The Compute Express Link (CXL) subsystem changes were merged today for the Linux 7.4 kernel. The fun work this cycle is preparing to support Dynamic Capacity Devices.

In CXL land, Dynamic Capacity Devices (DCD) are Type-3 CXL memory expansion hardware that allows the server memory capacity to scale either up or down dynamically without needing to reboot the hardware or physically reconfigure the hardware. With CXL DCD, it's possible to reduce idle/over-provisioned memory by pooling resourcces across a rack with multi-host memory sharing and dynamically bursting memory capacity on demand depending upon the workload.

Dynamic Capacity Device support is explained in more detail via the ComputeExpressLink.org blog.

For Linux 7.4 there is prep changes for the CXL infrastructure in the Linux kernel but for this merge window isn't yet all upstream with the rest of the code still undergoing review:

"The headline item is the first part support for Dynamic Capacity Devices (DCD). It covers flagging DCD support in the mailbox, reading the dynamic capacity configuration from the device, gathering DSMAS data for the DCD partitions, and configuring the dynamic capacity interrupts. It also adds a documentation update and a cleanup that enforces partition ordering. DCD is not enabled yet with these code changes. Patches are applied to reduce the whole set in order to relief the burden of very large patch set review.

Zero sized HDM decoders are now allowed, along with cxl_test support for them. There is also a set of decoder hardening patches for switch decoder enumeration. Latency calculations now account for link width and use consistent units. The GPF timeout programming has been reworked. The pull also has a number of misc patches."

More details on the merged CXL changes for Linux 7.4 via this pull request.