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

From: Andy Smith
Subject: Confusion regarding LZ4-compressed kernel support and Xen (PV)
Date: Sat, 25 Apr 2020 23:38:51 +0000
User-agent: Mutt/1.5.23 (2014-03-12)


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

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

Cloning just now and
rebuilding my PV grub image gives the same result.

I see the error message comes from:

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

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:

…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.


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