Getting a serial console on USB-only Qubes laptop

My goal here is to get boot printk’s to understand a hardware compat issue, so first I’m looking for info on how to setup a boottime serial console with qubes.

From what I saw:

  • console= kernel parameters are ignored, and we see through dmesg that printk get directed to tty0 and hvc0
  • some information exist on qubes website (Safe Remote Dom0 Terminals | Qubes OS), but they are only applicable to a running system, and won’t help getting early kernel logs

Then, as is not uncommon those days on a laptop, I do not have an RS232 interface. The kernel does support console=ttyUSB to cope with this, but how we can make it work in the qubes model, where we cannot even attach a USB/serial device do dom0, is not yet clear to me either.

Any hint to get those early printk’s to the outside world ?

You can, of course, attach a USB device to dom0, by removing the
“rd.qubes.hide_all_usb” from the boot parameters.
If you have a USB3 debug port, DbC is designed to log printk messages -
you’ll need a debugging kernel, of course, but it will give you
something before the console comes onstream.