Confusion regarding LZ4-compressed kernel support and Xen (PV)

[Andy Smith <andy@strugglers.net>]

Hi,

I am booting Xen PV guests using a grub image compiled as a PV guest
and then having that parse the /boot/grub/grub.cfg from within the
guest.

When this encounters an LZ4-compressed kernel I get this:

    Loading Linux 5.4.0-26-generic ...
    error: not xen image.                [ vmlinuz-5.4.0-26-gen  10B  0%  0.00B/s ]
    Loading initial ramdisk ...
    error: you need to load the kernel first.

    Press any key to continue...

I know it is a problem related to LZ4 compression because if I
manually decompress that kernel image then I am able to boot it
fine.

Cloning https://git.savannah.gnu.org/git/grub.git just now and
rebuilding my PV grub image gives the same result.

I see the error message comes from:

grub-core/loader/i386/xen_file.c
92:  grub_error (GRUB_ERR_BAD_OS, "not xen image");

I cannot see any evidence that there is LZ4 support in that file,
but I don't really know what I'm looking for so quite possibly I am
wrong.

Searching the web finds articles about Ubuntu's decision to switch
to LZ4-compressed kernels and some of these mention that this
required changes to grub. I was also unable to find any actual
details of the patch/commit for that. Again, if it's present at:

    https://git.launchpad.net/~ubuntu-core-dev/grub/+git/ubuntu

…then it's not obvious to me.

Does upstream grub have support for booting LZ4-compressed kernels?

Is that also expected to work when grub is booted as a Xen PV image?

I am just a bit lost as to where to look to find a solution to this,
so any pointers would be much appreciated.

Thanks,
Andy