Patent Publication Number: US-2006005190-A1

Title: Systems and methods for implementing an operating system in a virtual machine environment

Description:
CROSS-REFERENCE  
      This application is related by subject matter to the inventions disclosed in the following commonly assigned applications: U.S. patent application Ser. No. 10/685,051 (Atty. Docket No. MSFT-2570/305147.01), filed on Oct. 14, 2003 and entitled, “SYSTEMS AND METHODS FOR USING SYNTHETIC INSTRUCTIONS IN A VIRTUAL MACHINE”; U.S. patent application Ser. No. 10/734,450 (Atty. Docket No. MSFT-2772/305423.01), filed on Dec. 12, 2003 and entitled “SYSTEMS AND METHODS FOR BIMODAL DEVICE VIRTUALIZATION OF ACTUAL AND IDEALIZED HARDWARE-BASED DEVICES”; and U.S. patent application Ser. No. 10/274,298 (Atty. Docket No. MSFT-2564/304108.01), filed on Oct. 18, 2002 and entitled, “SOFTWARE LICENSE ENFORCEMENT MECHANISM FOR AN EMULATED COMPUTING ENVIRONMENT,” the entirety of said patent applications being hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      The present invention generally relates to the field virtual machines (also known as “processor virtualization”) and software that executes in a virtual machine environment. More specifically, the present invention relates to systems and methods for implementing an operating system that is able to ascertain whether it is running in a virtual machine environment and that is able to modify its behavior accordingly.  
     BACKGROUND OF THE INVENTION  
      Computers include general purpose central processing units (CPUs) that are designed to execute a specific set of system instructions. A group of processors that have similar architecture or design specifications may be considered to be members of the same processor family. Examples of current processor families include the Motorola 680×0 processor family, manufactured by Motorola, Inc. of Phoenix, Ariz.; the Intel 80×86 processor family, manufactured by Intel Corporation of Sunnyvale, Calif.; and the PowerPC processor family, which is manufactured by Motorola, Inc. and used in computers manufactured by Apple Computer, Inc. of Cupertino, Calif. Although a group of processors may be in the same family because of their similar architecture and design considerations, processors may vary widely within a family according to their clock speed and other performance parameters.  
      Each family of microprocessors executes instructions that are unique to the processor family. The collective set of instructions that a processor or family of processors can execute is known as the processor&#39;s instruction set. As an example, the instruction set used by the Intel 80×86 processor family is incompatible with the instruction set used by the PowerPC processor family. The Intel 80×86 instruction set is based on the Complex Instruction Set Computer (CISC) format. The Motorola PowerPC instruction set is based on the Reduced Instruction Set Computer (RISC) format. CISC processors use a large number of instructions, some of which can perform rather complicated functions, but which require generally many clock cycles to execute. RISC processors use a smaller number of available instructions to perform a simpler set of functions that are executed at a much higher rate.  
      The uniqueness of the processor family among computer systems also typically results in incompatibility among the other elements of hardware architecture of the computer systems. A computer system manufactured with a processor from the Intel 80×86 processor family will have a hardware architecture that is different from the hardware architecture of a computer system manufactured with a processor from the PowerPC processor family. Because of the uniqueness of the processor instruction set and a computer system&#39;s hardware architecture, application software programs are typically written to run on a particular computer system running a particular operating system.  
      Computer manufacturers want to maximize their market share by having more rather than fewer applications run on the microprocessor family associated with the computer manufacturers&#39; product line. To expand the number of operating systems and application programs that can run on a computer system, a field of technology has developed in which a given computer having one type of CPU, called a host, will include an emulator program that allows the host computer to emulate the instructions of an unrelated type of CPU, called a guest. Thus, the host computer will execute an application that will cause one or more host instructions to be called in response to a given guest instruction. Thus the host computer can both run software design for its own hardware architecture and software written for computers having an unrelated hardware architecture. As a more specific example, a computer system manufactured by Apple Computer, for example, may run operating systems and program written for PC-based computer systems. It may also be possible to use an emulator program to operate concurrently on a single CPU multiple incompatible operating systems. In this arrangement, although each operating system is incompatible with the other, an emulator program can host one of the two operating systems, allowing the otherwise incompatible operating systems to run concurrently on the same computer system.  
