ng0 <contact.ng0@cryptolab.net> writes:

> Hi, a short update on the multiple providers I'm talking with at the moment.
> This just in after the holidays from 1984 ehf, iceland.
>
> All in all not problematic, and they'd even add GuixSD to the list of available isos
> once there is an iso.
>
> So, what's the status on the ISO, who's working on it?

I've been looking into this off and on over the last few weeks, but I
haven't made any breakthroughs.  The closest I got was this:

1) Create a disk image for testing:

  ./pre-inst-env guix system --root=/tmp/disk-image-gc-root --fallback disk-image ~/guix/gnu/system/install.scm
  cp $the_resulting_path /tmp/disk-image
  
2) Try to boot it (with an attached hard disk), and watch it fail:

  qemu-img create -f qcow2 /tmp/test 10G
  sudo qemu-system-x86_64 -machine type=pc-i440fx-2.5,accel=kvm -boot order=dc,menu=on -m size=4G -k en-us -name guixsd -cdrom "/tmp/disk-image" "/tmp/test"

3) Mount it as loopback device:

  sudo losetup -P /dev/loop0 /tmp/disk-image
  sudo mkdir /mnt/disk-image-partition-1
  sudo mount /dev/loop0p1 /mnt/disk-image-partition-1
  
4) Make a bootable CD-ROM image of it (see (grub) Invoking grub-mkrescue):

  sudo grub-mkrescue -o /tmp/test-img.iso /mnt/disk-image-partition-1

5) Try to boot (partial success):

  sudo qemu-system-x86_64 -machine type=pc-i440fx-2.5,accel=kvm -boot order=dc,menu=on -m size=4G -k en-us -name guixsd -cdrom "/tmp/disk-image" "/tmp/test"

There appear to be (at least) two problem that prevent this naive
solution from working, which might point us in the right direction:

First, the GRUB menu is trying to find a file system with label
"gnu-disk-image" (via "search --label --set gnu-disk-image"), which
won't work because there is no file system with that label in the
resulting image.  Possible fix: the manual for grub-mkrescue says "The
root device will be set up appropriately on entering your 'grub.cfg'
configuration file", so perhaps we can simply omit our --search.  FYI,
the boot process continues successfully past this point precisely
because GRUB has already set the root; the fact that our search command
failed generates an error message but does not change the fact that it
succeeds in booting to the initrd.
  
Second, the init process from the initrd (I think that's what it's
called?) is trying to look for a file system with label
"gnu-disk-image", which it never finds.  It just sits there waiting to
find it, and it never shows up, so it freaks out.  Possible solution:
modify the behavior of our initrd's init process.  I'm not sure how to
customize the init process here, but there must be a way.  We'll
probably also need the kernel module that enables reading of iso9660
file systems, if it wasn't present already.

If you don't like grub-mkrescue, you can "roll your own" ISO generation
program, like Nix does by customizing the xorriso invocation [1]...  But
honestly, it looks pretty complicated [2].  So if we can let
grub-mkrescue do that work for us, that would be swell.

I've attached my raw notes that I took while trying to figure this out.
Hopefully somebody will find it useful.  Like most things that have to
do with computers, it's surprisingly complicated.  I'm very impressed by
what the NixOS folks were able to accomplish!

[1]
https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/installer/cd-dvd/iso-image.nix
https://github.com/NixOS/nixpkgs/blob/master/nixos/lib/make-iso9660-image.nix
https://github.com/NixOS/nixpkgs/blob/master/nixos/lib/make-iso9660-image.sh

[2] The xorriso-fs manual page (info xorriso-fs) explains most of the
options that Nix uses pretty well.  The xorriso manual (info xorriso) is
a bit more dense.  To be honest, I had a hard time figuring out what we
would have to do while reading xorriso's manual; xorriso-fs's manual is
more approachable.

-- 
Chris