Title: Changes to dracut kernel module/microcode handling
Author: Andrew Ammerlaan <andrewammerlaan@gentoo.org>
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 "
