Let's say you want a kernel argument like memmap='1G$12G' to preallocate 1GiB of RAM starting from the 12GiB mark for later use e.g. for a DMA device. If you are on a rpm-ostree based system like Fedora Silverblue (Fedora Atomic Desktop) you might try this:
rpm-ostree kargs --append="memmap='1G$12G'"Then you notice: