Windows Server 2012 virtual machine fails to recognize hot-added virtual CPUs

Products

• VMware vSphere ESXi

Issue/Introduction

Symptoms:
Windows Device Manager detects hot-added virtual CPUs (vCPUs) but vCPUs are not accessible when:

• You are running a Windows Server 2012 Datacenter Edition virtual machine.
• You are running a virtual machine with virtual machine hardware version 9 (virtual machine compatibility level ESXi 5.1 and later).

Cause

This is a known issue affecting virtual machine hardware version 9 (virtual machine compatibility level ESXi 5.1 and later).

Resolution

This issue is resolved in ESXi 5.1 Patch 2. For the latest version, see VMware Downloads.

Alternatively, you can use virtual machine hardware version 8 (virtual machine compatibility level ESXi 5.0 and later) which is not affected. To downgrade the hardware level, see Downgrading the virtual machine hardware version in ESX/ESXi (1028019).