Re: multiboot2 and module2 boot issues via GRUB2

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 30/03/2021 19:28, Roman Shaposhnik wrote:
> Hi!
>
> seems like I've run into an issue with multiboot2 and module2
> commands that I can't quite explain. Since it may be something
> super simply and silly -- I wanted to reach out here before I do
> a GRUB/Xen/LK source deepdive.
>
> So here's the deal: whenever I boot straight up Linux kernel
> I can do the following sequence of commands:
>    linux /kernel
>    initrd foo.cpio.gz bar.cpio.gz
> and have linux kernel effectively stack content of bar.cpio.gz
> on top of foo.cpio.gz and present a unified initramfs that way.
>
> I'm trying to replicate it with Xen, but:
>      multiboot2 /boot/xen.gz
>      module2 /kernel
>      module2 foo.cpio.gz
>      module2 bar.cpio.gz
> only seems to be picking up foo.cpio.gz
>
> Has anyone run into this issue before?

I can explain why that happens.  Not sure if it counts as a feature, bug
or mis-expectation, but CC'ing grub-devel for their input.

The initrd command is presumably concatenating those two files together
in memory, and presenting Linux a single initrd pointer.

For the module2 example, you're putting 3 distinct files in memory, and
giving Xen a list 3 modules.

Xen is capable of taking various things via modules, such as an
XSM/Flask policy, or microcode, so has logic to identify these if
present and separate them from "other stuff".  However, there is a
hardcoded expectation that the first module is the dom0 kernel, and the
next unrecognised module, if present, is *the* initrd.

I expect that Xen isn't handing bar.cpio.gz on to dom0, but I'm not sure
whether passing two distinct initrd-like-things to Linux is even possible.

What you presumably want is some `initrd` side effect in Grub so you can
write `module2 foo.cpio.gz bar.cpio.gz` and have it concatenate things
together in memory and present one MB2 module, but I suspect that exact
syntax might be ambiguous with command line handling.  I have no idea
whether such a command currently exists.

~Andrew