Ubuntu – How to boot load the kernel using EFI stub (efistub) loader


I have Ubuntu 14.04 running in UEFI mode as only operating system, no dual-boot here. The kernel version is 3.13.0-24-generic. There is an EFI partition. In this case the EFI partition is not at the default /dev/sda1 but at /dev/sda3 because I did actually convert BIOS mode to EFI mode. I have used the grub-efi-amd64 package, though that actually loads GRUB boot menu from UEFI firmware boot menu (UEFI boot loads \EFI\ubuntu\grubx64.efi).

I want to skip that double boot menu loading step, and boot faster, directly from UEFI into the kernel. The Ubuntu kernels since 12.10 have "Kernel EFI stub loader" feature.

I know I do need to copy the Ubuntu kernel to the EFI partition (possibly rename) and create an entry in UEFI boot menu (for instance using efibootmgr). Which exact terminal commands are necessary to do this?

Best Answer

The commands below are more generic then for kernel version 3.13.0-35 only.

1. Mount the efi partition and copy the kernel files there

$ mount /dev/sda3 /boot/efi

$ mkdir -pv /boot/efi/EFI/ubuntu/

$ cp -uv /boot/vmlinuz-* /boot/initrd.img-* /boot/efi/EFI/ubuntu/
'/boot/vmlinuz-3.13.0-35-generic' -> '/boot/efi/EFI/ubuntu/vmlinuz-3.13.0-35-generic'
'/boot/initrd.img-3.13.0-35-generic' -> '/boot/efi/EFI/ubuntu/initrd.img-3.13.0-35-generic'

2. Change the kernel file name

Shorten the kernel file name by removing -generic because there seems to be a 39 character length path limit and Rename kernel file(s) to end in .efi, this ensures compatibility with most systems

$ for f in /boot/efi/EFI/ubuntu/vmlinuz-*-generic; do mv -uv -- "$f" "${f//-generic/}.efi"; done
'/boot/efi/EFI/ubuntu/vmlinuz-3.13.0-35-generic' -> '/boot/efi/EFI/ubuntu/vmlinuz-3.13.0-35-generic.efi'`

The above name kernel file name shortening is not enough for a dpkg installed mainline kernel, because for example /EFI/ubuntu/vmlinuz-3.16.0-031600rc6.efi without -generic is still 40 characters long.

3. Add new entry to EFI boot menu

Replace 3.13.0-35 in this example with your specific kernel version

$ kv=3.13.0-35;efibootmgr -c -p 3 -L $kv -l \EFI\ubuntu\vmlinuz-$kv.efi -u root=/dev/sda1 initrd=\\EFI\\ubuntu\\initrd.img-$kv-generic ro rootfstype=ext4 debug ignore_loglevel libata.force=dump_id crashkernel=384M-:128M

This new boot menu entry will become your default new boot choice.

You might not need the extra debugging parameters debug, ignore_loglevel, libata.force=dump_id and crashkernel=384M-:128M. Initrd must be present, otherwise boot hangs at "Switched to clocksource tsc." because the root device sda1 cannot be opened.

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