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Chapter 3. Listing of kernel parameters and values

3.1. Kernel command-line parameters

Kernel command-line parameters, also known as kernel arguments, are used to customize the behavior of Red Hat Enterprise Linux at boot time only.

3.1.1. Setting kernel command-line parameters

This section explains how to change a kernel command-line parameter on AMD64 and Intel 64 systems and IBM Power Systems servers using the GRUB2 boot loader, and on IBM Z using zipl.

Kernel command-line parameters are saved in the boot/grub/grub.cfg configuration file, which is generated by the GRUB2 boot loader. Do not edit this configuration file. Changes to this file are only made by configuration scripts.

Changing kernel command-line parameters in GRUB2 for AMD64 and Intel 64 systems and IBM Power Systems Hardware.

  1. Open the /etc/default/grub configuration file as root using a plain text editor such as vim or Gedit.

  2. In this file, locate the line beginning with GRUB_CMDLINE_LINUX similar to the following: 

    GRUB_CMDLINE_LINUX="rd.lvm.lv=rhel/swap crashkernel=auto rd.lvm.lv=rhel/root rhgb quiet"
  3. Change the value of the required kernel command-line parameter. Then, save the file and exit the editor.

  4. Regenerate the GRUB2 configuration using the edited default file. If your system uses BIOS firmware, execute the following command: 

    # grub2-mkconfig -o /boot/grub2/grub.cfg

    On a system with UEFI firmware, execute the following instead: 

    # grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg

After finishing the procedure above, the boot loader is reconfigured, and the kernel command-line parameter that you have specified in its configuration file is applied after the next reboot.

Changing kernel command-line parameters in zipl for IBM Z Hardware

  1. Open the /etc/zipl.conf configuration file as root using a plain text editor such as vim or Gedit.
  2. In this file, locate the parameters= section, and edit the requiremed parameter, or add it if not present. Then, save the file and exit the editor.

  3. Regenerate the zipl configuration: 

    # zipl
    Note

    Executing only the zipl command with no additional options uses default values. See the zipl(8) man page for information about available options.

After finishing the procedure above, the boot loader is reconfigured, and the kernel command-line parameter that you have specified in its configuration file is applied after the next reboot.

3.1.2. What kernel command-line parameters can be controlled

For complete list of kernel command-line parameters, see https://www.kernel.org/doc/Documentation/admin-guide/kernel-parameters.txt.

3.1.2.1. Hardware specific kernel command-line parameters

pci=option[,option…​]

Specify behavior of the PCI hardware subsystem

SettingEffect

earlydump

[X86] Dump the PCI configuration space before the kernel changes anything

off

[X86] Do not probe for the PCI bus

noaer

[PCIE] If the PCIEAER kernel parameter is enabled, this kernel boot option can be used to disable the use of PCIE advanced error reporting.

noacpi

[X86] Do not use the Advanced Configuration and Power Interface (ACPI) for Interrupt Request (IRQ) routing or for PCI scanning.

bfsort

Sort PCI devices into breadth-first order. This sorting is done to get a device order compatible with older (⇐ 2.4) kernels.

nobfsort

Do not sort PCI devices into breadth-first order.

Additional PCI options are documented in the on disk documentation found in the kernel-doc-<version>.noarch package. Where '<version>' needs to be replaced with the corresponding kernel version.

acpi=option

Specify behavior of the Advanced Configuration and Power Interface

SettingEffect

acpi=off

Disable ACPI

acpi=ht

Use ACPI boot table parsing, but do not enable ACPI interpreter
This disables any ACPI functionality that is not required for Hyper Threading.

acpi=force

Require the ACPI subsystem to be enabled

acpi=strict

Make the ACPI layer be less tolerant of platforms that are not fully compliant with the ACPI specification.

acpi_sci=<value>

Set up ACPI SCI interrupt, where <value> is one of edge,level,high,low.

acpi=noirq

Do not use ACPI for IRQ routing

acpi=nocmcff

Disable firmware first (FF) mode for corrected errors. This disables parsing the HEST CMC error source to check if firmware has set the FF flag. This can result in duplicate corrected error reports.