Specifying common variables across multiple boot menu entries

Red Hat Enterprise Linux 8.6 LPAR mode z/VM guest KVM guest

The zipl environment feature is useful in the case of multiple boot menu entries.

Procedure

Create a zipl environment file that holds only the keyword-value pairs for common variables.
For example:
```
# cat /etc/ziplenv
COMMON=nosmt 
```

Introduce the ${COMMON} variable to the kernel parameters in the boot menu configurations.

[defaultboot]
defaultauto
target=/boot

# First boot configuration (DASD)
[boot1]
ramdisk=/boot/initrd
parameters='root=/dev/ram0 ro ${COMMON}' 
image=/boot/image-1
target=/boot

# Second boot configuration (DASD)
[boot2]
image=/boot/mnt/image-3
ramdisk=/boot/mnt/initrd
parmfile=/boot/mnt/parmf-3
target=/boot

...

For the boot2 configuration section, the parmfile option specifies a kernel parameter file that contains kernel parameters, including the ${COMMON} variable, for example:

# cat /boot/mnt/parmf-3
....
CRASH=256M
PANIC_TIMEOUT=panic=8
${COMMON}

Call zipl to install multiple boot configurations that include the ${COMMON} variable.
For example, to make zipl install both boot configuration boot1 and boot2:
```
# zipl 
```
At boot time, the ${COMMON} variable is replaced by nosmt, as specified in the installed zipl environment block by the keyword COMMON.

Results

The zipl environment block now defines the COMMON keyword:

# zipl-editenv --list
...
COMMON=nosmt

IPL any boot configuration that uses a command line with the ${COMMON} variable, and display the actual command line:

# cat /proc/cmdline
... nosmt ...

To not have nosmt set, first ensure that the environment file does not define the COMMON variable, or defines it as an empty string:

# cat /etc/ziplenv
COMMON=

Then install the boot record.

However, if you do not want to re-install the boot record, you can either set the COMMON variable to the empty string in the already installed zipl environment block:

# zipl-editenv -s COMMON=
# zipl-editenv -l
COMMON=

Or remove it from the zipl environment block:

# zipl-editenv -u COMMON
# zipl-editenv -l