Kernels 128, 131 and 132 won't boot on Pentium 3, attempted to kill the idle task

[Boris Gjenero]

I am running Ubuntu 18.04.6 LTS on an old PC with a Pentium 3 and Soltek SL-65KVB motherboard. The 5.4.0 generic kernels work fine up to and including release 126. Attempts to boot 128, 131 and 132 quickly lead to a panic every time. The error seems to be "Attempted to kill the idle task". I tried idle=halt and idle=poll kernel command line options without any change. The installation procedure for the kernels seems fine, because I installed kernel 125 after installing the problem kernels, and it worked.

I am attaching a photo of what the screen shows when the crash happens. I could try to capture more of that via a serial port if that would be helpful.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: linux-image-5.4.0-126-generic 5.4.0-126.142~18.04.1
ProcVersionSignature: Ubuntu 5.4.0-126.142~18.04.1-generic 5.4.203
Uname: Linux 5.4.0-126-generic i686
ApportVersion: 2.20.9-0ubuntu7.28
Architecture: i386
CurrentDesktop: XFCE
Date: Sat Nov 19 18:01:57 2022
InstallationDate: Installed on 2012-10-07 (3695 days ago)
InstallationMedia: Mythbuntu 12.04.1 "Precise Pangolin" - Release i386 (20120818.1)
SourcePackage: linux-hwe-5.4
UpgradeStatus: Upgraded to bionic on 2018-04-08 (1686 days ago)

[Boris Gjenero (boris-gjenero)]

After adding mitigations=off to the kernel command line, 5.4.0-132-generic boots successfully every time. This workaround satisfies me. I don't need those mitigations on that old PC, and turning them off probably speeds it up a bit.