Category Archives: Boot


Installing Gentoo on EFI and virtualbox is tough!

The problem is because the gentoo CD doesn’t come with a EFI boot, so you have to start from MBR. Well, it turns out that it’s not possible to change the EFI settings if you have booted through MBR.

That means you have to install gentoo without EFI support first and make a boot partition ready for the EFI boot loader. How do you do that without being able to configure EFI?

If you are just looking for the solution go to the short version.

The Fix – Long Version

First of all in Gentoo you must enable EFI stub so that the kernel can be executed as a boot loader (awesome functionality!).

Secondly if EFI has nothing specified it will look for a file named “boot<arch>.efi” in \efi\boot. So in most cases ‘\efi\boot\bootx64amd.efi‘. Notice that since EFI requires FAT the path uses backslash rather than forward slash (/). So you need to copy the kernel there and rename it.

After rebooting (and turning on EFI in virtualbox) the kernel loads. But panics:

VFS: Cannot open root device (null) or unknown-block(0 0): error -6
Please append a correct “root=” boot option; here are the available partitions:
(omitted sda partitions)
Kernel panic – not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

What does this mean? At first I thought it was because I didn’t set any EFI variables. It is possible to do so through the EFI Shell. You can start this by mashing F12 when virtualbox is booting up a VM. From here there are many commands you can run to tweak your boot up. And written by some drunk lemurs, really wtf Intel!

Let’s have a look at some useful commands:

ram -r: shows the boot options.

bcfg: adds boot options. Example is bcfg boot add <option nr> <efi file> “<label>” . In my case this command would look like blk3:\efi\boot\bootx64.efi

I never figured out how to add kernel arguments (there’s a “-opt” option to bcfg).

This did not help, I still got kernel panic. My second thought was that it was because I did not have the driver for the HDD. I recompiled the kernel with anything that rhymed with EFI,SATA ,PIIX or ACHI. It didn’t help. The live  CD could find the drives, so I took every module it used and put into /etc/conf.d/modules by lsmod | gawk ‘{print $1}’. It didn’t help (I got quite a lot of warnings though).

Thinking about it, since the kernel actually got loaded it must have found the device. The problem is probably the ‘correct “root=”…’ message.

It turns out that you can set the root partition by a kernel command line option. I had no idea, but this is something boot loaders usually do. I heard you could also add these options when you compile the kernel, but I don’t want to recompile my kernel. Instead I booted up in UEFI Shell again and started the efi file with a root option:

>blk3:\efi\root\bootx64amd.efi root=/dev/sda2

It worked! Gentoo boots, and I got a warm and nice feeling in my stomach. Since it has booted from EFI it should be possible to set the parameters through efibootmgr:

$efibootmgr -c -d /dev/sda -p 1 -L “gentoo Linux” -l ‘\efi\boot\bootx64amd.efi’  root=/dev/sda2
$efibootmngr -v

The output shows it worked. But after reboot still kernel panic. Checking the boot options in UEFI Shell I can see that there’s no record saved. Is it a bug in efibootmgr? I spent a few more hours trying to add options with bcfg when I realized there’s a really nice way to add boot options. Don’t boot the shell, instead enter Boot Maintenance Management. From here it’s very easy to add a record with extra arguments.

Starting is fine, but as soon as I reboot the EFI variables will be wiped! This must be a bug (feature?) in virtualbox.

There is one final resort. If EFI does not find an efi file in ESP it will try to execute startup.nsh in the efi\boot folder. Since it will execute the efi file first it’s important to rename it. I renamed mine to gentoo.efi.

Create a startup.nhs file with the UEFI Shell command:


save it and reboot.

Finally Gentoo can automatically boot up in EFI mode!

The fix – short version

  • First install gentoo and create a FAT32 boot partition (and all other things needed for EFI, such as GPT and ESP). Don’t forget to update fstab.
  • On first reboot enter the boot menu by pressing F12.
  • Go to Boot Maintenance manager -> Boot Options -> add boot options
  • Select the boot partition, probably the only one with a GUID. Find the location of your efi file (efi\boot\bootxamd64.efi)
  • Add description (any thing you like). Input Option Data: root=/dev/sda2
  • Commit changes and exit. Go back and select boot manager and select your created boot option.
  • Once gentoo boots up you can try to add a record from efibootmgr. If it doesn’t save after reboot continue with the steps below.
  • Rename the efi file to something like /efi/boot/gentoo.efi
  • Create startup.nsh in the same folder and add the UEFI Shell path to the efi file. Example: blk3:\efi\boot\gentoo.efi
  • Reboot.