Re: [PATCH v2] Documentation/driver-api/cxl: device hotplug section

[Alejandro Lucero Palau <alucerop@amd.com>]

Hi Gregory,


Thank you for this addition to documentation.

Just a comment below about something we talked about at LPC.


On 12/18/25 17:07, Gregory Price wrote:
> 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@gourry.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).


All this is fine, but my concern is what the BIOS will do when a device 
ends up being hotplugged. Assuming that programmability from the 
Host/user space for creating regions on demand based on requirements 
like bandwidth, and therefore relying on interleaving and granularity (I 
know this is not trivial but I think this is possible in the near 
future, or at least possible to be demanded) then the BIOS should not do 
any HDM programming at all ...


I think it is worth to document this somehow and maybe to discuss this 
with BIOSes vendors if we consider this convenient.


Thank you,

Alejandro


> +
> +
> +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.