Kernel

Linux Kernel 6.14 is Released: This is What's New for Compute Express Link (CXL)

The Linux Kernel 6.14 release brings several improvements and additions related to Compute Express Link (CXL) technology.

Here is the detailed list of all commits merged into the 6.14 Kernel for CXL and DAX. This list was generated by the Linux Kernel CXL Feature Tracker .

Linux Kernel 6.13 is Released: This is What's New for Compute Express Link (CXL)

The Linux Kernel 6.13 release brings several improvements and additions related to Compute Express Link (CXL) technology.

Here is the detailed list of all commits merged into the 6.13 Kernel for CXL and DAX. This list was generated by the Linux Kernel CXL Feature Tracker .

CXL related changes from Kernel v6.12 to v6.13:

Linux Kernel 6.10 is Released: This is What's New for Compute Express Link (CXL)

The Linux Kernel 6.10 release brings several improvements and additions related to Compute Express Link (CXL) technology.

Here is the detailed list of all commits merged into the 6.10 Kernel for CXL and DAX. This list was generated by the Linux Kernel CXL Feature Tracker .

Linux Kernel 6.9 is Released: This is What's New for Compute Express Link (CXL)

The Linux Kernel 6.9 release brings several improvements and additions related to Compute Express Link (CXL) technology.

New Features

Here is a list of new features for CXL:

Here is the detailed list of all commits merged into the 6.9 Kernel for CXL and DAX. This list was generated by the Linux Kernel CXL Feature Tracker .

Linux Kernel CXL Feature Tracker

I’m always watching the Linux Kernel for new and exciting features that are merged for Compute Express Link (CXL). There’s some great notes from the monthly developer meetup here , but the devil is always in the details, and not every commit is discussed in the meeting. So I wrote a simple Python script, called cxl_feature_tracker.py that looks in all commits to the Linus Torvalds Linux Kernel GitHub repository , and extracts any that mention “CXL” or “DAX”, or that make changes to the drivers/cxl or drivers/dax directories. The output is a very long list, but it has some gems amongst the list of fixes.

Using Linux Kernel Tiering with Compute Express Link (CXL) Memory

In this blog post, we will walk through the process of enabling the Linux Kernel Transparent Page Placement (TPP) feature with CXL memory mapped as NUMA nodes using the system-ram namespace. This feature allows the kernel to automatically place pages in different types of memory based on their usage patterns.

Prerequisites

This guide assumes that you are using a Fedora 36 system with Kernel 5.19.13, and that your system has a Samsung CXL device installed. You can confirm the presence of the CXL device with the following command:

Note: This article was updated on Thursday, July 31st, 2025 and will work with newer Ubuntu releases.

By default, Ubuntu systems run with the Ubuntu kernels provided by the Ubuntu repositories. To get unmodified upstream kernels that have new features or to confirm that upstream has fixed a specific issue, we often need to install the mainline Kernel. The mainline kernel is the most recent version of the Linux kernel released by the Linux Kernel Organization. It undergoes several stages of development, including merge windows, release candidates, and final releases. Mainline kernels are designed to offer the latest features and improvements, making them attractive to developers and power users. Kernel.org lists the available Kernel versions.

Using Linux Kernel Memory Tiering

In this post, I’ll discuss what memory tiering is, why we need it, and how to use the memory tiering feature available in the mainline v5.15 Kernel.

What is Memory Tiering?

With the advent of various new memory types, some systems will have multiple types of memory, e.g. High Bandwidth Memory (HBM), DRAM, Persistent Memory (PMem), CXL and others. The Memory Storage hierarchy should be familiar to you.

Memory Storage Hierarchy

How To Emulate CXL Devices using KVM and QEMU

What is CXL?

Compute Express Link (CXL) is an open standard for high-speed central processing unit-to-device and CPU-to-memory connections, designed for high-performance data center computers. CXL is built on the PCI Express physical and electrical interface with protocols in three areas: input/output, memory, and cache coherence.

CXL is designed to be an industry open standard interface for high-speed communications, as accelerators are increasingly used to complement CPUs in support of emerging applications such as Artificial Intelligence and Machine Learning.

How to Boot Linux from Intel® Optane™ Persistent Memory

How to , Linux

Introduction

In this article, I will demonstrate how to configure a system with Intel Optane Persistent Memory (PMem) and use part of the PMem as a boot device. This little known feature can reduce boot times for those that need it.

The basic steps include:

Configure the Persistent Memory in AppDirect Interleaved
Create two small SECTOR namespaces, one per Region
Install the OS and select one or both of the namespaces (single disk install, or mirrored LVM)

Configure the Persistent Memory

The following figure shows how we will provision the persistent memory.

How to Boot Linux from Intel® Optane™ Persistent Memory

How to , Linux

Introduction

The basic steps include:

Configure the Persistent Memory in AppDirect Interleaved
Create two small SECTOR namespaces, one per Region
Install the OS and select one or both of the namespaces (single disk install, or mirrored LVM)
Read More

How to build an upstream Fedora Kernel from source

I typically keep my Fedora system current, updating it once every week or two. More recently, I wanted to test the Idle Page Tracking feature, but this wasn’t enabled in the default kernel provided by Fedora.

# grep CONFIG_IDLE_PAGE_TRACKING /boot/config-$(uname -r)
# CONFIG_IDLE_PAGE_TRACKING is not set

To enable the feature, we need to build a custom kernel with the feature(s) we need. Thankfully, the process isn’t too difficult.

