I'm on a laptop with a

Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz

processor running Slackware 14.2 at the moment and kernel 4.18.0 (I compiled it).

I have iucode_tool and intel-microcode installed from SlackBuilds.

These used to work fine on my current kernel, with the microcode loading early as evidenced by the message to that effect in dmesg. However, after the latest intel-microcode update (to version 20180807), I am getting this when running 'dmesg | grep micro':

--
[ 0.000000] [Firmware Bug]: TSC_DEADLINE disabled due to Errata; please update microcode to version: 0x52 (or later)
[ 0.205052] microcode: sig=0x806e9, pf=0x80, revision=0x4e
[ 0.205270] microcode: Microcode Update Driver: v2.2
--

It has apparently stopped loading early. I get the same result even upon rolling back to earlier versions of intel-microcode (which I've kept in /tmp/, where they were saved upon initial installation).

Any ideas as to how I might debug / fix this?

There are some recent reports about your problem on the Internet and the single "plausible" resolution looks to be upgrading the BIOS.
The BIOS, if any, might contain new microcode and might bring your CPU to the "0x52 (or later)" required level.
Your CPU is pretty new - Kaby Lake U - latest microcode revision is 0x8E (Page 12):
https://www.intel.com/content/dam/ww...e-guidance.pdf

Thanks for that! It actually started working, with dmesg showing early loading:

--
[ 0.000000] microcode: microcode updated early to revision 0x8e, date = 2018-03-24
[ 0.803993] microcode: sig=0x806e9, pf=0x80, revision=0x8e
[ 0.804068] microcode: Microcode Update Driver: v2.2.
--

I'll monitor it for a bit and eventually mark this as solved (I'd like to keep an eye on it for a while longer). I don't know that anything I did helped, but for what it's worth, the sequence of events was:

- I tried booting the standard 14.2 huge kernel (4.4.whatever; I forget the precise version number, but whatever is currently the latest on 14.2, which I keep up to date), and the microcode loaded successfully on that.

- I then booted the newer kernels (some 4.18 release candidates from earlier and 4.18 proper) and the microcode loaded fine on those too (though it wasn't before, as reported in the original post).

--

I don't know if it even makes sense to expect that trying the older kernel first might make a difference, but the above is what I experienced and it may be that someone else finds this of some use.