Patent Publication Number: US-7904903-B2

Title: Selective register save and restore upon context switch using trap

Description:
FIELD OF THE INVENTION 
     An embodiment of the present invention relates generally to computing devices and, more specifically, to saving limited context information when transitioning between virtual machines. 
     BACKGROUND INFORMATION 
     Various mechanisms exist for context switching between processes controlled by an operating system. Similarly, in virtualization environments, context switching is necessary when the virtual machine (VM) scheduler, typically a virtual machine monitor/manager (VMM) switches between active and inactive virtual machines. In a virtualization environment, the VMM schedules the available processor cycles among running operating systems. The various operating systems (OSs) run in corresponding guest virtual machines. Before scheduling a guest VM to run, the context of the currently running VM must be saved to ensure that it will run properly when it is scheduled to run again. 
     Various processor registers must be saved as part of the context switch. Standard registers, stack pointers, MMX, single instruction multiple data (SIMD) and floating point (FP) registers must be saved, in addition to any processor-specific registers. When context is switched from the currently running VM, a context save is performed before the next scheduled VM is scheduled. Context registers for the next scheduled VM are restored prior to allowing the VM to run again. Context switching can be expensive in terms of time, power and other resources. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present invention will become apparent from the following detailed description of the present invention in which: 
         FIG. 1  is a block diagram of an exemplary virtualization platform on which may be practiced embodiments of the present invention; 
         FIG. 2  is a flow diagram of an exemplary process flow according to embodiments of the present invention; and 
         FIG. 3  is a block diagram of an exemplary platform on which a virtual machine monitor (VMM) may be executed, according to embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention is a system and method relating to limiting the context to be saved when switching among virtual machines (VMs) in a virtualization environment. In at least one embodiment, the present invention is intended to save only the register sets that are actually being used by the currently running VM, instead of saving all possible registers. 
     Reference in the specification to “one embodiment” or “an embodiment” of the present invention means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrase “in one embodiment” appearing in various places throughout the specification are not necessarily all referring to the same embodiment. 
     For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one of ordinary skill in the art that embodiments of the present invention may be practiced without the specific details presented herein. Furthermore, well-known features may be omitted or simplified in order not to obscure the present invention. Various examples may be given throughout this description. These are merely descriptions of specific embodiments of the invention. The scope of the invention is not limited to the examples given. 
     Existing systems will typically save all context for a VM before suspending it and scheduling another VM. In many cases, the suspended VM has not used all classes of registers. For instance, the VM may not perform floating point operations, so it would not need the context for the floating point (FP) registers when it is resumed by the VMM. Existing systems will save the FP registers anyway, prior to context switching. This operation is wasteful, when not necessary. 
     Referring now to  FIG. 1 , there is shown an exemplary virtualization platform  100  on which may be practiced embodiments of the present invention. The VMM  110  and guest VMs  101 ,  103 ,  105  and  107  execute on platform hardware  120 . The platform hardware  120  may include a processor  122 , memory  124  and one or more I/O devices  126  and  128 . Processor  122  may be any type of processor capable of executing software, such as a microprocessor, digital signal processor, microcontroller, or the like. The platform hardware  120  may be of a personal computer (PC), mainframe, handheld device, portable computer, set top box, or any other computing system. Though  FIG. 1  shows only one such processor  122 , there may be one or more processors in platform hardware  120  and one or more of the processors may include multiple threads, multiple cores or the like. 
     Memory  124  may be a hard disk, a floppy disk, random access memory (RAM), read only memory (ROM), flash memory, or any other type of medium readable by processor  122 . Memory  124  may store instructions for performing the execution of method embodiments of the present invention. 
     The one or more I/O devices  126  and  128  may be, for example, network interface cards, communication ports, video controllers, disk controllers on system buses (e.g., PCI, ISA, AGP), devices integrated into the chipset logic or processor (e.g., real-time clocks, programmable timers, performance counters) or any other device on the platform hardware  120 . The one or more I/O devices  126  and  128  may be accessed through I/O instructions, or memory mapped I/O accesses or through any other means known in the art. 
     The processor  122  has a variety of registers  123 . The context of registers  123  are typically saved during processor context switching. 
     In embodiments of the present invention, the VMM is notified when a guest VM uses a set of registers to indicate that the VMM needs to save the context of the used registers during a context switch. In an example, streaming SIMD extension (SSE) registers may be used by a guest VM. Certain instructions utilize a register set. For instance there exists a known instruction to perform a SIMD multiply. If a guest VM process executes this instruction, then the SIMD registers are used in this guest VM. 
