Modifying Kernel Boot Parameters
Kernel boot parameters configure how the SUSE Linux Enterprise Server for NVIDIA kernel starts and behaves, controlling areas such as hardware compatibility, performance and diagnostics. Set them temporarily from the GRUB 2 boot menu to test a change for a single boot. To apply a change to every subsequent boot, set it permanently in the boot loader configuration instead. Understanding the correct syntax and where to apply it lets you tune a system for a specific workload and resolve hardware-related boot problems. You can also roll back safely if a change causes issues.
Prerequisites:
To change kernel boot parameters on an experimental basis for the upcoming boot process, you need physical or remote console access to the system. Examples include a hypervisor console or a hardware management interface. This method does not work over SSH, since the system is not yet reachable over the network at that point in the boot process.
To change kernel boot parameters for all subsequent boot processes, you need
rootor equivalent administrative privileges.
1 Introduction to kernel boot parameters #
Kernel boot parameters tell the kernel how to behave at startup. Knowing where they come from and how GRUB 2 passes them to the kernel makes it easier to change them correctly. This knowledge also helps you reason about which change to make for a given problem.
1.1 What are kernel boot parameters? #
Kernel boot parameters are configurations passed to the kernel during the boot process managed by GRUB 2. The configurations are instructions to the GRUB 2 boot loader on how to configure certain features and make them behave in the intended manner. The method of modification depends on whether you want the change to affect only the next boot, or all subsequent boots.
1.2 Methods of modification #
You can modify the kernel boot parameters using the following methods:
- Temporarily
Changes are held in memory and apply only to the upcoming boot process. These changes made through the GRUB 2 boot menu are not written to any configuration file and are discarded after reboot.
- Permanently
Changes are written to the
/etc/default/grubfile and persist across all subsequent boot processes. For permanent changes, we recommend using theupdate-bootloadercommand instead of directly editing the/etc/default/grubfile.However, if you directly edit the
/etc/default/grubfile, modify theGRUB_CMDLINE_LINUXorGRUB_CMDLINE_LINUX_DEFAULTvariables. After modification, regenerate the GRUB 2 configuration by running the following command with administrative privileges.>sudogrub2-mkconfig -o /boot/grub2/grub.cfgThis command applies only to the standard, non-immutable system. It fails on the immutable mode of SLE for NVIDIA, whose read-only root file system is only writable inside a new transactional snapshot. There, run
update-bootloaderthroughtransactional-updateinstead of calling it directly:>sudotransactional-update run update-bootloader --add-option PARAMETER=VALUETo only regenerate the GRUB 2 configuration, without adding or removing a parameter, use the dedicated subcommand instead:
>sudotransactional-update grub.cfgIn immutable mode, changes take effect only after a reboot, since they live in a new snapshot until then.
Note: Snapshots do not stack without a rebootEach
transactional-update runcall branches its new snapshot from the currently running system, not from an earlier snapshot you have not yet booted into. Running the command again before rebooting does not combine the two changes: only the most recent snapshot becomes the one that boots next, and it does not include the earlier, not-yet-activated change. Reboot to activate each change before making the next one.
1.3 Benefits of modifying kernel boot parameters #
A few common reasons to modify kernel boot parameters:
Performance. Tune the kernel for a specific workload. For example, the
quietparameter suppresses most boot messages, which shortens boot time.Hardware compatibility. Work around a hardware quirk the kernel misdetects by default. For example, the
acpi=offparameter disables the Advanced Configuration and Power Interface (ACPI), which can resolve boot problems on older hardware.Troubleshooting. Isolate a driver or subsystem that is preventing the system from booting. For instance, the nomodeset parameter disables kernel mode setting, which helps when a graphics driver prevents the system from booting.
1.4 Examples of commonly modified kernel boot parameters #
A few of the most commonly modified kernel boot parameters:
- root=
Specifies the path to the root file system.
- quiet
Suppresses most boot messages.
- splash
Displays a graphical boot splash screen.
- nomodeset
Disables kernel mode setting.
- debug
Enables debugging output.
- acpi=
Toggles the Advanced Configuration and Power Interface (ACPI) settings.
- selinux=0
Disables SELinux, which is enabled by default on SLE for NVIDIA 16. For the recommended approach, the
permissivemode alternative, and how to relabel the file system after reenabling SELinux, see .