      When a guest computer system is emulated on a host computer system, the guest computer system is said to be a “virtual machine” as the guest computer system only exists in the host computer system as a pure software representation of the operation of one specific hardware architecture. The terms emulator, virtual machine, and processor emulation are sometimes used interchangeably to denote the ability to mimic or emulate the hardware architecture of an entire computer system. As an example, the Virtual PC software created by Connectix Corporation of San Mateo, Calif. emulates an entire computer that includes an Intel 80×86 Pentium processor and various motherboard components and cards. The operation of these components is emulated in the virtual machine that is being run on the host machine. An emulator program executing on the operating system software and hardware architecture of the host computer, such as a computer system having a PowerPC processor, mimics the operation of the entire guest computer system.  
      The emulator program acts as the interchange between the hardware architecture of the host machine and the instructions transmitted by the software running within the emulated environment. This emulator program may be a host operating system (HOS), which is an operating system running directly on the physical computer hardware. Alternately, the emulated environment might also be a virtual machine monitor (VMM) which is a software layer that runs directly above the hardware and which virtualizes all the resources of the machine by exposing interfaces that are the same as the hardware the VMM is virtualizing (which enables the VMM to go unnoticed by operating system layers running above it). A host operating system and a VMM may run side-by-side on the same physical hardware.  
      Current virtual machine software (such as Virtual Server and Virtual PC, sold by Microsoft Corporation) allow for virtualization as described above, but there is significant performance overhead associated with allowing for virtualization. The performance overhead can reach levels as high as 70%, particularly in software applications with heavy I/O workloads (with heavy disk access or network communications). This level of overhead is unacceptable in applications that require maximum processor speed. What is needed is a way to reduce processor overhead in a virtual machine environment.  
      In conventional operating systems (OSs), certain OS activities are performed with an assumption that the operating system is running on dedicated physical hardware. In a virtual machine environment, these activities can be detrimental to the other guest OSs that are running concurrently on the same physical hardware. These detrimental activities tie up operating system resources (designed to run on dedicated physical hardware, not in a virtual environment), because the operating system assumes that the hardware is dedicated to it, and has no knowledge of other operating systems using the resources or waiting to use them. What is needed is a way to modify the behavior of a guest OS such that it is not detrimental to other guest OSs that are running in a virtual machine environment.  
     SUMMARY OF THE INVENTION  
      The present invention includes systems for and methods of implementing an operating system that is capable of ascertaining whether it is operating in a virtual machine environment and is further capable of modifying its behavior to operate more efficiently in a virtual machine environment.  
