Regression in 7.1-rc1 in booting Ubuntu distro kernel

[Steve French]

I notice failures in booting Ubuntu 25.10 on my Lenovo Thinkpad laptop
since 7.1-rc1 kernel.  I have not seen this error before.

"KERNEL PANIC! Please reboot your computer.  VFS: Unable to mount root
fs on unknown-block(0,0)"

which doesn't make any sense because that isn't the root fs.  /etc/fstab shows:

/dev/disk/by-uuid/6ca90829-120e-4d09-be02-46422daffdcd / ext4 defaults 0 1


The last kernel that booted successfully was the daily build from the
day before (4/26/26
https://kernel.ubuntu.com/mainline/daily/2026-04-26/).  It also fails
on current daily builds and on 7.1-rc2 kernel
(https://kernel.ubuntu.com/mainline/v7.1-rc2/) so I have been stuck
doing my testing on out of tree builds booting from a few weeks old
kernel.

Any ideas how to find out what regressed in the mainline kernel in
7.1-rc1 (or in the ubuntu mainline daily build) or how to workaround
or debug this?
-- 
Thanks,

Steve