VMexits

VMexits are events in hardware-assisted virtualization where execution inside a guest virtual machine is interrupted by the hypervisor. They occur when the guest attempts a privileged operation or accesses resources that must be mediated for isolation and control. Technologies such as Intel VT-x and AMD-V implement VM exits, and each exit is associated with a reason code describing why the transition occurred.

When a VM exit happens, the processor switches from running the guest to executing a hypervisor-defined exit handler. The hypervisor saves the guest state, analyzes the exit reason, and emulates or handles the requested operation, such as performing an I/O port operation, reading or writing a model-specific register, or resolving a memory access. After handling, the hypervisor resumes the guest, typically via a VM resume instruction.

Common VM exit causes include: I/O instructions (in/out to ports), reads or writes of MSRs, CPUID, control register or debug register accesses, external interrupts and NMIs, HLT lifecycle events, and EPT violations or guest page faults. Exits can also occur for certain VMX instructions or for instructions that trap into the hypervisor for various reasons.

VMexits have a measurable impact on performance because each one involves a mode switch between the guest and the hypervisor. Virtualization systems minimize exits through techniques such as optimized device emulation, paravirtualized interfaces, and hardware-assisted features that reduce the frequency of privileged operations requiring mediation.

They are a foundational concept in popular hypervisors and cloud platforms, contributing to isolation, security, and efficient resource sharing in virtualized environments.