      Embodiments of the present invention are directed to a system for and method of providing operating systems that are aware that they are operating in a virtual machine environment and, as a result of this realization, are able to reduce some of the performance overhead which has been historically problematic with the virtual machine environment. The introduction of a shared communication area between the host operating system and the guest operating systems provides a mechanism for communication between guests and host without passing control of the computer between host and guests. One example of the type of communications passed between guest operating systems and host relates to thread scheduling. With the realization that the guest is operating in a VM environment and with the introduction of the shared communications area, guest operating systems send additional information (such as execution priorities) to the host operating system, which allows the host operating system to make more-efficient thread scheduling decisions, because the host has more information regarding the overall demand (including that of guests) for processor time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing summary, as well as the following detailed description of preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings exemplary constructions of the invention; however, the invention is not limited to the specific methods and instrumentalities disclosed. In the drawings:  
       FIG. 1  is a block diagram representing a computer system in which aspects of the present invention may be incorporated;  
       FIG. 2  illustrates the logical layering of the hardware and software architecture for an emulated operating environment in a computer system;  
       FIG. 3A  illustrates a virtualized computing system;  
       FIG. 3B  illustrates an alternative embodiment of a virtualized computing system comprising a virtual machine monitor running alongside a host operating system;  
       FIG. 4  illustrates a virtualized computing system from  FIG. 3A  further comprising a host operating system with VM-aware guest operating systems;  
       FIG. 5  is a flowchart that illustrates a method of implementing a VM-aware guest operating system with the capability to modify its behavior in order to improve efficiency in a virtual machine environment;  
       FIG. 6  illustrates an exemplary virtualized computing system comprising a host operating system with shared communication areas between the host operating system and VM-aware guest operating systems; and  
       FIG. 7  is a flowchart that illustrates an exemplary method of scheduling threads in a VM-aware operating system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS  
      The inventive subject matter is described with specificity to meet statutory requirements. However, the description itself is not intended to limit the scope of this patent. Rather, the inventor has contemplated that the claimed subject matter might also be embodied in other ways, to include different steps or combinations of steps similar to the ones described in this document, in conjunction with other present or future technologies. Moreover, although the term “step” may be used herein to connote different elements of methods employed, the term should not be interpreted as implying any particular order among or between various steps herein disclosed unless and except when the order of individual steps is explicitly described.  
      Computer Environment  
      Numerous embodiments of the present invention may execute on a computer.  FIG. 1  and the following discussion is intended to provide a brief general description of a suitable computing environment in which the invention may be implemented. Although not required, the invention will be described in the general context of computer executable instructions, such as program modules, being executed by a computer, such as a client workstation or a server. Generally, program modules include routines, programs, objects, components, data structures and the like that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the invention may be practiced with other computer system configurations, including hand held devices, multi processor systems, microprocessor based or programmable consumer electronics, network PCs, minicomputers, mainframe computers and the like. The invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices.  
      As shown in  FIG. 1 , an exemplary general purpose computing system includes a conventional personal computer  20  or the like, including a processing unit  21 , a system memory  22 , and a system bus  23  that couples various system components including the system memory to the processing unit  21 . The system bus  23  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory includes read only memory (ROM)  24  and random access memory (RAM)  25 . A basic input/output system  26  (BIOS), containing the basic routines that help to transfer information between elements within the personal computer  20 , such as during start up, is stored in ROM  24 . The personal computer  20  may further include a hard disk drive  27  for reading from and writing to a hard disk, not shown, a magnetic disk drive  28  for reading from or writing to a removable magnetic disk  29 , and an optical disk drive  30  for reading from or writing to a removable optical disk  31  such as a CD ROM or other optical media. The hard disk drive  27 , magnetic disk drive  28 , and optical disk drive  30  are connected to the system bus  23  by a hard disk drive interface  32 , a magnetic disk drive interface  33 , and an optical drive interface  34 , respectively. The drives and their associated computer readable media provide non volatile storage of computer readable instructions, data structures, program modules and other data for the personal computer  20 . Although the exemplary environment described herein employs a hard disk, a removable magnetic disk  29  and a removable optical disk  31 , it should be appreciated by those skilled in the art that other types of computer readable media which can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, random access memories (RAMs), read only memories (ROMs) and the like may also be used in the exemplary operating environment.  
      A number of program modules may be stored on the hard disk, magnetic disk  29 , optical disk  31 , ROM  24  or RAM  25 , including an operating system  35 , one or more application programs  36 , other program modules  37  and program data  38 . A user may enter commands and information into the personal computer  20  through input devices such as a keyboard  40  and pointing device  42 . Other input devices (not shown) may include a microphone, joystick, game pad, satellite disk, scanner or the like. These and other input devices are often connected to the processing unit  21  through a serial port interface  46  that is coupled to the system bus, but may be connected by other interfaces, such as a parallel port, game port or universal serial bus (USB). A monitor  47  or other type of display device is also connected to the system bus  23  via an interface, such as a video adapter  48 . In addition to the monitor  47 , personal computers typically include other peripheral output devices (not shown), such as speakers and printers. The exemplary system of  FIG. 1  also includes a host adapter  55 , Small Computer System Interface (SCSI) bus  56 , and an external storage device  62  connected to the SCSI bus  56 .  
