UEFI booting

Last Friday (Black Friday, since I was in the US this year), I ended up buying for myself an early birthday present; I finally got the ZenBook UX31A that I was looking at since September, after seeing the older model being used by J-B of VLC fame. Today it arrived, and I decided to go the easy route: I already prepared a DVD with Sabayon, and after updating the “BIOS” from Windows (since you never know), I wiped it out and installed the new OS on it. Which couldn’t be booted.

Now before you run around screaming “conspiracy”, I ask you to watch Jo’s video (Jo did you really not have anything with a better capture? My old Nikon P50 had a better iris!) and notice that Secure Boot over there works just fine. Other than that, this “ultrabook” is not using SecureBoot because it’s not certified for Windows 8 anyway.

The problem is not that it requires Secure Boot or anything like that but much more simply, it has no legacy boot. Which is what I’m using on the other laptop (the Latitude E6510), since my first attempt at using EFI for booting failed badly. Anyway this simply meant that I had to figure out how to get this to boot.

What I knew from the previous attempt is this:

  • grub2 supports UEFI both 32- and 64-bit mode, which is good — both my systems run 64-bit EFI anyway;
  • grub2 requires efibootmgr to set up the boot environment;
  • efibootmg requires to have access to the EFI variables, so it requires a kernel with support for EFI variables;
  • but there is no way to access those variables when using legacy boot.

This chicken-and-egg problem is what blown it for me last time — I did try before the kernel added EFI Stub support anyway. So what did I do this time? Well, since Sabayon did not work out of the box I decided to scratch it and I went with good old fashioned Gentoo. And as usual to install it, I started from SysRescueCD — which to this day, as far as I can tell, still does not support booting as EFI either. It’s a good thing then that Asus actually supports legacy boot… from USB drives as well as CDs.

So I boot from SysRescueCD and partition the SSD in three parts: a 200MB, vfat EFI partition; a root-and-everything partition; and a /home partition. Note that I don’t split either /boot or /usr so I’m usually quite easy to please, in the boot process. The EFI partition I mount as /mnt/gentoo/boot/efi and inside it I create a EFI directory (it’s actually case-insensitive but I prefer keeping it uppercase anyway).

Now it’s time to configure and build the kernel — make sure to enable the EFI Stub support. Pre-configure the boot parameters in the kernel, make sure to not use any module for stuff you need during boot. This way you don’t have to care about an initrd at all. Build and install the kernel. Then copy /boot/vmlinuz-* as /boot/efi/EFI/kernel.efi — make sure to give it a .efi suffix otherwise it won’t work — the name you use now is not really important as you’ll only be using it once.

Now you need an EFI shell. The Zenbook requires the shell available somewhere, but I know that at least the device we use at work has some basic support for an internal shell in its firmware. The other Gentoo wiki has a link on where to download the file; just put it in the root of the SysRescueCD USB stick. Then you can select to boot it from the “BIOS” configuration screen, which is what you’ll get after a reboot.

At this point, you just need to execute the kernel stub: FS1:EFIkernel.efi will be enough for it to start. After the boot completed, you’re now in an EFI-capable kernel, booted in EFI mode. And the only thing that you’re left to do is grub-install --efi-directory=/boot/grub/efi. And .. you’re done!

When you reboot, grub2 will start in EFI mode, boot your kernel, and be done with it. Pretty painless, isn’t it?