Linux 5.10 can't be booted by Xen pvgrub2

[alyssais]

Booting NixOS from Xen using pvgrub2 fails with a 5.10 kernel. A 5.4 kernel works fine. I don't have the error message to hand, but will keep this updated as I track down what's going on. Wanted to open an issue ASAP to have this documented to make it easier for other people who might run into this.

[alyssais]

The error is:

error: not a xen image.
error: you need to load the kernel first.

[alyssais]

The problem is that pvgrub2 doesn't support zstd compression yet, which we're using on >=5.9 kernels.

See also:

• https://bugs.gentoo.org/753728
• https://bugs.archlinux.org/task/68445

(Thanks @puckipedia.)

I don't think there's anything we can do on the NixOS side about this.