      The personal computer  20  may operate in a networked environment using logical connections to one or more remote computers, such as a remote computer  49 . The remote computer  49  may be another personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the personal computer  20 , although only a memory storage device  50  has been illustrated in  FIG. 1 . The logical connections depicted in  FIG. 1  include a local area network (LAN)  51  and a wide area network (WAN)  52 . Such networking environments are commonplace in offices, enterprise wide computer networks, intranets and the Internet.  
      When used in a LAN networking environment, the personal computer  20  is connected to the LAN  51  through a network interface or adapter  53 . When used in a WAN networking environment, the personal computer  20  typically includes a modem  54  or other means for establishing communications over the wide area network  52 , such as the Internet. The modem  54 , which may be internal or external, is connected to the system bus  23  via the serial port interface  46 . In a networked environment, program modules depicted relative to the personal computer  20 , or portions thereof, may be stored in the remote memory storage device. It will be appreciated that the network connections shown are exemplary and other means of establishing a communications link between the computers may be used. Moreover, while it is envisioned that numerous embodiments of the present invention are particularly well-suited for computerized systems, nothing in this document is intended to limit the invention to such embodiments.  
      Virtual Machines  
      From a conceptual perspective, computer systems generally comprise one or more layers of software running on a foundational layer of hardware. This layering is done for reasons of abstraction. By defining the interface for a given layer of software, that layer can be implemented differently by other layers above it. In a well-designed computer system, each layer only knows about (and only relies upon) the immediate layer beneath it. This allows a layer or a “stack” (multiple adjoining layers) to be replaced without negatively impacting the layers above said layer or stack. For example, software applications (upper layers) typically rely on lower levels of the operating system (lower layers) to write files to some form of permanent storage, and these applications do not need to understand the difference between writing data to a floppy disk, a hard drive, or a network folder. If this lower layer is replaced with new operating system components for writing files, the operation of the upper layer software applications remains unaffected.  
      The flexibility of layered software allows a virtual machine (VM) to present a virtual hardware layer that is in fact another software layer. In this way, a VM can create the illusion for the software layers above it that said software layers are running on their own private computer system, and thus VMs can allow multiple “guest systems” to run concurrently on a single “host system.” 
       FIG. 2  is a diagram representing the logical layering of the hardware and software architecture for an emulated operating environment in a computer system. An emulation program  94  runs on a host operating system and/or hardware architecture  92 . Emulation program  94  emulates a guest hardware architecture  96  and a guest operating system  98 . Software application  100  in turn runs on guest operating system  98 . In the emulated operating environment of  FIG. 2 , because of the operation of emulation program  94 , software application  100  can run on the computer system  90  even though software application  100  is designed to run on an operating system that is generally incompatible with the host operating system and hardware architecture  92 .  
       FIG. 3A  illustrates a virtualized computing system comprising a host operating system software layer  104  running directly above physical computer hardware  102 , and the host operating system (host OS)  104  virtualizes all the resources of the machine by exposing interfaces that are the same as the hardware the host OS is virtualizing (which enables the host OS to go unnoticed by operating system layers running above it).  
      Alternately, a virtual machine monitor, or VMM, software layer  104 ′ may be running in place of or alongside a host OS  104 ″, the latter option being illustrated in  FIG. 3B . For simplicity, all discussion hereinafter (specifically regarding the host OS  104 ) shall be directed to the embodiment illustrated in  FIG. 3A ; however, every aspect of such discussion shall equally apply to the embodiment of  FIG. 3B  wherein the VMM  104 ′ of  FIG. 3B  essentially replaces, on a functional level, the role of the host OS  104  of  FIG. 3A  described herein below.  
