I am using Devuan on an HP laptop with intel 64bit cpu. Everything
worked very well, I did a lot of compilation and it is very stable,
never had a freeze in months!

[    0.100000] smpboot: CPU0: Intel(R) Core(TM)2 CPU T7200  @ 2.00GHz
(family: 0x6, model: 0xf, stepping: 0x6)
[    0.100000] Performance Events: PEBS fmt0-, Core2 events, Intel PMU
driver.
[    0.100000] core: PEBS disabled due to CPU errata

Yesterday I installed a kernel upgrade, bad things happened

1) after the first reboot with the new kernel, I get up to my desktop,
check out sources ad start building Arctic Fox browser, come back after
a time and find the machine completely frozen - no disk activity, no
mouse possible, no errors. No response to power button pressed (had to
press 5 seconds)

2) at reboot, machine freezes quite early in the boot process

3) I retry and it still freezes

I tried selecting in GRUB the older kernel and it boots. It goes past
the last error, starts file system check/journal replay and the machine
seems stable again.

This is the last good kernel version:

4.9.0-11-amd64 #1 SMP Debian 4.9.189-3+deb9u2 (2019-11-11) x86_64 GNU/Linux

the unstable version must be the version 4.9.210-1 installed

What could the issue be? I read about backports of spectre mitigations
being possible issues.