
+---------------------------------------------
+A device present at boot likely had its capacity reported in the
+:doc:`CEDT<acpi/cedt>`.  If a device is removed and a new device hotplugged,
+the capacity of the new device will be limited to the original CFMWS capacity.
+
+Adding capacity larger than the original device will cause memory region
+creation to fail if the region size is greater than the CFMWS size.
+
+The CFMWS is *static* and cannot be adjusted.  Platforms which may expect
+different sized devices to be hotplugged must allocate sufficient CFMWS space
+*at boot time* to cover all future expected devices.
+
+Multi-Endpoint Memory Device Present at Boot
+--------------------------------------------
+A hot-plug capable CXL memory device, such as one which presents multiple
+expanders as a single large-capacity device, should report the maximum
+*possible* capacity for the device at boot. ::
+
+                  HB0
+                  RP0
+                   |
+     [Multi-Endpoint Memory Device]
+              _____|_____
+             |          |
+        [Endpoint0]   [Empty]
+
+
+Limiting the size to the capacity preset at boot will limit hot-add support
+to replacing capacity that was present at boot.
+
+No CXL Device Present at Boot
+-----------------------------
+When no CXL memory device is present on boot, some platforms omit the CFMWS
+in the :doc:`CEDT<acpi/cedt>`.  When this occurs, hot-add is not possible.
+
+For a platform to support hot-add of a full memory device, it must allocate
+a CEDT CFMWS region with sufficient memory capacity to cover all future
+potentially added capacity.
+
+To support memory hotplug directly on the host bridge, or on a switch
+downstream of the host bridge (but not contained within a CXL memory device),
+a platform must construct a CEDT CFMWS at boot with sufficient resources to
+support the max possible (or expected) hotplug memory capacity. ::
+
+         HB0                 HB1
+      RP0    RP1             RP2
+       |      |               |
+     Empty  Empty            USP
+                      ________|________
+                      |    |    |     |
+                     DSP  DSP  DSP   DSP
+                      |    |    |    |
+                         All  Empty
+
+For example, a BIOS/EFI may expose an option to configure a CEDT CFMWS with
+a pre-configured amount of memory capacity (per host bridge, or host bridge
+interleave set), even if no device is attached to Root Ports or Downstream
+Ports at boot (as depicted in the figure above).
+
+
+Interleave Sets
+===============
+
+Host Bridge Interleave
+----------------------
+Host-bridge interleaved memory regions are defined *statically* in the
+:doc:`CEDT<acpi/cedt>`.  To apply cross-host-bridge interleave, a CFMWS entry
+describing that interleave must have been provided *at boot*.  Hotplugged
+devices cannot add host-bridge interleave capabilities at hotplug time.
+
+See the :doc:`Flexible CEDT Configuration<example-configurations/flexible>`
+example to see how a platform can provide this kind of flexibility regarding
+hotplugged memory devices.  BIOS/EFI software should consider options to
+present flexible CEDT configurations with hotplug support.
+
+HDM Interleave
+--------------
+Decoder-applied interleave can flexibly handle hotplugged devices, as decoders
+can be re-programmed after hotplug.
+
+To add or remove a device to/from an existing HDM-applied interleaved region,
+that region must be torn down an re-created.
-- 
2.52.0

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