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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..e4a065fdd3ec --- /dev/null +++ b/Documentation/driver-api/cxl/platform/device-hotplug.rst @@ -0,0 +1,130 @@ +.. 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). + +BIOS/EFI software is expected to configure sufficient resources **at boot +time** to allow hotplugged devices to be configured by software (such as +proximity domains, HPA regions, and host-bridge configurations). + +BIOS/EFI is not expected (**nor suggested**) to configure hotplugged +devices at hotplug time (i.e. HDM decoders should be left unprogrammed). + +This document covers some examples of those resources, but should not +be considered exhaustive. + +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 may 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 to +have sufficient HPA 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 +-------------------------------------------- +Non-switch-based Multi-Endpoint devices are outside the scope of what the +CXL specification describes, but they are technically possible. We describe +them here for instructive reasons only - this does not imply Linux support. + +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. + +This describes the base case for any given device not being present at boot. +If a future possible device is not described in the CEDT at boot, hot-add +of that device is either limited or 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 (along with any relevant CEDT CHBS entry). + +To support memory hotplug directly on the host bridge/root port, or on a switch +downstream of the host bridge, 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. |