Re: grub2 + xen + lz4 kernels

[Juergen Gross]

On 30.11.21 00:25, Shaun Reitan wrote:
> I currently use XEN to boot PV (paravirt) virtual server instances for 
> our customers. Grub2 introduced support for booting a xen kernel 
> directly from a guests disk image which has worked great for years. We 
> use the following command to build our image
>
> grub-mkstandalone -O x86_64-xen -o grub2-x86_64.gz boot/grub/grub.cfg
>
> What we have been seeing more and more is newer distros like Ubuntu 
> 20.04 using lz4 compressed kernel images which will not boot and 
> displays the error "not xen image."
>
> If i use the 
> https://raw.githubusercontent.com/torvalds/linux/master/scripts/extract-vmlinux 
> <https://raw.githubusercontent.com/torvalds/linux/master/scripts/extract-vmlinux> utility 
> to decompress the kernel image I am able to boot it without any issues 
> using our current grub2 xen loader.
>
> My question is how can I get lz4 support added into grub2 for xen? We 
> are willing to pay for a dev to add this support if needed as I have 
> limited time to really dig into this.
>
> There are a few workarounds out their, most involve a hook that 
> decompresses the kernel after an update but I'm not wanting to add 
> complexity to the kernel update process. It leaves too much room for 
> error and the possibility of a clients server failing to boot after what 
> should of been a simple kernel upgrade.
>
> Any help, even if just pointing me in the right direction would be 
> appreciated!

Hmm, Grub2 already supports some compression methods, so adding LZ4
shouldn't be that hard. Especially as there already is some LZ4
support hidden in the ZFS handling.

I guess this LZ4 support wants to be put into a grub module of its
own and then be added to the filter list.

You should have a look at:

- GRUB_FILE_FILTER_GZIO for an example how a compression filter is
  added via grub_file_filter_register()

- the file grub-core/fs/zfs/zfs_lz4.c for current LZ4 support in
  Grub2

This should basically do the job.

I've added Daniel to Cc: as he might have some more thoughts.


Juergen

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