      In regard to  FIG. 3 , it is important to note that VM A  108  and VM B  110  are virtualized computer hardware representations that exist only as software constructions and which are made possible due to the presence of specialized software code that not only presents VM A  108  and VM B  110  to Guest OS A  112  and Guest OS B  114  respectively, but which also performs all of the software steps necessary for Guest OS A  112  and Guest OS B  114  to indirectly interact with the real physical computer hardware  102 . This complete functionality can generally be referred to as a virtual machine monitor (VMM) (shown only in  FIG. 3B ) where, for certain embodiments (such as the one illustrated in  FIG. 3A ), the VMM comprises part of the host operating system  104 . However, in other embodiments (not shown) the VMM may be an application running above the host operating system  104  and interacting with the computer hardware only through said host operating system  104 . In yet other embodiments (such as shown in  FIG. 3B ), the VMM may comprise a partially independent software system that on some levels interacts indirectly with the computer hardware  102  via the host operating system  104  but on other levels the VMM interacts directly with the computer hardware  102  (similar to the way the host operating system interacts directly with the computer hardware). And in yet other embodiments (similar to that shown in  FIG. 3B ), the VMM may comprise a fully independent software system that on all levels interacts directly with the computer hardware  102  (similar to the way the host operating system interacts directly with the computer hardware) without utilizing the host operating system  104  (although still interacting with said host operating system  104  insofar as coordinating use of said computer hardware  102  and avoiding conflicts and the like).  
      All of these variations for implementing the VMM are anticipated to form alternative embodiments of the present invention as described herein, and nothing herein should be interpreted as limiting the invention to any particular VMM configuration. In addition, any reference to interaction between applications  116 ,  118 , and  120  via VM A  108  and/or VM B  110  respectively (presumably in a hardware emulation scenario) should be interpreted to be in fact an interaction between the applications  116 ,  118 , and  120  and a VMM. Likewise, any reference to interaction between applications VM A  108  and/or VM B  110  with the host operating system  104  and/or the computer hardware  102  (presumably to execute computer instructions directly or indirectly on the computer hardware  102 ) should be interpreted to be in fact an interaction between the VMM and the host operating system  104  or the computer hardware  102  as appropriate.  
      Referring again to  FIG. 3A , above the host OS  104  (or VMM  104 ′) are two virtual machine (VM) implementations, VM A  108 , which may be, for example, a virtualized Intel 386 processor, and VM B  110 , which may be, for example, a virtualized version of one of the Motorola 680×0 family of processors. Above each VM A  108  and  110  are guest operating systems (guest OSs) A  112  and B  114  respectively. Above guest OS A  112  are running two applications, application A 1   116  and application A 2   118 , and above guest OS B  114  is application B 1   120 .  
      Historically, virtual machines have been based upon the illusion that guest OSs (e.g., guest OS A  112 ) are running on dedicated hardware, when in fact they are sharing the hardware with other guest OSs (e.g., guest OS B  114 ). Host OS  104  is responsible for maintaining this illusion. The present invention relaxes this illusion and allows the guest OSs to ascertain whether they are running in a VM, and, subsequently, to alter their behavior, based on this realization. Behavior modifications are described that will increase the efficiency of guest OSs operating in a VM environment.  
      Operating in a VM Environment  
       FIG. 4  illustrates a virtualized computing system similar to that shown in  FIG. 3A , but in  FIG. 4 , a VM-aware guest OS A  132  and a VM-aware guest OS B  134  have replaced guest OS A  112  and guest OS B  114 , respectively. VM-aware guest OS A  132  and VM-aware guest OS B  134  are operating systems that are able to ascertain whether they are operating in a virtual machine environment and, if so, are able to modify their behavior to operate more efficiently.  
