[syslinux] Is efiboot.img required?

[Ady]

> Quoting http://www.syslinux.org/wiki/index.php/Isohybrid#UEFI:
> 
> "The additional isohybrid feature for UEFI adds a partition to the MBR
> partition table pointing to the same file in the ISO 9660 filesystem
> as does the El Torito catalog entry for EFI."
> 
> According to this (and can be see using fdisk) it's being created a
> partition-inside-partition just by setting its start and end sectors,
> so this could be done to be equal to the initial MBR partition
> generated by IsoHybrid, so there would not be duplicated files
> (kernel, initram and syslinus.cfg). Is that possible, currently or
> with some patches on isohybrid command? In what situations it would
> not work (iso9660 can be seen as a read-only FAT filesystem)? Also,
> could it be used directly the same MBR entry just by setting the
> partition entry flags or with some other minor tune-ups?
> 
> 
> 
> -- 
 
I am replying to this email, but the content of your prior email might 
be relevant too.

IMHO, the current content of the Isohybrid page in the Syslinux Wiki is 
not clear enough (in the past we have had here some discussion about 
the target audience of such page). The current generic information is 
rather focused on _potential_ capabilities of _isohybrid_ tools and 
some of their internal technical characteristics, and less focused on 
actual practical steps for ends users.

The actual structure of the resulting partition table depends on the 
building tool in use. For example, there are several variants of the 
'isohybrid' tool, and the 'xorriso' tool also provides isohybrid 
capabilities. Each tool might generate different partition tables for 
each isohybrid image (also depending on the options in use, of course).

More importantly, part of the UEFI-related information in the 
aforementioned isohybrid wiki page also assumes a mix of preconditions, 
and some of them are not currently achievable by 'syslinux.efi'.

Currently (as of v.6.03), 'syslinux.efi' does not support multiple 
volumes in the way that is currently described in the 'isohybrid' wiki 
page, so 'syslinux.efi' cannot simultaneously access both volumes, the 
ISO9660 volume and a FAT volume in an "efi.img". Additionally, 
currently 'syslinux.efi' has no support for ISO9660.

So, even before evaluating whichever potential conflicts with specific 
hardware or with any variant of (U)EFI (e.g. EFI-based Macs) or with 
specific kernels, the fact is that 'syslinux.efi' could help you boot, 
for example, a USB flash drive formatted with FAT32, or a FAT EFI 
System Partition in your local disk drive, or network booting UEFI 
clients, but currently 'syslinux.efi' has no use for "efi.img" in a 
ISO9660 volume, and it cannot boot optical media in UEFI mode (with or 
without an "efi.img").

If you want to be able to boot a (FAT) storage media (such as a USB 
drive, among others) in UEFI mode, or network boot UEFI clients, then 
'syslinux.efi' v.6.03 might be useful for you. The same ISO(hybrid) 
image could be used in systems with CSM mode in the same way as it is 
used in BIOS-based hardware (by means of ISOLINUX, for example).

If you really need to boot optical media in UEFI mode, then you/we 
could wait some (long) time for potential developments in Syslinux, or 
you might want to consider other UEFI bootloaders, unfortunately.

Regards,
Ady.
 
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