Re: [PATCH] core/partmap: Add El Torito boot catalog parsing

["Thomas Schmitt" <scdbackup@gmx.net>]

Hi,

not that i would want to interfere with GRUB2 development
decisions. Nevertheless, i have some comments from the view
of the specs.

Vladimir Serbinenko wrote:
> El torito catalog contains bootable binaries.

UEFI 2.4 9.3.6.2 "CD-ROM Media Device Path"
"The CD-ROM Media Device Path is used to define a system partition
 that exists on a CD-ROM. The CD-ROM is assumed to contain an
 ISO-9660 file system and follow the CD-ROM “El Torito” format.
 [...] In EFI the bootable entity is an EFI System Partition that
 is pointed to by the Boot Entry."

According to El Torito 1.0 the boot images may be emulated
disk-like devices or executable binaries for the given
platform:

2.0 "ISO-9660 and the Booting Catalog"
"The image may be virtualized into INT 13 drive number 00 or 80
 for bootable disk emulation, or [... some BIOS shortcut ...].
 The image may also simply be some code which will be loaded at
 boot time (no emulation)."

UEFI pierces a little hole into the theory of running on an
emulated disk by prescribing the "no emulation" mark in the
boot catalog which for BIOS means: load and execute binary.

12.3.2.1 "ISO-9660 and El Torito"
"[...] To boot from a CD-ROM or DVD-ROM in the boot services
 environment, an EFI System partition is stored in a “no emulation”
 mode as defined by the “El Torito” specification. [...]
 EFI differs from “El Torito” “no emulation” mode in that it does
 not load the “no emulation” image into memory and jump to it.
 EFI interprets the “no emulation” image as an EFI system partition."

------------------------------------------------------------------

So i think the general role of an El Torito boot image is not a
strong reason to reject its view as full flegded EFI system
partition.
But i see other reasons to restrict its role to an opaque launcher
for software in a directory of the ISO:

The El Torito catalog format implies size restriction to 32 MB.
EFI allows open end by size 0, but that's really not very neat.

I assume the higher boot stages of BIOS and EFI can share a lot of
GRUB2 software. The GRUB2 software needed for BIOS traditionally
resides in ISO 9660 directories.
So it is easier and less error prone to re-use it there rather
than changing the BIOS boot procedure to finding the ESP and
using its software.


Have a nice day :)

Thomas