      The operation of VM-aware guest OS A  132  and VM-aware guest OS B  134  of  FIG. 4  is described in reference to  FIG. 5 , which is a flowchart that illustrates a method  140  of implementing a VM-aware operating system with the capability to modify its behavior in order to improve efficiency in a virtual machine environment. At step  142 , the method first comprises starting the VM-aware operating system (e.g., VM-aware guest OS A  132  or VM-aware guest OS B  134 ).  
      At step  144 , the VM-aware OS determines whether it is operating in a VM environment. This determination is done by any of a variety of methods, including the use of synthetic instructions, as described in U.S. patent application Ser. No. 10/685,051 filed on Oct. 14, 2003 and entitled, “SYSTEMS AND METHODS FOR USING SYNTHETIC INSTRUCTIONS IN A VIRTUAL MACHINE” (hereinafter the &#39;051 patent application). The &#39; 051  patent application describes a method for an operating system to determine whether it is running on a virtualized processor or running directly on an x86 processor, by executing a synthetic instruction (e.g., VMCPUID) for returning a value representing an identity for the central processing unit. If a value is returned, the guest OS concludes that the operating system is running on a virtualized processor; if an exception occurs in response to the synthetic instruction, the guest OS concludes that the operating system is running directly on an x86 processor. Another method for determining whether the guest OS is running in a VM environment include running a series of tests threads and comparing performance of the current environment to historical results. In any event, if the VM-aware OS determines that it is not operating in a VM environment, method  140  proceeds to step  146 . Alternatively, if the VM-aware OS determines that it is operating in a VM environment, method  140  proceeds to step  150 .  
      At step  146 , the VM-aware OS operates in its “traditional” manner, because it is operating on dedicated hardware and is not in a VM environment. At step  148 , the VM-aware operating system determines whether a “shut down” command has been received. If a “shut down” command is received, the VM-aware OS shuts down and method  130  ends. If no “shut down” command has been received, the VM-aware OS continues to operate in the “traditional” manner, as described in step  146 .  
      At step  150 , the VM-aware OS modifies its behavior in order to operate more efficiently in a VM environment. Examples of behavior modifications include, but are not limited to, 1) thread scheduling; 2) using bimodal devices to increase efficiency of devices, as described in U.S. Patent Application No. 1-734,450, filed on Dec. 12, 2003, entitled “SYSTEMS AND METHODS FOR BIMODAL DEVICE VIRTUALIZATION OF ACTUAL AND IDEALIZED HARDWARE-BASED DEVICES” (hereinafter the &#39;450 patent application); and 3) utilizing synthetic instructions (as in the &#39;051 patent application) that contain the execution priorities to host OS  104 .  
      At step  152 , the VM-aware OS determines whether a “shut down” command has been received. If a “shut down” command is received, the VM-aware OS shuts down and method  130  ends. If no “shut down” command has been received, the VM-aware OS continues to operate in its modified, high-efficiency mode, as described in step  150 .  
      Scheduling Example  
      In operating systems, a scheduler assigns processors to an execution context or thread. The scheduler reviews all ready threads and then schedules the threads for processing. If there is no work to be done, the scheduler loops while looking for work for a period of time before finally entering a busy-wait zone. When operating systems are not operating in a virtual machine, this behavior is not detrimental to performance. However, in a virtual machine environment, this behavior is detrimental to other guest operating systems that have work ready, but for which the processor is occupied. This looping and busy-waiting time contributes to the high-overhead level associated with running operating systems in a virtual machine environment.  
      The example shown in  FIG. 6  is an exemplary system and method for behavior modification. However, the present invention is not limited to thread scheduling behavior modifications.  
