Troubleshooting: Using grub: modify a grub entry, change Dom0 commandline, add or remove kernel parameters,

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Note: this is a first try, made because forum search was not helping. I propose it as a potential Community guide.

Troubleshooting: Use grub for temporary changes when Qubes-OS is booting

For some problems, it can be useful to change the way Qubes starts at boot time.

This guide is about changing the very first part of boot, when xen and the Dom0 kernel are being started. In most cases, they are started by the grub bootloader.

Interrupt grub

  • Restart your computer, and watch the screen, waiting for the grub to display
    • Normally this is a blue screen
    • It shows some different “entries”
    • There is a short time when you can interrupt it, using the keyboard
    • If you do nothing, the default entry is automatically started
  • To interrupt, you can do one of these:
    • press the key ‘e’. This will allow to change the “default”. You can press “Esc” or “Escape” to return to the menu.
    • Press or hold SHIFT to go to the menu.

In the menu, you can select a different “entry”, using the Up and Down arrow keys. There may be troubleshooting entries, or ones to use different kernels. After booting, you can verify which kernel was used in Dom0 terminal, by typing uname -a.

In the edit mode, can modify the entry to have different kernel parameters.

Change the Dom0 kernel parameters

This is almost always why we are here.

In the edit mode:

  • Each item can be quite long, and may be split over several lines.
  • Find the item that begins like this: module2 vmlinuz. Use the Up/Down arrow keys to go to it.
  • Go to the very end of the item, using right arrow.
  • To add a parameter, type a space, then add the parameter. See below for some examples.
  • To remove a parameter, Go left in the line, to the end of it, and use “Backspace” key to remove it.

When you have finished, Press the F10 key, to continue the boot, using your changes. Remember that the changes are temporary. They will not be there next time you restart.

If you make a mistake, “Escape” cancels all changes and returns to the grub menu, where you can press e and try again.

If necessary, you can press Ctrl-Alt-Delete to reboot your computer without continuing the boot.

You can verify changes to the kernel parameters in a Dom0 terminal, after Qubes has started, by typing:
cat /proc/cmdline.

If you find a good setting, make a careful note of the changes, because you may want to use the same changes again, or you may want to add some of them permanently.

Some typical parameters we might add or remove

USB keyboard problems

  • Add qubes.skip_autostart : prevents all qubes from automatic starting. This is useful when sys-usb has a problem and does not send keyboard/mouse data to Dom0. Normally, the next two items are used at the same time.
  • Remove rd.qubes.hide_all_usb : makes the USB controllers be visible to Dom0. It may not be enough to allow use of a USB keyboard
  • Add usbcore.authorized_default=0 : if Dom0 can see a USB controller, then only mouse and keyboard devices will be usable.

Get better information for debugging

  • Remove “rhgb” and “quiet” : you will see lots of progress messages scroll past as the system starts up. This is often useful to see the last few messages before it stops.

Prevent some drivers from loading

  • blacklist a kernel module
  • ToDo

Display is black or corrupted

  • nomodeset or modeset=0 : prevent Kernel Mode Setting from using GPU drivers
  • … ToDo more to add here.

Some more technical changes used in rare cases

  • Boot the Dom0 kernel without xen. Why…?
  • Hide some devices for Passthrough
  • Todo…
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True … and this will increase your drive’s “Unsafe Shutdowns” count by one. Just saying.
[user@dom0 ~]$ sudo smartctl --all /dev/YOURDRIVE | grep ^Unsafe
(where YOURDRIVE is most probably nvme0 … or else sda)

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Is that right? I thought a soft reboot before any write operations would be the best option if it is undesirable to continue with a possibly undesirable grub boot. I will add an “if necessary” for now. Do you have any reliable docs?

Of course, nothing would surprise me in the rats nest of PC hardware…

Edit: all further comments/remarks/criticism is welcome.

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Nah. Just my experience, and confirmed by a few other forum users. There is a thread somewhere. from last year or the year before.

Edit: here it is!

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