Built a new WDL_Arch64 a week or so ago using new f_plugin - booted fine. Modified that plugin but slightly (to include a few alternative apps and remove some others) and built new WDL_Arch64 today. Odd... last week's fine, this week Kernel panic...
Known major dIfference: kernel/modules; last week's used kernel/modules version 5.12.15-arch1-1, the newest uses 5.13.5-arch1-1
Certainly the newest build works fine as long as I swap kernel/modules (in the initrd.gz and firstrib_rootfs) back to 5.12.15-arch1-1 and reboot.
However, may or may not be the kernel itself that is the problem. I note a major difference in packaging - Arch now providing the modules in compressed .zst form, previously used .xz
My latest build initrd.gz certainly seems to modprobe the main module list okay (watched via wd_debug grub kernel addition), but didn't find the boot partition so modules not loaded correctly actually (and also said (later in initrd/init script) that overlay module was 'wrong format' (that's when I checked and noticed zstd compressed). Not sure at the moment if the kernel is at fault (or somehow, but unlikely I feel, incompatible with my computer) - thought it might instead by busybox modprobe not being able to handle the .zst compressed module, but swapped the busybox.net one for the latest official arch static busybox and still same errors trying to boot with kernel/modules 5.13.5-arch1-1. The Arch Linux kernels have themselves supported zstd for some time. In the probably unlikely event Arch themselves have blundered in the xz to zstd format changeover for modules compression I'll wait a bit and see if any online reports (and probably try another new build next week). For now will just use the new build with the older kernel since working fine.
If no go, later I'll try using Arch's mkinitcpio (along with Arch's initrd special non-static busybox from mkinitcpio-busybox package) and experiment with simply using WDL init/w_init in there - may be a better way to do it anyway. EDIT: Actually, I'm looking into that alternative now, and yes, I think that is the likely issue - Arch's official initramfs uses kmod and not busybox modprobe and kmod includes zstd support. I'll try modifying that initramfs to use WDL init and w_init, which may prove to be a good solution.