(SOLVED) Optimizing GRUB in UEFI multiboot scenario

[Helena]

I have managed recently to install Gentoo on an UEFI multiboot system. At the moment I have Windows 10, Linux Mint and Gentoo. It took a lot of hard work as the Gentoo documentation is hardly adequate for this situation.

Now that it's up and running I find that the size of the ESP partition is a limiter. Currently I have both grub and the binary kernels there. Obviously I want them on another partition, could be the system (/) partition or a separate /boot-partition. Linux Mint has it this way, already, I believe. However I would like to avoid experimenting again.

Could it be this simple:

1. Boot normally
2. Unmount /boot (as it points to the ESP partition)
3. Create /boot (or use as is)
4. Recompile a kernel (which will then install in /boot)
5. Update /etc/fstab according to step 3.
5. Run grub-mkconfig -o /boot/grub/grub.cfg
6. Reboot

After that, if desired move already existing binary kernels to the new location or delete them.

Help is appreciated.

[charles17]

[Helena]

Thank you for replying quickly

[charles17]

[Helena]

Unfortunately I still have not made my situation clear. I will try to rephrase.

I have this partition layout:

[charles17]

So your main topic is about grub.
Are you having two grub installations, one from Linux Mint and one from Gentoo? Or just only one?
I seem to recall that rEFInd supports Kernel image outside ESP. For grub I have no idea.

Edit
This section from archlinux wiki might give you some help

[Ant P.]

refind autodetects kernels in my btrfs /boot/. Bit annoying because its filesystem driver is slow with a lot of files in there, but it means the ESP never runs out of space or needs mounting r/w. /EFI/refind/ is 2.3MB total.

[Helena]

Thx charles17 and Ant P for pointing out rEFind. I've looked into the creator's webpages and he confirms clearly (as far as I understand) that grub should be able to load a kernel from another partition. As I already guessed in my starting post.

I don't want to change my boot loader at the moment so I am still looking for a gentoo-compliant procedure for grub. The ArchLinux WiKi is good background, as always, but doesn't provide me with the solution yet.

[charles17]

Helena,

Not yet clear to me. Are you using the bootloader ( grub? ) provided by Linux Mint, or did you install grub from Gentoo?
And what is the exact location of your kernels? Are they in /boot/EFI/ or in /boot/?

Maybe you could change the ESP's mountpoint to /boot/whatever/EFI and have stuff in /boot but outside the (space limited) ESP.

[Helena]

My ESP partition is:

[charles17]

Someone more familiar with Grub to step in here.
But you'd better change the topic (in the top post) to better reflect "need help with multiboot Grub"

[GDH-gentoo]

Yes, GRUB can boot kernels that are not in the ESP, as long as it can read the filesystem of the partition(s) they are installed in. And it definitely can read ext2 and ext4 filesystems.

[Helena]

I'll try to answer your questions.

[CaptainBlood]

Largely off topic: grub2 vulnerabilty
Unclear how much grub.cfg from EFI boot partition are exposed...
The article points out:

[GDH-gentoo]

[Helena]

Topic title changed

[Helena]

So back to my original question: which steps to take?

Currently, /boot is linked to the root of the ESP partition, where also the kernels reside. GRUB finds them there. GRUB's configuration file and modules are found in the grub subdirectory of the ESP. GRUB itself in the EFI/gentoo subdirectory.

This means that whenever a new kernel is compiled, I will have to mount the ESP, in order to update grub.cfg. Also, when GRUB itself receives an update (which happens less often). In all other situations I should't need the ESP when working from Gentoo, as it is only used during booting. Correct?

Right now, this is not the case because I have the kernels in the ESP. Therefore I want to move them to the Gentoo system partition, or maybe a separate boot partition.

If this sound right then the next question will be how to update /etc/fstab.

[Helena]

So I see 4 options altogether, with a slight adaptation of Rod Smith's tutorial:

1A: GRUB binary+configuration file+modules, binary kernels all on the ESP
1B: GRUB binary+configuration file+modules on the ESP; binary kernels on a separate partition
2: GRUB binary on the ESP; GRUB configuration file+modules, binary kernels on a separate partition
3: GRUB binary on the ESP with a special prefix configuration file; second GRUB configuration file+modules, binary kernels on a separate partition

I'm now on 1A and want to move to 1B.

Notwithstanding the attractiveness of options 2 and 3. especially option 3 is interesting because maintenance would be very simple.

[charles17]

[Helena]

[charles17]

As long you keep it with /boot/efi you wouldn't change anything, since /boot/efi of course is inside /boot. And you can have /boot only once.

But you could use Alternative UEFI firmware location /boot/efi/EFI/BOOT/

[Helena]

Hurray! It works!

[charles17]

[Helena]

Done. Thx again.

For the record, here is what I've found out (Note that this applies to UEFI/GPT only, and I've not checked Secure Boot):

• In an initial install, mount the ESP to /boot/efi (this and following references where necessary preceded by /mnt/gentoo)
  
• (create and) mount a separate partition for holding kernels to /boot
  
• (create and) mount a large partition for holding everything else to /
  
• install GRUB using the following command:

  Code:

  grub-install --efi-directory=/boot/efi

  (--target and --dev parameters don't seem necessary)
  

Of course this is only a minor addition to the Handbook which still applies.

[pietinger]