VMREADVMWRITE

VMREAD and VMWRITE are privileged instructions in Intel's VMX (Virtual Machine Extensions) technology, used to access fields within the Virtual Machine Control Structure (VMCS). The VMCS is a per-virtual-machine data structure that stores guest-state, host-state, and VM-execution control information that governs how a guest OS executes on the physical processor.

Operation and purpose:

VMREAD reads the value of a specified VMCS field into a 64-bit general-purpose register, while VMWRITE writes a 64-bit value from a general-purpose register into a specified VMCS field. Each instruction identifies the target VMCS field by its encoding and uses a destination or source register to transfer the value. In typical assembly syntax, you will see forms like VMREAD rax, VMCS_FIELD and VMWRITE VMCS_FIELD, rax, where VMCS_FIELD denotes the field to access.

Requirements and behavior:

These instructions require the processor to be in VMX operation (enabled via VMXON). They are privileged when not executing inside a VMX-enabled context, and improper use (such as invalid or read-only fields) can generate faults or VM exits. VMREAD/VMWRITE do not by themselves cause VM entries or exits; instead, they manipulate the VMCS, which in turn controls behavior during VM-entry and VM-exit transitions.

Usage context:

Hypervisors and virtual machine monitors use VMREAD and VMWRITE to inspect and configure VMCS fields, such as guest-state fields (for example, guest RIP or RSP) and VM-execution control fields (for example, interruptibility states or control masking). They are fundamental tools for managing VM state and behavior in VMX-based virtualization.

See also:

VMX, VMCS, VM-entry, VM-exit, Intel 64 and IA-32 Architectures Software Developer's Manual.