efibootmgr questions

houtworm · Posted: Wed Jan 04, 2023 9:38 am Post subject: efibootmgr questions

I have a few questions about the efibootmgr.
I looked at the wiki page https://wiki.gentoo.org/wiki/Efibootmgr

questions:
1/ what does the efibootmgr do what grub can't do?
2/ do I have to rename my kernel to kernel.efi ?
3/ why do I have to remount efivars to rw ? When rebooting, the changes are lost.

comment: I think it is a bad idea to put the kernel in the efi partition. With multiple linux distro's the efi partition it too small. I put my kernels in the partition of each distribution, in /boot and mount efi at /boot/efi like other distro's do. That is also where grub can find it.
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Kees

NeddySeagoon · Posted: Wed Jan 04, 2023 9:58 am Post subject:

houtworm,

efibootmgr manages the efivars, stored in the EFI non volatile memory.
The efivars tell the EFI firmware what the boot order is and for each entry, where to try to boot from.
They do other things too but in the context of your question, that's the main one.
As such, they are consulted before grub, or any other EFI compatible program is loaded.

Grub comes later, if the efivars instruct the EFI firmware to load grub.

The EFI firmware will only load EFI compatible binaries with filenames ending in .efi.
To make the kernel an EFI compatible binary, select the EFI_STUB option in the kernel and give it a name ending in .efi when its copied to the EFI System Partition.

The EFI firmware can only read the ESP, so anything it is to load has to go there.
If its a boot loader, the boot loader can load things from other places, provided that it can read the filesystem(s).
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Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

sMueggli · Guru Joined: 03 Sep 2022 Posts: 374

houtworm · Posted: Wed Jan 04, 2023 12:30 pm Post subject: Re: efibootmgr questions

Ah so the nvram is on my motherboard! I have never used an OS that writes on the motherboard so this is new to me. It makes a lot of sense!
Thank you both for the answers, I am seeing the light now!
Ok so grubx64.efi is enough, no kernel needed.
I have never used UEFI but now with a few linux distro's it became very complicated with using a single boot partition for everything. And now I could install windows without ruining the bootsector :-)

Yeah got you there Bill Gates!
It tooko me a few days of reading and trying and I am so glad with your help, now just the finishing touch and it is done!
_________________
niemand is onbekwamer, dan een timmerman zonder hamer

Kees

NeddySeagoon · Posted: Wed Jan 04, 2023 12:50 pm Post subject:

houtworm,

Perhaps a contrast between BIOS bootinh anh EFI booting is useful.
The (non-EFI) BIOS can read exactly one sector from the boot drive. That's LBA 0. A long time ago that was known as Cylinder 0, Head 0, Sector 0.
If LBA 0 failed, the drive could not be used as a boot drive.
The code there had to be enough to start the system. It was just enough to load a better loader ...

With EFI, the is no boot sector any more. EFI can read exactly one filesystem type. VFAT.
It loads an EFI compliant binary from the ESP. The EFI compliant binary can do whatever you want.

With both systems its possible to enter the firmware and change the boot order.
With BIOS, you choose a different drive as each drive has at most one boot sector.
With EFI, you choose a different boot entry in the efivars, and maybe define a new one. Different entries may be on different drives but need not be.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.