Title: Changes to dracut kernel module/microcode handling Author: Andrew Ammerlaan Posted: 2024-05-17 Revision: 2 News-Item-Format: 2.0 Display-If-Installed: sys-kernel/dracut Display-If-Installed: virtual/dist-kernel Display-If-Installed: sys-kernel/linux-firmware Display-If-Installed: sys-firmware/intel-microcode Impact ==================== Several changes were made regarding out-of-tree kernel modules, CPU microcode, and how these are handled in initial RAM file systems (initramfs) generated by sys-kernel/dracut for distribution kernels. Depending on the local Dracut and USE flag configuration, some configuration adjustments may be required as a result of these changes. Background (the problem) ==================== Previously Dracut implicitly included all out-of-tree kernel modules it could find. This leads to several problems: - It unnecessarily increases the size of the initramfs - It creates a bit of a mess when using distribution kernels, consider the following: 1) Distribution kernel is upgraded 2) Initramfs for the new kernel is generated, it does not include any out-of-tree kernel modules. 3) Portage triggers rebuild of the out-of-tree kernel modules 4) If zfs is installed, its rebuild will trigger an initramfs re-installation. Otherwise no rebuild is triggered. Problem: What is and is not included in the initramfs is now ambiguous. It depends on the emerge order of the kernel modules when zfs is used. And will completely change if at some later stage regeneration of the initramfs is triggered manually via e.g.: emerge --config sys-kernel/gentoo-kernel As a result, Dracut's "--reproducible" setting is not working. And the functionality of the initramfs may change (seemingly) at random. Background (the fix) ==================== Several things have been changed: - Out-of-tree kernel modules installed by portage are explicitly omitted from the initramfs generated by Dracut by default. - Packages that install a kernel module for which it might make sense to have it in the initramfs, have gained the "initramfs" USE flag. When this flag is enabled, Dracut is instructed to include the installed kernel modules. Packages for which it is essential that its kernel modules are included in the initramfs have this new flag enabled by default. - When distribution kernels are used (USE=dist-kernel), and a module that should be in the initramfs is installed (USE=initramfs) the initramfs is always re-generated. - The packages installing CPU microcode (sys-kernel/linux-firmware and sys-firmware/intel-microcode) have been adjusted to mirror the above changes for out-of-tree kernel modules. Both packages have gained the "dist-kernel" USE flag, and the "initramfs" flag is now enabled by default. When both flags are enabled, Dracut is configured to include the installed microcode in the initramfs, and then the initramfs is regenerated. When the "dist-kernel" flag is disabled, the "initramfs" flag behaves as it previously did. User Action Required (Dracut and/or Distribution Kernel users) ==================== Users of sys-kernel/dracut and/or Distribution Kernels should double check two things: 1) Please ensure that you are *not* globally enabling or disabling the "initramfs" USE flag. Enabling it globally might result in an unnecessarily large initramfs. Disabling it globally might result in missing functionality in the initramfs. Which could lead to boot failure if, for example, the zfs module is missing while the root partition is a zfs. 2) Any add_drivers, or omit_drivers lines in /etc/dracut.conf or /etc/dracut.conf.d/* may override the Dracut configuration snippets installed by the kernel module packages in /usr/lib/dracut/dracut.conf.d. Please review your Dracut configuration files to ensure that you are not unintentionally overriding the settings set by Portage. User Action Required (other users) ==================== Other users may wish to disable the "initramfs" USE flag on sys-kernel/linux-firmware and/or sys-firmware/intel-microcode if they already have other mechanisms in place for updating the CPU microcode (such as kernel built-in CPU microcode). Users who do not use sys-kernel/dracut or Distribution Kernels can safely disable the "initramfs" USE flag globally. Frequently Asked Questions ==================== A package installing a kernel module I would like in my initramfs has not gained the "initramfs" USE flag. How do I proceed? Please report a new bug on bugs.gentoo.org, requesting that the package maintainer consider adding support to the package for including the modules in the initramfs. In the meantime you can locally override the configuration provided by the package (see below). Note though that when distribution kernels are used, regeneration of the initramfs must be triggered manually via e.g.: emerge --config sys-kernel/gentoo-kernel How do I override the provided Dracut configuration snippets to include/exclude a custom list of modules? To override the provided configuration snippet, create a new file /etc/dracut.conf.d/10-PACKAGENAME.conf, replacing PACKAGENAME with the name of the package providing the module. Add to this file: omit_drivers+=" my list of drivers to omit " and/or add_drivers+=" my list of drivers to include "