For a detailed understanding of all available kernel parameters and their possible values, source documentation is available at https://www.kernel.org/doc/Documentation/admin-guide/kernel-parameters.txt.
1.5 General rules for specifying values of kernel boot parameters #
When modifying kernel boot parameters, follow these essential formatting rules:
- Basic Parameter Spacing
Separate individual parameters with a single space:
With
update-bootloader, add each parameter with its own--add-optioncall. The command inserts the spacing for you:>sudoupdate-bootloader --add-option quiet>sudoupdate-bootloader --add-option splashIn the
/etc/default/grubconfiguration file for GRUB 2, use spaces between kernel parameters when passing them as string values to theGRUB_CMDLINE_LINUXorGRUB_CMDLINE_LINUX_DEFAULTvariables.At the boot menu, press E to edit, locate the line starting with
linux, and add parameters with spaces.
Example 1: Separate individual parameters with a single space #root=/dev/sda1 quiet splash
- Parameter Value Assignment
When assigning values, use the equals sign (
=) without spaces:Example 2: Correct method of assigning values to kernel parameters #root=/dev/sda1 video=1920x1080
Example 3: Incorrect method of assigning values to kernel parameters #root = /dev/sda1 video = 1920x1080
- Values Containing Spaces
For values containing spaces, the handling differs between methods:
update-bootloader- Warning: Configuration corruption from unescaped spaces
update-bootloader --add-optiondoes not quote or escape a value for you: a value containing a space is split into two separate boot parameters, corrupting the configuration. For a value containing spaces, edit/etc/default/grubdirectly or use the GRUB 2 boot menu instead, both described below. - GRUB 2 configuration file
Use either double quotes with escape characters, or use backslashes.
Example 4: Kernel parameter values containing spaces in the GRUB 2 configuration file #GRUB_CMDLINE_LINUX="root=\"/dev/mapper/USER VOLUME\""
GRUB_CMDLINE_LINUX="root=/dev/mapper/USER\ VOLUME"- GRUB 2 Boot Menu
Use only double quotes for values containing spaces:
Example 5: Kernel parameter values containing spaces in the GRUB 2 boot menu #root="/dev/mapper/USER VOLUME"
- Multiple values for a kernel boot parameter
Use commas without spaces to separate multiple values for a single parameter in all methods:
Example 6: Multiple values for a kernel boot parameter #module_blacklist=nouveau,radeon
2 Difference between kernel boot parameters and linuxrc parameters #
Older SUSE documentation distinguishes kernel boot parameters from
linuxrc parameters. On SUSE Linux Enterprise Server for NVIDIA 16, that distinction no longer applies to
an installed system: linuxrc is gone, and the parameters that used to
be associated with it are ordinary kernel boot parameters handled by dracut.
2.1 What happened to linuxrc on SLE for NVIDIA 16? #
linuxrc was the boot-time parameter parser used by the installer on
earlier SLE for NVIDIA releases. It ran only during installation, before the target
system's root file system existed, so it played no part in an already-installed system. It
is not present on SLE for NVIDIA 16 at all: SLE for NVIDIA 16 installs exclusively with Agama, which does not use
linuxrc. Agama provides a compatibility layer for a handful of
linuxrc option names, for administrators migrating scripts or profiles
built around the old parser. For the installer-time options and the compatibility mapping,
see Section 6, “For more information”.
What remains is dracut, the framework that builds the
initial RAM disk (initramfs). It brings up the environment the kernel needs before the real
root file system is mounted, such as loading storage drivers or unlocking encrypted
devices. Unlike linuxrc, dracut is not specific to installation. It also
runs on every boot of an installed system.
The options that configure dracut are not a separate parameter category with their own syntax. They are ordinary kernel boot parameters, entered on the same GRUB 2 command line and following the same rules described in this article. Among these, dracut simply acts on the ones it recognizes before handing off to the rest of the boot process.