       FIG. 6  illustrates a virtualized computing system comprising a shared communication area A  162  arranged between host OS  104  and VM-aware guest OS A  132 . Similarly, a shared communication area B  164  is arranged between host OS  104  and VM-aware guest OS B  134 . This virtualized computing system provides a way for guest OSs to operate more efficiently. In a scheduling example, the virtualized computing system described in  FIG. 6  includes a way for a guest OS to provide additional thread information (such as information regarding the priority of thread and expected duration of time for the thread to run). When this information is combined with information from other guest OSs, the host OS has a much clearer picture of all of the demand for resources within the system and is, therefore, able to make decisions that will greatly improve the efficiency of the system.  
      Shared communication areas A  162  and B  164  are mechanisms which provide VM-aware guest OS A  132  and VM-aware guest OS B  134  an efficient way to transfer information to host OS  104  without passing control to host OS  104 . Passing control to host OS  104  is time consuming, therefore detrimental to overall system performance, and therefore to be avoided, if possible. In one example, shared communication area A  162  and shared communication area B  164  are embodied with shared memory space. In another example, shared communication area A  162  and shared communication area B  164  are embodied by a direct communications link between VM-aware guest OS A  132  and VM-aware guest OS B  134 , respectively.  
      VM-aware guest OS A  132  and VM-aware guest OS B  134  further contain a VM-aware scheduler A  166  and a VM-aware scheduler B  168 , respectively. VM-aware schedulers A  166  and B  168  operate on a set of ready threads that have execution properties (such as priority, deadline, and reserve (a portion of processor assigned to a thread). The execution properties are placed into shared communication areas A  162  and B  164 .  
      The virtualized computing system described in  FIG. 6  further includes a host scheduler  172  within host OS  104 . Host scheduler  172  makes scheduling decisions that are more efficient, based on two new features of the system: first, VM-aware guest OS A  132  and VM-aware guest OS B  134  are aware that they are operating in a VM environment and are able to send execution priorities to host scheduler  172  to enable more-efficient execution of all threads from a plurality of VM-aware guest OSs; second, shared communication areas A  162  and B  164  provide an efficient way to send information to host OS  104  without passing control to host OS  104 .  
      Host scheduler  172  assigns a plurality of virtual processors  170 A- 170 N to process the threads, according to the execution priorities placed into shared communication areas A  162  and B  164 . Host scheduler  172  reviews the execution priorities for all VM-aware guest OSs (e.g., VM-aware guest OS A  132  and VM-aware guest OS B  134 ), creates a composite run-list based on the priorities from all VM-aware guests OSs, and assigns virtual processors  170  to process the threads accordingly.  
      The operation of the VM-aware guest OSs of  FIG. 6  is described in reference to  FIG. 7 , which is a flowchart that illustrates an exemplary method  180  of scheduling threads in an operating system in accordance with the invention. At step  182 , the method starts with the VM-aware guest OS A  132  determining whether it is operating in a VM environment. This determination is done by any of a variety of methods including but not limited to the use of synthetic instructions as described in the &#39;051 application (described above). If yes, method  180  proceeds to step  192 ; if no, method  180  proceeds to step  184 .  
      At step  184 —which is a default operating mode for an operating system on dedicated hardware-VM-aware guest OS A  132  processes a thread. At step  186 , VM-aware guest OS A  132  determines whether more threads are ready to be processed. If yes, method  180  returns to step  184 ; if not, method  180  proceeds to step  188 . At step  188 , VM-aware guest OS A  132  determines whether a “shut down” command has been received; if so, VM-aware guest OS A  132  shuts down and method  180  ends; if no “shut down” command has been received, method  180  proceeds to step  190 . At step  190 , VM-aware guest OS A  132  enters a busy-wait status while it waits for more threads to be processed and, after a specified amount of time, method  180  returns to step  186  for the guest OS A  132  to check for more threads to process.  
