grub2-install: error: /boot/efi doesn't look like an EFI partition on a fresh Gentoo install
Solution 1:
The answer is simple. EFI System Partition (ESP) should be FAT32 (FAT16 or even FAT12 will also do for most proper UEFI; they are mostly shown as vfat in Linux) instead of ext2:
mkfs.fat -F32 /dev/sda2
FWIW, if you are not going to install grub i386-pc (for BIOS/CSM boot), you don't need the "BIOS boot" partition. It is only required by grub i386-pc (but not grub x86_64-efi or i386-efi) on GPT.
It doesn't really matter whether you use the ESP for /boot. If you do, you should run:
grub2-install --efi-directory /boot
If you mount it on /boot/efi instead, then you should run:
grub2-install --efi-directory /boot/efi
--boot-directory /boot is implied (i.e. default); It doesn't matter whether /boot is the ESP, another separate partition, or a directory on the / filesystem.
You may not even need to chroot again to perform grub2-install; For example, you mounted sda4, your partition for /, on /mnt; AND THEN, mounted sda2, your ESP, on /mnt/boot/efi, then you can simply run:
grub2-install --boot-directory /mnt/boot --efi-directory /mnt/boot/efi
Although grub2-mkconfig needs to be run in chroot AFAIK. But if you plan on writing a simple and clean grub.cfg yourself instead (which is the only graceful way to use grub2), then this will be out of your concern.
Solution 2:
The problem seemed to be mixing and matching BIOS, EFI, MBR, and GPT. I tried to follow the guides and use the GPT, but there were some dependencies that I could not resolve because I am too inexperienced. Its not even clear to me the kernel was built with GPT support (via config option CONFIG_EFI_PARTITION).
When I dropped GPT and EFI and switched to purely BIOS and MBR, I was able to boot the resulting machine. BIOS and GPT are supposed to be a valid combination, but I'm wondering if it works in practice (see, for example, Bootloader Options).
Here's the configuration I was able to run the machine with:
$ parted
GNU Parted 3.2
Using /dev/sda
(parted) print
Model: ATA VMware Virtual I (scsi)
Disk /dev/sda: 21.5GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:
Number Start End Size Type File system Flags
1 1049kB 135MB 134MB primary ext2 boot
2 135MB 19.5GB 19.3GB primary ext4
3 19.5GB 21.5GB 2012MB primary linux-swap(v1)
Partition 1 is /boot, Partition 2 is /, and Partition 3 is swap.
Solution 3:
I just had this issue and found that /boot existed but didn't have my boot partition mounted to it. Just had to mount the boot partition to fix this
mnt /dev/sda1 /boot
Solution 4:
If you are installing this on virtual-box make sure you have the enabled the option of EFI under the category of Setting/System