Comment 7 for bug 2012763

I launched a VM via LXC using qemu and verified that it does only create / attach a single CPU core:
root@maximum-porpoise:~# lscpu
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
Address sizes: 39 bits physical, 48 bits virtual
CPU(s): 1
On-line CPU(s) list: 0
Thread(s) per core: 1
Core(s) per socket: 1
Socket(s): 1
NUMA node(s): 1
Vendor ID: GenuineIntel
CPU family: 6
Model: 165
Model name: Intel(R) Core(TM) i9-10900F CPU @ 2.80GHz

Note my machine has a single 10 core CPU with HT enabled:
Architecture: x86_64
  CPU op-mode(s): 32-bit, 64-bit
  Address sizes: 39 bits physical, 48 bits virtual
  Byte Order: Little Endian
CPU(s): 20
  On-line CPU(s) list: 0-19
Vendor ID: GenuineIntel
  Model name: Intel(R) Core(TM) i9-10900F CPU @ 2.80GHz
    CPU family: 6
    Model: 165
    Thread(s) per core: 2
    Core(s) per socket: 10

So I do suspect that qemu itself simply fails on systems with more than 288 cores regardless of the config of the VM...

The servers that are failing have dual 96 core AMD EPYC 9654 96-Core Processor, which, with hyperthreading provides 384 CPU cores to the system.

I've gone back and asked them to disable hyperthreading to get the CPU count down to 192 cores to see if qemu works then or not... if the same test succeeds with that config, I think that would certainly confirm the issue.