As shared back in 2022 in my Homelab Considerations for vSphere 8.x blog post, if you attempt to install ESXi on system where the CPU is no longer being supported by the OEM vendor, who then informs VMware on what to publish on the VMware Hardware Compatibility Guide (VCG), you will see the following error message preventing you from proceeding.
Also mentioned in that article, you can override this default behavior with the following ESXi boot option: allowLegacyCPU=true which would convert this from an error to warning message but allowing you to proceed in an installation or upgrade.
Obviously, this is not officially supported by VMware and its behavior was never guarantee but it certainly was a nice gesture, in my humble opinion from Engineering, as they could have easily not allowed any override based on on our official VCG. For homelab use cases, it might be okay to use a CPU that is 5 to 6 years old, knowing that it should be replaced hopefully in the near future.
I just had a couple of users report issues while upgrading to the latest ESXi 8.0 Update 2 release when using the allowLegacyCPU boot option and saw the following error message
VMB: 716: Unsupported CPU
Since this was only the second occurrence of this issue, still waiting to hear back from the first reported issue, I thought I would check with Engineering to see if there was any changes in latest ESXi 8.0 Update 2 release because I was not able to reproduce this with my oldest system, which was a 2011 Apple Mac Mini.
It looks like with ESXi 8.0 Update 2, we now have a minimum requirement for a CPU processor to support the XSAVE instruction, which was introduced back in 2011 with both Intel Sandy Bridge and AMD Bulldozer processors.
The CPU that this user was attempting to install ESXi is an Intel Westmere, which was prior generation to Sandy Bridge and does NOT support the XSAVE CPU instruction and this is why the installer is preventing the installation even when the override boot option is provided. As of publishing this blog post, you can still go up to ESXi 8.0 Update 1c and anything that maybe released in future that incorporate this requirement, you will not be able to install and/or upgrade to.
While I am sympathetic in prolonging hardware for homelab use cases, especially what looks to be functional hardware, but I think a refresh is probably needed if you are trying to use a CPU that is more than a decade old 🙂