I have a relatively new install. When I upgraded to 6.10 my server only shows a single core, but when I rolled back to 6.9 it properly showed the 4 core 8 thread CPU. Mind you this is a I7-6700K. Unraid even see's the correct CPU in the dashboard but only shows the single core, CPU 0. 

 

The motherboard is a MSI Z170A GAMING M7 (MS-7976) with a BIOS of BIOS dated: Thu 28 Jun 2018 12:00:00 AM CDT. 

 

I have kept the server current on Unraid supported releases, so I am now at 6.11.5, but haven't had my quad core CPU shown since 6.9. I have attached diagnostics to review. I can see when it boots that there are errors on the CPU, but I'm unsure how to resolve those. 

hawkeye-diagnostics-20230413-0829.zip

MCE errors are being logged and some CPUs offlined:

 

On-line CPU(s) list:             0
Off-line CPU(s) list:            1-7

 

One thing you can try is to disable mitigations, see if that helps.

I was able to add the Disable Mitigations plugin and reboot the server. The dashboard still shows only 1 cpu core. Attaching the updated diagnositics. 

hawkeye-diagnostics-20230413-1129.zip

Still errors, looks like a CPU problem:

 

Apr 13 11:15:31 Hawkeye  mcelog: CPU 0 on socket 0 has large number of corrected cache errors in Level-3 Instruction
Apr 13 11:15:31 Hawkeye  mcelog: System operating correctly, but might lead to uncorrected cache errors soon
Apr 13 11:15:31 Hawkeye  mcelog: Running trigger `cache-error-trigger' (reporter: yellow)
Apr 13 11:15:31 Hawkeye mcelog: Offlining CPU 0 due to cache error threshold
Apr 13 11:15:31 Hawkeye mcelog: Offlining CPU 0 failed
Apr 13 11:15:31 Hawkeye mcelog: Offlining CPU 1 due to cache error threshold
Apr 13 11:15:31 Hawkeye mcelog: Offlining CPU 2 due to cache error threshold
Apr 13 11:15:31 Hawkeye mcelog: Offlining CPU 3 due to cache error threshold
Apr 13 11:15:31 Hawkeye mcelog: Offlining CPU 4 due to cache error threshold
Apr 13 11:15:31 Hawkeye mcelog: Offlining CPU 5 due to cache error threshold
Apr 13 11:15:31 Hawkeye mcelog: Offlining CPU 6 due to cache error threshold
Apr 13 11:15:31 Hawkeye mcelog: Offlining CPU 7 due to cache error threshold

 

How would this prove to be a hardware issue if I was able to see all 4 cores and 4 logical cores on 6.9, but moving to 6.10 only shows a single core? I had also reverted my upgrade back to 6.9 to prove it wasn't a hardware issue and all 4 cores and 4 logical cores were present. Upgraded again, and again only a single core showed. 

Newer kernel might be detecting a new error, in any case that's a Linux error, nothing much LT or Unraid can do.