I'm currently struggling with an update of the kernel on our servers. If I try to update to 4.0.2 + grsec patch (at the moment latest: grsecurity-3.1-4.0.2-201505101122.patch) the system does not boot any more.
The problem is: There is hardly anything that gives a hint what's going on. On normal boot it does not print out anything, it will just reboot and return to grub again and again. When I pass early_printk parameter I get some boot messages, it stops at the NR_IRQS message (this seems to be a default on every kernel boot). I noted that past that the first thing that happens on a working system the next thing that is happening is the initialization of the console:
[ 0.000000] NR_IRQS:4352 nr_irqs:256 0
[ 0.000000] Console: colour VGA+ 80x25
[ 0.000000] console [ttyS0] enabled
Kernel 4.0 introduces some new options regarding console (CONFIG_DUMMY_CONSOLE_COLUMNS, CONFIG_DUMMY_CONSOLE_ROWS), so it might have something to do with that and is failing at the console initialization.
Some further info that might be relevant:
* Right now the system is running kernel 3.19.5 with patch grsecurity-3.1-3.19.5-201504270827.patch, which is working.
* Disabling the grsec patch the system boots again.
* It's a virtualized system (kvm), with a virtual serial console (virsh console).