OSs are not most software and are not designed around the instruction set changing underneath them during normal operation. Why would they be? You can't physically swap out a processor while the system is booted and you can't swap a virtual processor either.
It works fine if you shut everything down and reboot the system, but that is often undesirable.
The whole point of the feature is that the VM can be migrated around different physical hardware without having to interrupt service. It just suddenly is running on a different host instance. But it has to be the same type of processor... or at least the same feature set. "Close" is not good enough, it needs to be a 1:1 match.
You can manually disable features until you have found the lowest common denominator between the feature sets of the different processors. But obviously the more types of processors you have in your cluster, the more problematic this is. In very few clusters will you find servers of mixed types, you buy 10,000 of the same server and operate them as a "unit". You don't just add in servers after the fact, sometimes you don't even replace failed servers.
And that hardware decision will have been made years ago, very often. The server market is hugely inertial, it's nothing like you putting together a build one evening and then going out and buying parts and putting it together.
It works fine if you shut everything down and reboot the system, but that is often undesirable.
The whole point of the feature is that the VM can be migrated around different physical hardware without having to interrupt service. It just suddenly is running on a different host instance. But it has to be the same type of processor... or at least the same feature set. "Close" is not good enough, it needs to be a 1:1 match.
You can manually disable features until you have found the lowest common denominator between the feature sets of the different processors. But obviously the more types of processors you have in your cluster, the more problematic this is. In very few clusters will you find servers of mixed types, you buy 10,000 of the same server and operate them as a "unit". You don't just add in servers after the fact, sometimes you don't even replace failed servers.
And that hardware decision will have been made years ago, very often. The server market is hugely inertial, it's nothing like you putting together a build one evening and then going out and buying parts and putting it together.