Windows Server 2025 and Windows 11 fail to boot on ESXi 8.x Intel Hosts with "Intel Merom" EVC Mode

Products

• VMware vSphere ESXi

Issue/Introduction

Windows Server 2025 or Windows 11 Virtual Machines (VMs) power up on an ESXi 8.x host but become stuck in a continuous boot loop or hang at the boot menu. This occurs in vSphere clusters where Enhanced vMotion Compatibility (EVC) is enabled and set to the legacy "Intel Merom" baseline.

• The VM boots normally when migrated to a host/cluster with EVC disabled or set to a modern generation.

• The guest OS may briefly display an UNSUPPORTED_PROCESSOR Blue Screen of Death (BSOD) or a black screen before restarting.

• In the vmware.log of the affected VM, a Triple Fault is recorded:

In(05) vcpu-0 - Triple fault.
In(05) vcpu-0 - MsgHint: msg.monitorEvent.tripleFault
In(05)+ vcpu-0 - A fault has occurred causing a virtual CPU to enter the shutdown state. If this fault had occurred outside of a virtual machine, it would have caused the physical machine to restart. The shutdown state can be reached by incorrectly configuring the virtual machine, a bug in the guest operating system, or a problem in VMware ESX.---------------------------------------

Environment

VMware vSphere ESXi 8.x

Cause

When using EVC mode with the old set of CPU features (especially Intel Merom), attempting to boot Windows Server 2025 on a newer CPU masquerading as a Merom resulted in an UNSUPPORTED_PROCESSOR BSOD and subsequent triple fault.  

Modern Windows operating systems (Windows Server 2025 and Windows 11) require specific CPU instruction sets that are not present in the "Intel Merom" (2006) architecture.

Specifically, these OS versions require SSE4.2 and the POPCNT (Population Count) instruction. When a VM is restricted to the Merom EVC baseline, the hypervisor masks these features from the Guest OS. The Windows kernel detects the missing instructions during boot, triggers an UNSUPPORTED_PROCESSOR exception, and executes a hard reset, leading to the triple fault.

Resolution

To resolve this issue, do not attempt to run Windows Server 2025 or Windows 11 on a CPU baseline that does not meet minimum architectural requirements.

1. Upgrade EVC Mode: Increase the EVC baseline of the cluster to a generation that supports SSE4.2 and POPCNT (typically Nehalem or newer).

2. Verify Hardware Requirements: Ensure the underlying physical hardware and the configured EVC mode meet the official Microsoft requirements for the guest operating system.

   • For Windows Server 2025 requirements, see: Microsoft Hardware Requirements

   • For Windows 11 requirements, see: Microsoft Hardware Requirements