If your Ubuntu Kernel is updated your kernel parameters will be lost. In my case one of my IBM systems will not boot unless iommu=soft exists as a kernel boot parameter. To make this change survive after kernel updates do the following.
edit /etc/default/grub and add the entry to GRUB_CMDLINE_LINUX_DEFAULT:
.. code-block:: grub
GRUB_CMDLINE_LINUX_DEFAULT=“quiet splash iommu=soft”
You can also make changes to grub 2 during system start-up by pressing ‘e’ and then ‘f10’ to boot. I had to change hd(1,msdos1) to hd(1,5). To make this change permanent when the OS booted I ran:
.. code-block:: grub
update-grub