2.2 Examples of dracut parameters #
The following kernel boot parameters are handled by dracut during the initrd stage, before the root file system is mounted:
rd.driver.blacklist=: Specifies drivers to be blocked in the initrd. For example,rd.driver.blacklist=nouveauprevents thenouveaugraphics driver from loading during the early stages of boot. This can be useful when troubleshooting driver-related boot issues.rd.break: Interrupts the boot process and drops the system to a shell within the initrd for debugging purposes. This parameter is helpful for diagnosing issues related to the early boot process, such as driver loading or file system mounting errors.rd.retry=: Specifies how long, in seconds, dracut retries configuring devices before giving up. For example,rd.retry=240extends the default 180-second timeout, which can help on hardware that takes longer than usual to initialize storage devices.rd.luks=0: Disables LUKS-encrypted device detection in the initrd. Detection is enabled by default on a system with an encrypted root file system. Use this parameter only if you need to turn it off, for example while troubleshooting an unrelated storage issue.
Because these are kernel boot parameters like any other, the temporary and permanent modification methods described elsewhere in this article apply to them. They work the same way as for quiet or nomodeset.
3 Temporarily modifying kernel boot parameters #
To change kernel boot parameters on an experimental basis for the upcoming boot process, edit the options in the GRUB 2 boot menu. This method requires physical or remote console access to the system and does not work over an SSH connection. Such changes apply only to the upcoming boot process and the resulting session, and do not persist after reboot.
As an example of modifying kernel boot parameters for the upcoming boot process, we disable the splash screen that you can see during the boot.
Restart the system. By default, the GRUB 2 menu appears with a brief countdown, and the saved entry boots automatically once it expires. If your system has been configured to hide the menu instead, press Esc while the SLE for NVIDIA boot logo shows to reveal it.
In the GRUB 2 boot menu, highlight the entry you want to modify using the arrow keys ↑ and ↓.
Press the E key. You are presented with an editor and the content of the selected boot entry. It looks similar to the following:
setparams 'SLE for NVIDIA 16.1' ... ... echo 'Loading Linux KERNEL_VERSION ...' linux /boot/vmlinuz-KERNEL_VERSION root=UUID=56d052b3-9148-4161-8065-3d97378d5783 ${extra_cmdline} splash=silent resume=/dev/disk/by-uuid/69ff0e54-23a7-4ba8-8983-5a29c54ffa5e quiet security=selinux selinux=1 crashkernel=377M,high crashkernel=72M,low mitigations=auto
Search for the string splash=silent and remove it.
To boot the entry, press F10 or Ctrl–X.
To discard the changes and start anew, press the Esc key.
After executing the procedure and continuing with the boot process, no splash screen is observed for the current boot process. On the next boot, the splash screen reappears.
4 Permanently modifying kernel boot parameters #
To change kernel boot parameters persistently for all subsequent boot processes, use the
update-bootloader command as root or a user with
equivalent administrative privileges.
--get-option PARAMETERPrints the current value of a parameter, if set.
--add-option PARAMETER=VALUEAdds a parameter, or replaces its value if it is already set.
--del-option PARAMETERRemoves a parameter.
--configApplies pending changes to the boot loader configuration.
On a production system, test the parameter temporarily first, as described in Section 3, “Temporarily modifying kernel boot parameters”. Confirm it has the effect you expect before committing to a change that applies to every subsequent boot.
As an example of modifying kernel boot parameters permanently, we disable the splash screen that appears during boot.
Before modifying kernel boot parameters, create a copy of the existing stable GRUB 2
configuration (/etc/default/grub) somewhere outside of your system.
If the boot loader gets corrupted, or you face problems after booting, your system might
fail to start. The backup lets you compare and restore the last known working
configuration.
Log in as root, or switch to a user with equivalent administrative privileges.
Check whether the parameter you want to change is already set:
#update-bootloader --get-option splashThe command prints the currently configured value, for example
splash=silent.Remove the parameter from the default boot options:
#update-bootloader --del-option splashTo add or change a parameter instead of removing one, use
update-bootloader --add-option PARAMETER=VALUE. Setting an already-present parameter again replaces its value rather than adding a duplicate entry.Apply the change to the boot loader configuration:
#update-bootloader --configReboot to confirm the splash screen no longer appears.
After executing the procedure and rebooting the system, we do not observe any splash screen for all subsequent boot processes.
update-bootloader works consistently across the boot loaders SUSE
supports, including GRUB 2 and the systemd-boot preview. If you prefer
to edit /etc/default/grub directly instead, see
Section 1.2, “Methods of modification” for the variables to modify and
the command required to apply the change.
