Modifying Kernel Arguments

Modifying Kernel Arguments via Ignition

You can specify kernel arguments in a Butane config using the kernel_arguments section.

Example: Staying on cgroups v1

Starting from June 2021, cgroups v2 is the default on new installations of Fedora CoreOS. Here’s an example kernelArguments section which adds the systemd.unified_cgroup_hierarchy=0 kernel argument so that the machine keeps using cgroups v1:

variant: fcos
version: 1.4.0
kernel_arguments:
  should_exist:
    - systemd.unified_cgroup_hierarchy=0

Example: Disabling all CPU vulnerability mitigations

Here’s an example kernelArguments section which switches mitigations=auto,nosmt to mitigations=off to disable all CPU vulnerability mitigations:

variant: fcos
version: 1.4.0
kernel_arguments:
  should_exist:
    - mitigations=off
  should_not_exist:
    - mitigations=auto,nosmt

Modifying Kernel Arguments on Existing Systems

Kernel arguments changes are managed by rpm-ostree via the rpm-ostree kargs sub command. Changes are applied to a new deployment and a reboot is necessary for those to take effect.

Adding kernel arguments

You can append kernel arguments. This is useful with e.g. console= that can be used multiple times. An empty value for an argument is allowed:

$ sudo rpm-ostree kargs --append=KEY=VALUE
Example: Add reserved memory for Kdump support
$ sudo rpm-ostree kargs --append='crashkernel=256M'

See also Debugging kernel crashes using kdump.

Removing existing kernel arguments

You can delete a specific kernel argument key/value pair or an entire argument with a single key/value pair:

$ sudo rpm-ostree kargs --delete=KEY=VALUE
Example: Remove console parameters to enable kernel auto-detection
$ sudo rpm-ostree kargs --delete 'console=ttyS0,115200n8'

See also Emergency console access.

Example: Update an existing system from cgroupsv1 to cgroupsv2 and immediately reboot
$ sudo rpm-ostree kargs --delete=systemd.unified_cgroup_hierarchy --reboot

Replacing existing kernel arguments

You can replace an existing kernel argument with a new value. You can directly use KEY=VALUE if only one value exists for that argument. Otherwise, you can specify the new value using the following format:

$ sudo rpm-ostree kargs --replace=KEY=VALUE=NEWVALUE
Example: Disable all CPU vulnerability mitigations
$ sudo rpm-ostree kargs --replace=mitigations=auto,nosmt=off

This switches mitigations=auto,nosmt to mitigations=off to disable all CPU vulnerability mitigations.

Interactive editing

To use an editor to modify the kernel arguments:

$ sudo rpm-ostree kargs --editor