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CXL

This page documents a CXL memory configuration workflow for RamRyder. To let software manage CXL-backed memory at the memory-channel or device level, the CXL memory exposed to Linux should be brought into a DAX-usable form that can be mapped and managed explicitly by software.

Goal​

The target setup is:

  • identify the current CXL memory mode on the host
  • make sure the device is exposed in a DAX-usable form
  • allow software to manage the device explicitly instead of treating it only as normal system RAM

This setup is intended for systems where memory placement and allocation are managed by software rather than by default kernel NUMA policies alone.

Background​

On Linux, CXL memory is commonly exposed in one of two modes:

  • Device DAX mode: the CXL memory appears as a DAX device such as /dev/dax0.0, which can be accessed directly by software through mmap.
  • System RAM mode: the CXL memory is attached to one or more NUMA nodes and behaves like regular system memory from the OS point of view.

For RamRyder-style software-defined memory management, Device DAX mode is often the more direct representation because the memory can be managed as an explicit device instead of being merged into general-purpose system RAM.

For detailed CXL provisioning, inspection, and DAX conversion steps, we recommend following the CXL Memory Resource Kit (cxl-reskit).

In particular, the repository provides practical guidance for:

  • examining the current CXL memory configuration
  • understanding whether the device is currently in Device DAX mode or System RAM mode
  • converting between Device DAX and System RAM modes with daxctl
  • validating the resulting DAX device layout

Relevant sections in the repository include:

Guidance for RamRyder Users​

When preparing CXL memory for RamRyder software, first verify how Linux currently enumerates the device. If the memory is already exposed as a DAX device, it can be integrated more directly into the software-managed workflow. If it appears only as system RAM, follow the cxl-reskit guidance to determine whether it can be reconfigured into Device DAX mode on your platform.

Because BIOS behavior, firmware support, kernel support, and daxctl capabilities vary across platforms, we intentionally do not duplicate the full step-by-step setup flow here. Please follow the upstream cxl-reskit documentation for the exact commands and validation procedure.