5 Troubleshooting and FAQs #
A wrong or unsupported kernel boot parameter can leave a system unstable or unable to boot. This section covers how to identify a bad parameter, recover a system that no longer boots, and confirm stability after a change.
5.1 Troubleshooting modifications to kernel boot parameters #
The following steps cover the most common issues: an incorrect parameter, a system that fails to boot, and a system that boots but behaves unpredictably afterward.
5.1.1 Identifying and correcting parameter issues #
Validate parameter syntax and configuration:
Verify the spelling accuracy of all kernel parameters.
Check for correct formatting without unnecessary spaces.
Confirm that the parameters are recognized by the kernel. If the parameter is already part of the standard configuration, check the current value with
cat /proc/cmdline. For a complete list of allowed parameters, refer to the kernel documentation at https://www.kernel.org/doc/Documentation/admin-guide/kernel-parameters.txt.
Investigate system logs for detailed error information:
>sudodmesg | grep -i error>sudojournalctl -k -bThe
journalctl -k -boutput includes aCommand line:entry showing the exact kernel boot parameters the kernel received for the current boot, which you can compare against what you intended to set.Temporarily enable or disable suspicious parameters to isolate issues. For example, use nomodeset to troubleshoot device driver issues related to graphics cards.
5.1.2 Recovering from boot failures #
A system that fails to boot is no longer reachable over the network. You need physical or remote console access (for example, a hypervisor console or a hardware management interface) to perform the following steps.
Boot into recovery mode:
Restart the system.
By default, the GRUB 2 menu appears automatically with a brief countdown. If your system has been configured to hide the menu instead, press Esc while the SLE for NVIDIA boot logo shows to reveal it.
At the GRUB 2 menu, select .
Select the
(recovery mode)entry for the kernel version you want to boot.
Recovery mode boots the system in single-user mode, without the parameters set for normal boot. It gives you a minimal working environment to fix the configuration.
Fix the configuration. Recovery mode does not use
GRUB_CMDLINE_LINUX_DEFAULT, so you have a working root shell regardless of what it currently contains:If you have a pre-modification backup, restore it:
>sudocp /etc/default/grub.bak /etc/default/grubIf you do not have a backup, remove the offending parameter directly:
>sudoupdate-bootloader --del-option PARAMETER
Regenerate the GRUB 2 configuration:
>sudoupdate-bootloader --config
5.1.3 Managing system stability #
Before making further changes, back up the current, still-working configuration:
>sudocp /etc/default/grub /etc/default/grub.bak>sudocp /boot/grub2/grub.cfg /boot/grub2/grub.cfg.bakCheck the system logs for errors or warnings introduced by the change:
>sudojournalctl -xe>sudodmesg | tailIf a parameter change made the boot noticeably slower, check which units took the longest to start:
systemd-analyze blameRemove the recently added parameters one at a time, rebooting after each, to find the cause.
If the issues persist, restore the backup from the first step, or boot from a previously working system snapshot.
6 For more information #
For a full list of kernel parameters and their possible values, see the https://www.kernel.org/doc/Documentation/admin-guide/kernel-parameters.txt upstream kernel documentation.
For the boot options recognized by the Agama installer at install time, see . These replace the
linuxrcparameters used by earlier SUSE installers.
7 Legal Notice #
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Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or (at your option) version 1.3; with the Invariant Section being this copyright notice and license. A copy of the license version 1.2 is included in the section entitled “GNU Free Documentation License”.
For SUSE trademarks, see https://www.suse.com/company/legal/. All other third-party trademarks are the property of their respective owners. Trademark symbols (®, ™ etc.) denote trademarks of SUSE and its affiliates. Asterisks (*) denote third-party trademarks.
All information found in this book has been compiled with utmost attention to detail. However, this does not guarantee complete accuracy. Neither SUSE LLC, its affiliates, the authors, nor the translators shall be held liable for possible errors or the consequences thereof.
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If a section in the Document is Entitled "Acknowledgements", "Dedications", or "History", the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
9. TERMINATION #
You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
10. FUTURE REVISIONS OF THIS LICENSE #
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See https://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.
ADDENDUM: How to use this License for your documents #
Copyright (c) YEAR YOUR NAME. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:
with the Invariant Sections being LIST THEIR TITLES, with the Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.