While tinkering with a Linux VM I regularly get kernel panics, that push up all the helpful output with stuff I don't understand and that is probably not helpful to me.
The panics mostly occur during the initramfs phase. I'm using VirtualBox.
The normal Shift+Pg Up does not work (in my case).
Is there another way to scroll back up and look at the output of whatever came before?
Best Answer
Serial port
The serial port is an old and reliable communication protocol hardware that the Linux kernel supports and most emulators emulate.
You can stream the kernel messages to a host file or console through it:
VirtualBox: How does one Capture the Entire Kernel Panic on Boot | Stack Overflow
QEMU:
Here's a minimal setup to reproduce the problem: https://github.com/cirosantilli/linux-kernel-module-cheat/blob/b366bac0c5410ceef7f2b97f96d93d722c4d9ea6/kernel_module/panic.c
real hardware: the serial port is not exposed on most modern laptops, which is a shame... but on desktops it looks like this:
Source.
And on the Raspberry Pi:
More details at: What is the difference between ttys0, ttyUSB0 and ttyAMA0 in Linux?
Serial alternatives
There are even fancier methods mentioned at: Determining cause of Linux kernel panic | Unix & Linux Stack Exchange
netdump: sends trace over network.
Supposes panic didn't break networking, which is more likely than the serial.
The advantages over serial are:
works for systems that don't have serial exposed such as modern laptops
serial cables have quite limited maximum wire lengths, which is problematic if you want to have all the boards of your company on a remote room to share resources across developers.
There are however serial connectors with an Ethernet server which I would recommend instead if your target exposes serial, e.g. this one:
kdump: boots a secondary Linux kernel that inspects the panicked kernel. What could possibly go wrong?
Those methods are more flexible, but less reliable.
See also: Scrolling up the failed screen with kernel panic | Super User