Set kernel parameters via GRUB

Update the master config at /etc/default/grub:

// Disable intel meltdown/spectre mitigations
GRUB_CMDLINE_LINUX_DEFAULT="mitigations=off"

Then run update-grub, which updates the config used during boot:

set -e
exec grub-mkconfig -o /boot/grub/grub.cfg "$@"

To check the parameters are active, read /proc/cmdline:

$ cat /proc/cmdline
BOOT_IMAGE=/vmlinuz-5.3.0-21-generic … mitigations=off …