For this walk through, I’ll be building a customised version of the Fedora 32 kernel version I already have installed (5.8.7-200.fc32.x86_64), using some of the instructions from https://fedoraproject.org/wiki/Building_a_custom_kernel .

Linux Device Mapper WriteCache (dm-writecache) performance improvements in Linux Kernel 5.8

The Linux ‘dm-writecache’ target allows for writeback caching of newly written data to an SSD or NVMe using persistent memory will achieve much better performance in Linux Kernel 5.8.

Red Hat developer Mikulas Patocka has been working to enhance the dm-writecache performance using Intel Optane Persistent Memory (PMem) as the cache device.

The performance optimization now queued for Linux 5.8 is making use of CLFLUSHOPT within dm-writecache when available instead of MOVNTI. CLFLUSHOPT is one of Intel’s persistent memory instructions that allows for optimized flushing of cache lines by supporting greater concurrency. The CLFLUSHOPT instruction has been supported on Intel servers since Skylake and on AMD since Zen.

Issue

Executing ipmctl show-dimm to get device information shows the persistent memory modules in a ‘Non-functional’ health state, eg:

# ipmctl show -dimm

 DimmID | Capacity | HealthState    | ActionRequired | LockState | FWVersion
=============================================================================
 0x0001 | 0.0 GiB  | Non-functional | N/A            | N/A       | N/A
 0x0011 | 0.0 GiB  | Non-functional | N/A            | N/A       | N/A
 0x0021 | 0.0 GiB  | Non-functional | N/A            | N/A       | N/A
 0x0101 | 0.0 GiB  | Non-functional | N/A            | N/A       | N/A
 0x0111 | 0.0 GiB  | Non-functional | N/A            | N/A       | N/A
 0x0121 | 0.0 GiB  | Non-functional | N/A            | N/A       | N/A
 0x1001 | 0.0 GiB  | Non-functional | N/A            | N/A       | N/A
 0x1011 | 0.0 GiB  | Non-functional | N/A            | N/A       | N/A
 0x1021 | 0.0 GiB  | Non-functional | N/A            | N/A       | N/A
 0x1101 | 0.0 GiB  | Non-functional | N/A            | N/A       | N/A
 0x1111 | 0.0 GiB  | Non-functional | N/A            | N/A       | N/A
 0x1121 | 0.0 GiB  | Non-functional | N/A            | N/A       | N/A

Other ipmctl commands may fail and return “No functional DIMMs in the system.”, eg:

How To Verify Linux Kernel Support for Persistent Memory

Linux Kernel support for persistent memory was first delivered in version 4.0 of the mainline kernel, however, it was not enabled by default until version 4.2.

If you use a Linux distribution that uses kernel 4.2 or later, or the distro backports features in to an older kernel, you will almost certainly have persistent memory support enabled by default. It is still worth verifying what features are enabled and disabled as this may vary by distro and release version for the very latest persistent memory features.

How To Extend Volatile System Memory (RAM) using Persistent Memory on Linux

Intel(R) Optane(TM) Persistent Memory delivers a unique combination of affordable large capacity and support for data persistence. Electrically compatible with DDR4, large capacity modules up to 512GB each can be installed in compatible servers alongside DDR on the memory bus.

Intel’s persistent memory product can be provisioned in a volatile “Memory Mode” which replaces DRAM volatile capacity with the persistent memory capacity, and persistent “AppDirect” mode which presents both DRAM and persistent memory to the operating system and applications. Both modes are explained in more detail here . It is possible to configure a system to utilize a percentage of persistent memory as volatile and persistent, but this mixed-mode still provisions all the DRAM capacity as a Last-Level Cache.

Kernel

Linux Kernel 6.14 is Released: This is What's New for Compute Express Link (CXL)

Linux Kernel 6.13 is Released: This is What's New for Compute Express Link (CXL)

Linux Kernel 6.10 is Released: This is What's New for Compute Express Link (CXL)

Linux Kernel 6.9 is Released: This is What's New for Compute Express Link (CXL)

New Features

Linux Kernel CXL Feature Tracker

Using Linux Kernel Tiering with Compute Express Link (CXL) Memory

How To Install a Mainline Linux Kernel in Ubuntu

Using Linux Kernel Memory Tiering

What is Memory Tiering?

How To Emulate CXL Devices using KVM and QEMU

What is CXL?

How to Boot Linux from Intel® Optane™ Persistent Memory

Introduction

Configure the Persistent Memory

How to Boot Linux from Intel® Optane™ Persistent Memory

Introduction

How to build an upstream Fedora Kernel from source

Linux Device Mapper WriteCache (dm-writecache) performance improvements in Linux Kernel 5.8

Intel Optane Persistent Memory Modules report "Non-functional" state in ipmctl

Issue

How To Verify Linux Kernel Support for Persistent Memory

How To Extend Volatile System Memory (RAM) using Persistent Memory on Linux

Kernel

CXL related changes from Kernel v6.13 to v6.14

CXL related changes from Kernel v6.12 to v6.13:

CXL related changes from Kernel v6.9 to v6.10:

New Features

CXL related changes from Kernel v6.8 to v6.9:

What is Memory Tiering?

What is CXL?

Introduction

Configure the Persistent Memory

Introduction

Issue