A Virtual machine monitor (VMM) is a thin layer that provides a virtual x86 hardware environment to the guest operating system on a VM. This hardware includes a virtual CPU, virtual I/O devices, and timers. A VMM leverages key technologies in VMkernel, such as scheduling, memory management, and the network and storage stacks.
Each VMM is devoted to one VM. To run multiple VMs, VMkernel starts multiple VMM instances, also known as worlds. Each VMM instance partitions and shares the CPU, memory, and I/O devices to successfully virtualize the system. A VMM can be implemented using hardware virtualization, software virtualization (binary translation), or paravirtualization (which is deprecated) techniques.
Paravirtualization refers to the communication between the guest operating system and the hypervisor to improve performance and efficiency. The value proposition of paravirtualization is in lower virtualization overhead, but the performance advantage of paravirtualization over hardware or software virtualization can vary greatly depending on the workload. Because paravirtualization cannot support unmodified operating systems (for example, Windows 2000/XP), its compatibility and portability are poor.
Paravirtualization can also introduce significant support and maintainability issues in production environments because it requires deep modifications to the operating system kernel, and for this reason, it was most widely deployed on Linux-based operating systems.
To step through this recipe, you need a running ESXi Server, a VM, a vCenter Server, and vSphere Web Client. No other prerequisites are required.
Perform the following steps to get started:
- Open up vSphere Web Client.
- On the home screen, navigate to
Hosts and Clusters. - Expand the left-hand navigation list.
- In the VM inventory, right-click on
virtual machine, then click onEdit Settings. TheVirtual Machine Edit Settingsdialog box appears. - On the
Virtual Hardwaretab, expand theCPUsection.
- Change the
CPU/MMU Virtualizationoption underAdvancedto one of the following options:
AutomaticSoftware CPU and MMUHardware CPU, Software MMUHardware CPU and MMU
- Click on
OKto save your changes.
- For the change to take effect, perform one of these actions:
- Power cycle or reset the VM
- Suspend and then resume the VM
- vMotion the VM
The VMM determines a set of possible monitor modes to use and then picks one to use as the default monitor mode unless something other than Automatic has been specified. The decision is based on:
- The physical CPU's features and guest operating system type
- Configuration file settings
There are three valid combinations for the monitor mode, as follows:
- BT: Binary translation and shadow page tables
- HV: AMD-V or Intel VT-x and shadow page tables
- HWMMU: AMD-V with RVI or Intel VT-x with EPT (RVI is inseparable from AMD-V, and EPT is inseparable from Intel VT-x)
BT, HV, and HWMMU are abbreviations used by ESXi to identify each combination.
When a VM is powering on, the VMM inspects the physical CPU's features and the guest operating system type to determine the set of possible execution modes. It first finds the set of modes allowed. Then it restricts the allowed modes by configuring file settings. Finally, among the remaining candidates, it chooses the preferred mode, which is the default monitor mode. This default mode is then used if you have Automatic selected.
For the majority of workloads, the default monitor mode chosen by the VMM works best. The default monitor mode for each guest operating system on each CPU has been carefully selected after a performance evaluation of the available choices. However, some applications have special characteristics that can result in better performance when using a non-default monitor mode. These should be treated as exceptions, not rules.
The chosen settings are honored by the VMM only if the settings are supported on the intended hardware. For example, if you select Software CPU and MMU for a 64-bit guest operating system running on a 64-bit Intel processor, the VMM will choose Intel VT-x for CPU virtualization instead of BT. This is because BT is not supported by the 64-bit guest operating system on this processor.
The virtual CPU consists of the virtual instruction set and the virtual memory management unit (MMU). An instruction set is a list of instructions that a CPU executes. MMU is the hardware that maintains the mapping between the virtual addresses and physical addresses in the memory.
The combination of techniques used to virtualize the instruction set and memory determines the monitor execution mode (also called the monitor mode). The VMM identifies the VMware ESXi hardware platform and its available CPU features and then chooses a monitor mode for a particular guest operating system on that hardware platform. It might choose a monitor mode that uses hardware virtualization techniques, software virtualization techniques, or a combination of hardware and software techniques.
We have always had a challenge in hardware virtualization. x86 operating systems are designed to run directly on bare metal hardware, so they assume that they have full control over the computer hardware. The x86 architecture offers four levels of privileges to operating systems and applications to manage access to the computer hardware: ring 0, ring 1, ring 2, and ring 3. User-level applications typically run in ring 3; the operating system needs to have direct access to the memory and hardware and must execute its privileged instructions in ring 0.
Binary translation allows the VMM to run in ring 0 for isolation and performance while moving the guest operating system to ring 1. Ring 1 is a higher privilege level than ring 3 and a lower privilege level than ring 0.
VMware can virtualize any x86 operating system using a combination of binary translation and direct execution techniques. With binary translation, the VMM dynamically translates all guest operating system instructions and caches the results for future use. The translator in the VMM does not perform a mapping from one architecture to another; that would be emulation, not translation. Instead, it translates from the full unrestricted x86 instruction set issued by the guest operating system to a subset that is safe to execute inside the VMM. In particular, the binary translator replaces privileged instructions with sequences of instructions that perform the privileged operations in the VM rather than on the physical machine. This translation enforces encapsulation of the VM while preserving the x86 semantics as seen from the perspective of the VM.
Meanwhile, user-level code is directly executed on the processor for high-performance virtualization. Each VMM provides each VM with all of the services of the physical system, including a virtual BIOS, virtual devices, and virtualized memory management.
In addition to software virtualization, there is support for hardware virtualization. This allows some of the work of running virtual CPU instructions to be offloaded onto the physical hardware. Intel has the Intel Virtualization Technology (Intel VT-x) feature. AMD has the AMD Virtualization (AMD-V) feature. Intel VT-x and AMD-V are similar in aim but different in detail. Both designs aim to simplify virtualization techniques.