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Chapter 12. Kernel

This chapter lists the most notable changes to kernel between RHEL 8 and RHEL 9.

12.1. Notable changes to kdump memory allocation

The kexec-tools package now supports the default crashkernel memory reservation values for RHEL 9

The kexec-tools package now maintains the default crashkernel memory reservation values. The kdump service uses the default value to reserve the crashkernel memory for each kernel. This implementation also improves memory allocation for kdump when a system has less than 4GB of available memory.

If the memory reserved by the default crashkernel value is not sufficient on your system, you can increase the crashkernel parameter using the default value as a reference.

To query the default crashkernel value:

 $ kdumpctl get-default-crashkernel

Note that the crashkernel=auto option in the boot command line is no longer supported on RHEL 9 and later releases.

For more information, see the /usr/share/doc/kexec-tools/crashkernel-howto.txt file.

12.2. Notable changes to TPM 1.2 secure cryptoprocessor support on RHEL 9

The TPM 1.2 secure cryptoprocessor is no longer supported on RHEL 9

The Trusted Platform Module (TPM) secure cryptoprocessor version 1.2 has been removed and is no longer supported on RHEL 9 and later versions. TPM 2.0 replaces TPM 1.2 and provides many improvements over TPM 1.2. TPM 2.0 is not backward compatible.

Note that for applications that require support for TPM 1.2, Red Hat recommends that you use RHEL 8.

12.3. Notable changes to kernel

cgroup-v2 enabled by default in RHEL 9

The control groups version 2 (cgroup-v2) feature implements a single hierarchy model that simplifies the management of control groups. Also, it ensures that a process can only be a member of a single control group at a time. Deep integration with systemd improves the end-user experience when configuring resource control on a RHEL system.

Development of new features is mostly done for cgroup-v2, which has some features that are missing in cgroup-v1. Similarly, cgroup-v1 contains some legacy features that are missing in cgroup-v2. Also, the control interfaces are different. Therefore, third party software with direct dependency on cgroup-v1 may not run properly in the cgroup-v2 environment.

To use cgroup-v1, you need to add the following parameters to the kernel command-line:

systemd.unified_cgroup_hierarchy=0
systemd.legacy_systemd_cgroup_controller
Note

Both cgroup-v1 and cgroup-v2 are fully enabled in the kernel. There is no default control group version from the kernel point of view, and is decided by systemd to mount at startup.

Kernel changes potentially affecting third party kernel modules

Linux distributions with a kernel version prior to 5.9 supported exporting GPL functions as non-GPL functions. As a result, users could link proprietary functions to GPL kernel functions through the shim mechanism. With this release, the RHEL kernel incorporates upstream changes that enhance the ability of RHEL to enforce GPL by rebuffing shim.

Important

Partners and independent software vendors (ISVs) should test their kernel modules with an early version of RHEL 9 to ensure their compliance with GPL.

Core scheduling is supported in RHEL 9

With the core scheduling functionality users can prevent tasks that should not trust each other from sharing the same CPU core. Likewise, users can define groups of tasks that can share a CPU core.

These groups can be specified:

  • To improve security by mitigating some cross-Symmetric Multithreading (SMT) attacks
  • To isolate tasks that need a whole core. For example for tasks in real-time environments, or for tasks that rely on specific processor features such as Single Instruction, Multiple Data (SIMD) processing

For more information, see Core Scheduling.

The kernelopts environment variable has been removed in RHEL 9

In RHEL 8, the kernel command-line parameters for systems using the GRUB2 bootloader were defined in the kernelopts environment variable. This variable was stored in the /boot/grub2/grubenv file for each kernel boot entry. However, storing the kernel command-line parameters using kernelopts was not robust. Therefore, Red Hat removed kernelopts and the kernel command-line parameters are now stored in the Boot Loader Specification (BLS) snippet, instead of in the /boot/loader/entries/<KERNEL_BOOT_ENTRY>.conf file.

Red Hat protects kernel symbols only for minor releases

Red Hat guarantees that a kernel module will continue to load in all future updates within an Extended Update Support (EUS) release, only if you compile the kernel module using protected kernel symbols. There is no kernel Application Binary Interface (ABI) guarantee between minor releases of RHEL 9.

12.4. Notable changes to boot loader

Boot loader configuration files are unified across CPU architectures

Configuration files for the GRUB boot loader are now stored in the /boot/grub2/ directory on all supported CPU architectures. The /boot/efi/EFI/redhat/grub.cfg file, which GRUB previously used on UEFI systems, is now a symbolic link to the /boot/grub2/grub.cfg file.

This change simplifies the layout of the GRUB configuration file, improves user experience, and provides the following notable benefits:

  • You can boot the same installation with either EFI or legacy BIOS.
  • You can use the same documentation and commands for all architectures.
  • GRUB configuration tools are more robust, because they no longer rely on symbolic links and they do not have to handle platform-specific cases.
  • The usage of the GRUB configuration files is aligned with images generated by CoreOS Assembler (COSA) and OSBuild.
  • The usage of the GRUB configuration files is aligned with other Linux distributions.

RHEL no longer boots on 32-bit UEFI

Support for the 32-bit UEFI firmware was removed from the GRUB and shim boot loaders. As a consequence, RHEL 9 requires a 64-bit UEFI, and can no longer boot on 64-bit systems that use a 32-bit UEFI.

The following packages have been removed as part of this change:

  • grub2-efi-ia32
  • grub2-efi-ia32-cdboot
  • grub2-efi-ia32-modules
  • shim-ia32