After a default Linux (in my case Oracle Linux 7.4) installation you won’t see anything after grub finished with “start /HOST/console”
Trick:
Worked on linux 7.4 running on Oracle Server X7-2
Spawn a TTY service on your serial console device port as a service:
# systemctl --now enable getty@ttyS0
Created symlink from /etc/systemd/system/getty.target.wants/getty@ttyS0.service to /usr/lib/systemd/system/getty@.service.
Another way is to tell grub2 to use the console... then you would also see booting and shutdown messages:
# grubby --remove-args="rhgb quiet" --args=console=ttyS0,9600 --update-kernel=ALL