Comment # 3 on bug 1137061 from Javier de San Pedro

Yeah, same thing in 20190529. 

I should elaborate: this does not result in unbootable system because linuxefi
is builtin. However, no other Grub modules are loadable, so one cannot boot
other OS, even if secure boot is disabled.

To reproduce:
1. Install TW on UEFI system , using Grub2
2. Disable secureboot 
3. On Grub2 prompt, type 'c' to go into Grub console
4. Enter "insmod linux" + <ENTER>
5. Enter "linux" + <TAB>

Expected result:
"Possible commands are:
  linux linuxefi"

Actual result:
"error: file `/boot/grub2/x86_64-efi/linux.mod' not found"
Only linuxefi command is available

Workaround: 
btrfs-mount-subvol ($root) /boot/grub2/x86_64-efi @/boot/grub2/x86_64-efi
before running "insmod linux"


Additionally, setting
 GRUB_USE_LINUXEFI="false"
in /etc/default/grub 
WILL result in an unbootable system, because with this setting "linux" is used
rather than "linuxefi" for the TW kernels, and "linux" module (as per the
above) will fail to insmod. 

I am actually forced to use GRUB_USE_LINUXEFI=false on some workstations (due
to multiple early UEFI firmware bugs *cough* AMI) .