VMXON

VMXON is an x86 instruction used to enable VMX (Virtual Machine Extensions) operation on a processor that supports Intel VT-x. It transitions the processor into VMX root operation, establishing the virtualization environment on that core. VMXON requires a 64-bit physical address to a memory region known as the VMXON region. The region must be 4 KB in size and aligned to a 4 KB boundary, and it must be writable by the executing software.

Before VMXON can be executed, VMX support must be enabled and permitted. The processor requires VMX to be enabled via the VMXE bit in CR4, and the IA32_FEATURE_CONTROL MSR must allow VMX operation (typically configured to permit VMX outside of SMX). The first 4 bytes of the VMXON region must contain the VMX revision identifier, which comes from the IA32_VMX_BASIC MSR. If the revision does not match, or if the region is not properly prepared, VMXON will fail.

On successful execution, the processor enters VMX operation; virtualization state is then managed using a separate data structure called the Virtual Machine Control Structure (VMCS). The VMXON region itself is not the VMCS, but it enables VMX operation that allows VMCS-based control of guest and host states. VMXON remains active while the processor operates in VMX mode and is paired with VMXOFF to exit VMX operation.

VMXON is a per-core operation and is typically invoked by a hypervisor or the operating system during initialization as part of enabling hardware-assisted virtualization. Related concepts include VMCS, VMXOFF, and the various VMX instructions that manage entries, exits, and transitions between host and guest contexts.