A VM exit is a fundamental concept in virtualization that describes a specific type of transition within a system running a virtual machine.
At its core, a VM exit is the switch back from the guest environment (where the virtual machine is running, typically in a less privileged "non-root" mode) to the host environment (where the hypervisor or Virtual Machine Monitor (VMM) operates, typically in a more privileged "root" mode).
Think of it as the virtual machine temporarily relinquishing control back to the software managing it (the hypervisor).
Understanding the Context: VM Entry and Exit
Virtualization involves two primary modes or states for the processor:
- Root Mode: This is the privileged mode where the hypervisor runs. It has full control over the hardware.
- Non-Root Mode: This is the less privileged mode where the guest operating system and its applications run inside the VM. It has limited direct hardware access.
The transition between these modes is key:
- VM Entry: The switch from the root mode (hypervisor) to the non-root mode (guest VM).
- VM Exit: The switch back from the non-root mode (guest VM) to the root mode (hypervisor).
What Causes a VM Exit?
VM exits don't happen randomly. They are typically triggered by specific actions or events occurring within the guest VM. The control over which guest operations will cause VM exits is managed by the VMCS (Virtual Machine Control Structure).
The VMCS is a data structure used by the processor to control VM operations. It includes settings that define which guest actions are "sensitive" and require hypervisor intervention.
Common reasons for a VM exit might include:
- Attempting to execute certain privileged instructions that the guest is not allowed to perform directly.
- Accessing specific I/O ports or memory-mapped I/O regions.
- Handling external interrupts or events directed at the guest.
- Internal exceptions within the guest that the hypervisor needs to handle (e.g., page faults requiring device emulation).
- Voluntary actions by the guest requesting hypervisor services (e.g., paravirtualization calls).
When one of these configured events occurs in the guest, the processor automatically triggers a VM exit, transferring control back to the hypervisor.
Role of the VMCS During VM Exit
The VMCS plays a crucial role during a VM exit:
- State Saving: The processor saves the state of the guest (registers, instruction pointer, etc.) into a dedicated area within the VMCS.
- State Loading: The processor loads the state of the hypervisor from another area within the VMCS, preparing the hypervisor to resume execution.
- Exit Information: The VMCS also contains information about the cause of the VM exit, allowing the hypervisor to understand what happened and respond appropriately.
By controlling which actions cause exits, the VMCS allows the hypervisor to intercept sensitive guest operations and emulate them safely or provide necessary services, maintaining isolation and control over the hardware.
