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Re: GRUB doesn't recognize partitions on El-Torito image during EFI boot

From: Jacek Konieczny
Subject: Re: GRUB doesn't recognize partitions on El-Torito image during EFI boot from CD
Date: Mon, 11 Nov 2013 20:40:55 +0100

On Mon, 11 Nov 2013 23:15:50 +0400
Andrey Borzenkov <address@hidden> wrote:
> > For EFI boot it should be straightforward – use the disk image as
> > a hard-disk-emulation El-Torito EFI boot image.
> Do you mean - in addition to EFI boot image? Because EFI requires ESP
> in "no emulation" mode to boot.

Does it? Can you give me any pointers to relevant documentation?
I did that (using disk emulation image) with other loaders (plain EFI
shell and ELILO) and it just worked ('no emulation' did not work with my
disk image). But it that is explicitely forbidden, then I will try
something else.

> Why "msdos"? El-Torito CD does not have standard DOS label, so it
> would be wrong name. And if you place whole image of partitioned disk
> there, it becomes nested partition (dos label in El-Torito
> "partition").
> Something like (cd,eltorito0,msdos1) probably.

Hmmm. I expected GRUB to see the eltorito image as EFI disk – separate
device. But if GRUB ignores the EFI file system access interface (and I
am afraid it does), then that may not work.

> No. As it stands currently grub does not expose El-Torito image as 
> partition of CD media.

I would not expect that.

> But you need to place image on CD either way. Are you using this image
> for anything else, like directly booting on non-EFI?

The image is used directly (not in ISO) for booting (EFI and non-EFI)
from USB disk, and for both EFI and non-EFI boot from CD.

BTW… my 'workaround' (with calling 'configfile' first with first
'search' result and continuing with loopback if that fails) worked well
in VirtualBox, but failed on a real machine (manual loopback setup in
the rescue shell still worked)… strange.

> >                                  GRUB should be able to see the
> > El-Torito contents, shouldn't it?
> > 
> So far there was not any use case. But yes, technically EFI does
> expose it as "partition" so grub probably should too.

I see. That would not techincally differ much from using the loopback

My initial idea was a bit different. Having experience
with other EFI loaders I expected GRUB to see files on the FAT partition
of the image, like other loaders and EFI shell see. That seemed like a
trivial solution - converting a bootable HDD image into a bootable CD 
was just adding it as an El-Torito there. It works for EFI shell and the
shell sees the files on the FAT partition the same way when booted from
the CD as when it was booted from the USB stick.

But it seems GRUB does not use the EFI file-system access at all. Is
that right? Are there any plans to change that?

Anyway… it seems I need to change a boot loader, use separate GRUB boot
image for EFI and use the loopback, or do things like everybody seems to
do – make the ISO the 'primary' boot image and store that in other
filesystems to make a USB stick bootable (using GRUB loopback)… 


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