VMCS

VMCS stands for Virtual Machine Control Structure. It is a data structure used by Intel's virtualization technology (VMX) to manage transitions between a guest operating system (the virtual machine) and the hypervisor or host. The VMCS stores both the state of the guest and the state to be loaded on exit, along with a set of control fields that govern how VM entries and exits occur. Each logical processor that runs a virtual machine maintains its own VMCS.

A VMCS resides in memory as a region with a VMCS revision identifier and a collection of fields organized into three groups: guest-state fields, host-state fields, and control fields. The guest-state fields capture the current state of the running guest, such as segment registers, control registers, and instruction pointers. The host-state fields define what state to restore when the guest is paused or exits, including the host's stack pointer and instruction pointer. The control fields include VM-entry and VM-exit controls (such as pin-based, processor-based, and secondary processor-based controls), as well as bitmaps for allowed or blocked events (exception bitmap, I/O bitmap, MSR bitmap), and other behavior modifiers like the TSC offset and VM-exit information.

Operations with VMCS typically involve the VMX instructions VMXON/VMXOFF to enable or disable VMX operation, VMCS loading with VMPTRLD to select the active VMCS region, and VMREAD/VMWRITE to read or modify fields. During execution, a VMEXIT transfers control to the host, with state saved in the VMCS, and on VMRESUME the guest state is restored. Proper setup of the VMCS is essential for correct and secure virtualization. Nested virtualization can leverage a VMCS linking pointer to chain multiple VMCS regions.