     In existing virtualization environments a virtualization trap may occur in some circumstances. This trap causes an exit to the VMM from processing on a guest VM. In some environments, this trap is called a VM_exit. Embodiments of the invention cause a trap, or VM_exit, to occur when predetermined instructions are executed in a guest VM. Control transfers to the VMM, which makes a note of this usage. In an embodiment, the VMM clears the trap mechanism, so that subsequent usage of the predetermined instruction does not cause additional traps during the current session. Thus, only the first usage of the predetermined instruction will cause a trap. 
     Referring now to  FIG. 2 , there is shown an exemplary process flow according to embodiments of the present invention. Standard registers are comprehended during context switches in  201 . In other words, a standard set of registers may always be saved with the guest VM context. The VMM sets a predetermined set of operations or instructions to cause a trap in order to notify the VMM that corresponding registers are being used, in  203 . It will be apparent to one of ordinary skill in the art that a variety of methods may be used to identify which instructions should cause traps. 
     In some embodiments running in virtualization environments, a virtual machine control structure (VMCS) may be used to identify these target operations. A VMCS is a data structure allocated to each VM for each processor that is assigned to the specific VM. In multi-processor systems, the VMM may choose to assign a processor to specific VM and not assign it to other VMs. In other embodiments, the VMM may allow all VMs to use all processors. The VMCS contains a number of control fields to define which events cause a VM_exit (trap) and which events do not cause a trap. VMCS data structures in existing systems do not accommodate fields for trapping instructions. 
     VMCS data structures in existing systems do not accommodate fields for a trapping mechanism for groups of instructions that access specific register sets. Setting up the VMCS to accommodate traps for events as described herein may be implemented by allocating additional bit fields to the VMCS. The additional bit fields will identify FP, MMX, SSE, SIMD instructions, and the like, to determine whether a process in a guest VM has used a specific register set. 
     The VMCS may be dynamically modifiable. As a guest VM is scheduled to run, the VMCS fields may be set as desired so that target instructions and events may be identified. For instance, the VMCS may have a bit corresponding to the set of instructions that utilize the floating point registers. In this scenario, setting this bit and then execution of an FDIV instruction in the corresponding VM would cause a VM_exit, or hypervisor trap. Once the traps are identified and set, normal OS operation proceeds in  205 . 
     In an example execution, an FP instruction, like FDIV, may be executed, as in  207 . Control is transferred to the VMM to take appropriate action based on the operation causing the trap in  209 . In this case, the usage of FDIV is associated with the guest VM that executed the instruction to indicate that FP register context must be saved for this VM. Once the VMM has identified that the guest VM has a need for the FP register context to be saved, the trap is deactivated in  211  so that each subsequent FP instruction will not cause a trap in the current session. 
     In some embodiments, the trap may remain deactivated until the guest VM exits, or is shutdown. Thus, each context switch to/from this guest VM will require (in this example) the FP registers to be saved or restored. However, some guest VMs may only occasionally use a set of registers. In this case, it may be more efficient to reset the trap at each context switch. In some embodiments, the trap is reactivated when a guest VM is launched or resumed. The VMM will then also “forget” that the guest VM used the register set in previous sessions. If the guest VM does not use a specific register set during this active session, then the VMM will not have identified the register set as requiring a context save. Thus, context saving/resuming may be minimized. 
     In an embodiment, target events may be identified in the VMCS. However, when the event occurs, a bit may be set or cleared in the VMCS to indicate that this event has occurred without requiring a full trap of VM_exit. This bit may then be checked at the onset of context switching to determine whether a register set must be saved/restored. The processor hardware may be configured to automatically set this bit upon the occurrence of the event. This will typically require the instruction set architecture to be configured to trap on specific instructions. Other embodiments may set the bit in software, firmware or a combination of a hardware, firmware and software. Some embodiments of the present invention may be implemented by further extending the VT-x (virtualization technology extension) architecture. More information about VT-x may be found in “Intel® Virtualization Technology Specification for the IA-32 Intel® Architecture”, and may be found on the public Internet at URL cache-www*intel*com/cd/00/00/19/76/197666 — 197666.pdf. (Note that periods have been replaced with asterisks in URLs contained within this document in order to avoid inadvertent hyperlinks). 
     In another embodiment, the VMM is aware of whether more than one guest VM uses a specific register set. For instance, there may be five guest VMs operating in the virtualization environment, but only one is using FP operations. In this case, only one guest VM accesses the FP registers, so the FP register context need not be saved or restored. In addition to keeping track of whether a specific guest VM uses the register set, the VMM may also keep track of whether the register set is at risk of being modified by a second guest VM, and if not, does not save/restore the register set during a context switch. When a second guest OS accesses the register set, then the VMM will save/restore that register set upon a context switch for the two guest VMs which use the register set. 
