How kernel parameters from different sources are combined

Ubuntu 22.04 LTS LPAR mode z/VM guest KVM guest

If kernel parameters are specified in a combination of methods, they are concatenated in a specific order.

  1. Kernel parameters that have been included in the boot configuration.

    The kernel parameters in the boot configuration cannot exceed 895 characters. If more then 895 characters are specified, the excessive characters are truncated.

  2. LPAR or z/VM®: Kernel parameters that you specify through the HMC or through z/VM interfaces).

    For DASD boot devices you can specify up to 64 characters (z/VM only); for SCSI boot devices you can specify up to 3452 characters.

In total, the combined kernel parameter string that is passed to the Linux® kernel for booting can be up to 4096 characters.

Multiple specifications for the same parameter

For some kernel parameters, multiple instances in the kernel parameter string are treated cumulatively. For example, multiple specifications for cio_ignore= are all processed and combined.

Conflicting kernel parameters

If the kernel parameter string contains kernel parameters with mutually exclusive settings, the last specification in the string overrides preceding ones. Thus, you can specify a kernel parameter when booting to override an unwanted setting in the boot configuration.
Example: If the kernel parameters in your boot configuration include possible_cpus=8 but you specify possible_cpus=2 when booting, Linux uses possible_cpus=2.

Parameters other than kernel parameters

Parameters on the kernel parameter string that the kernel does not recognize as kernel parameters are ignored by the kernel and made available to user space programs. How multiple specifications and conflicts are resolved for such parameters depends on the program that evaluates them.