      At step  192 , VM-aware guest OS A  132 —which is operating now operating in an enhanced “VM-aware” mode—starts by processing a thread and then, at step  194 , VM-aware guest OS A  132  determines whether more threads are ready to be processed. If yes, method  180  returns to step  192  for further processing; if not, method  180  proceeds to step  195 . At step  195 , guest OS A  132  determines whether a “shut down” command has been received and, if so, guest OS A shuts down and method  180  ends; if not, then at step  196 , VM-aware guest OS A  132  indicates to host OS  104  (or, for certain alternative embodiments, to VMM  104 ′) that guest OS A  132  currently has no work to do (that is, no threads to process). In one example, this indication is sent by VM-aware guest OS A  132  via shared communications area A  162  as described above. In another example, this indication is performed by VM-aware guest OS A  132  sending a synthetic instruction to host OS  104  (which is programmed to understand said synthetic instruction, of course) and then, at step  198 , host OS  104  determines whether work from other VMs is ready to be processed.  
      If VM-aware guest OS A  132  indicates to host OS  104  in step  196  that it does not have any work (via shared communications area B  132  or via a synthetic instruction, for example), host OS  104  determines whether there is a need for processor resources elsewhere and, if not, then host OS  104  allows guest OS A  132  to keep receiving processor resources which essentially enables guest OS A  132  to continue processing even though guest OS A  132  has no work to do at present and, thus, guest OS A  132  will enter a busy-wait loop at step  199  before returning to step  194  to see if there are more threads to execute. On the other hand, if host OS  104  determines that there is a need for processor resources elsewhere, then host OS  104 , at stet  200 , takes the processor resources for guest OS A  132  and temporarily gives them to another process running on host OS A  104  (such as another VM and guest operating system, e.g., guest OS B  134 ) before given them back to host OS A  104 , effectively suspending guest OS A  132  until host OS  104  provides it with processor resources once again and then, at that time, guest OS A optionally continues to a busy-wait state (which is skipped for certain alternative embodiments) before proceeding back to step  194 .  
     CONCLUSION  
      The various systems, methods, and techniques described herein may be implemented with hardware or software or, where appropriate, with a combination of both. Thus, the methods and apparatus of the present invention, or certain aspects or portions thereof, may take the form of program code (i.e., instructions) embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other machine-readable storage medium, wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the invention. In the case of program code execution on programmable computers, the computer will generally include a processor, a storage medium readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and at least one output device. One or more programs are preferably implemented in a high level procedural or object oriented programming language to communicate with a computer system. However, the program(s) can be implemented in assembly or machine language, if desired. In any case, the language may be a compiled or interpreted language, and combined with hardware implementations.  
      The methods and apparatus of the present invention may also be embodied in the form of program code that is transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via any other form of transmission, wherein, when the program code is received and loaded into and executed by a machine, such as an EPROM, a gate array, a programmable logic device (PLD), a client computer, a video recorder or the like, the machine becomes an apparatus for practicing the invention. When implemented on a general-purpose processor, the program code combines with the processor to provide a unique apparatus that operates to perform the indexing functionality of the present invention.  
      While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating there from. For example, while exemplary embodiments of the invention are described in the context of digital devices emulating the functionality of personal computers, one skilled in the art will recognize that the present invention is not limited to such digital devices, as described in the present application may apply to any number of existing or emerging computing devices or environments, such as a gaming console, handheld computer, portable computer, etc. whether wired or wireless, and may be applied to any number of such computing devices connected via a communications network, and interacting across the network. Furthermore, it should be emphasized that a variety of computer platforms, including handheld device operating systems and other application specific hardware/software interface systems, are herein contemplated, especially as the number of wireless networked devices continues to proliferate. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the appended claims.  
      Finally, the disclosed embodiments described herein may be adapted for use in other processor architectures, computer-based systems, or system virtualizations, and such embodiments are expressly anticipated by the disclosures made herein and, thus, the present invention should not be limited to specific embodiments described herein but instead construed most broadly. Likewise, the use of synthetic instructions for purposes other than processor virtualization are also anticipated by the disclosures made herein, and any such utilization of synthetic instructions in contexts other than processor virtualization should be most broadly read into the disclosures made herein.