Re: multiboot2 and module2 boot issues via GRUB2

[Roman Shaposhnik <roman@xxxxxxxxxx>]

Hi Andrew!

first of all -- thanks for pointing me in the right direction. So after reading relevant sources: comments inline.

On Tue, Mar 30, 2021 at 12:08 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:

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.

It is indeed what seems to be happening.

 

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

And that is also correct -- nothing like that is possible with modules.

Kernel actually needs to be aware of them. So the question then

becomes...

 

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.

...I guess there's no mechanism out-of-the box to achieve what I want?

 

And the obvious next question: is my EVE usecase esoteric enough that

I should just go ahead and do a custom GRUB patch or is there a more

general interest in this?

Thanks,
Roman.