     In another embodiment, an operating system running in a guest VM cooperates with the VMM. The guest OS may be knowledgeable of its processes to know which processes use which register sets. For instance, if the OS knows that only one executing process uses SIMD instructions, when that process exits, SIMD registers no longer need to be saved/restored upon a context switch of this guest VM. In this embodiment, the OS may notify the VMM when a process exits that has previously caused a trap, and the VMM may then reset the trap control structure to indicate that the guest VM does not use the specific register set. 
     Referring now to  FIG. 3 , there is shown an exemplary platform  300  on which the previously described VMM may be executed, according to embodiments of the invention. Processor  310  communicates with a memory controller hub (MCH)  314 , also known as North bridge, via the front side bus  301 . The MCH  314  communicates with system memory  312  via a memory bus  303 . The MCH  314  may also communicate with an advanced graphics port (AGP)  316  via a graphics bus  305 . The MCH  314  communicates with an I/O controller hub (ICH)  320 , also known as South bridge, via a peripheral component interconnect (PCI) bus  307 . The ICH  320  may be coupled to one or more components such as PCI hard drives (not shown), legacy components such as IDE  322 , USB  324 , LAN  326  and Audio  328 , and a Super I/O (SIO) controller  356  via a low pin count (LPC) bus  356 . 
     Processor  310  may be any type of processor capable of executing software, such as a microprocessor, digital signal processor, microcontroller, or the like. Though  FIG. 1  shows only one such processor  310 , there may be one or more processors in platform hardware  300  and one or more of the processors may include multiple threads, multiple cores, or the like. 
     Memory  312  may be a hard disk, a floppy disk, random access memory (RAM), read only memory (ROM), flash memory, or any other type of medium readable by processor  310 . Memory  312  may store instructions for performing the execution of method embodiments of the present invention. 
     Non-volatile memory, such as Flash memory  352 , may be coupled to the IO controller via a low pin count (LPC) bus  309 . The BIOS firmware  354  typically resides in the Flash memory  352  and boot up will execute instructions from the Flash, or firmware. 
     In some embodiments, platform  300  is a server enabling server management tasks. This platform embodiment may have a baseboard management controller (BMC)  350  coupled to the ICH  320  via the LPC  309 . 
     The techniques described herein are not limited to any particular hardware or software configuration; they may find applicability in any computing, consumer electronics, or processing environment. The techniques may be implemented in hardware, software, or a combination of the two. The techniques may be implemented in programs executing on programmable machines such as mobile or stationary computers, personal digital assistants, set top boxes, cellular telephones and pagers, consumer electronics devices (including DVD players, personal video recorders, personal video players, satellite receivers, stereo receivers, cable TV receivers), and other electronic devices, that may include a processor, a storage medium accessible by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and one or more output devices. Program code is applied to the data entered using the input device to perform the functions described and to generate output information. The output information may be applied to one or more output devices. One of ordinary skill in the art may appreciate that the invention can be practiced with various system configurations, including multiprocessor systems, minicomputers, mainframe computers, independent consumer electronics devices, and the like. The invention can also be practiced in distributed computing environments where tasks or portions thereof may be performed by remote processing devices that are linked through a communications network. 
     Each program may be implemented in a high level procedural or object oriented programming language to communicate with a processing system. However, programs may be implemented in assembly or machine language, if desired. In any case, the language may be compiled or interpreted. 
     Program instructions may be used to cause a general-purpose or special-purpose processing system that is programmed with the instructions to perform the operations described herein. Alternatively, the operations may be performed by specific hardware components that contain hardwired logic for performing the operations, or by any combination of programmed computer components and custom hardware components. The methods described herein may be provided as a computer program product that may include a machine accessible medium having stored thereon instructions that may be used to program a processing system or other electronic device to perform the methods. The term “machine accessible medium” used herein shall include any medium that is capable of storing or encoding a sequence of instructions for execution by the machine and that cause the machine to perform any one of the methods described herein. The term “machine accessible medium” shall accordingly include, but not be limited to, solid-state memories, and optical and magnetic disks. Furthermore, it is common in the art to speak of software, in one form or another (e.g., program, procedure, process, application, module, logic, and so on) as taking an action or causing a result. Such expressions are merely a shorthand way of stating the execution of the software by a processing system cause the processor to perform an action of produce a result. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the spirit and scope of the invention.