Re: multiboot2 and module2 boot issues via GRUB2

[Roman Shaposhnik <roman@zededa.com>]

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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@citrix.com>
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.

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