Tracking current time on multiprocessor hosts and virtual machines

• US 7,870,411 B2
• Filed: 07/17/2007
• Issued: 01/11/2011
• Est. Priority Date: 07/17/2006
• Status: Active Grant

• Current Assignee: Citrix Systems, Inc. (Cloud Software Group)
• Original Assignee: XenSource, Inc. (Cloud Software Group)
• Inventors: Fraser, Keir, Pratt, Ian Alexander
• Primary Examiner(s): SURYAWANSHI, SURESH
• Application Number:
  US11/879,338
• Publication Number:
  US 20080126820A1
• Time in Patent Office: 1,274 Days
• Field of Search: 713/375
• US Class Current: 713/375
• CPC Class Codes: G06F 1/12 Synchronisation of different clock signals provided by a plurality of clock generators
• CPC Class Codes: G06F 1/14 Time supervision arrangements, e.g. real time clock

Abstract

An operating system in a virtual environment can obtain the current time of the processor that the OS is utilizing through a method for synchronizing timers on multiple processors with a standard reference time, such as the Coordinated Universal Time (UTC). A hypervisor controlling the processors obtains a number of synchronization values that, together with a local timer counter value, are utilized by the guest operating system to determine the physical processor time.

Claims

1. A method for synchronizing timers on multiple hypervisor controlled processors to a standard reference time, the method comprising:

obtaining, by a synchronization program executing on a computing system comprising a processor communicating with a guest operating system via a hypervisor, synchronization values,

wherein the computing system comprises a platform timer communicatively coupled to the hypervisor and the processor comprises a timer;

calculating, by the hypervisor, a calibration constant using a relative frequency of a standard reference time, a relative frequency of the processor timer, and an error correction factor;

calculating a synchronized time value calibrated using the calibration constant; and

storing the synchronized time value in an accessible location.

2. The method of claim 1, further comprising calculating the error correction factor by counter-balancing an absolute error of the standard reference time.

3. The method of claim 1, further comprising calculating the standard reference time using an extrapolated reference time of the platform timer.

4. The method of claim 1, further comprising synchronizing the guest operating system with the synchronized time value.

5. The method of claim 1, wherein obtaining synchronization values further comprises:

obtaining the relative frequency of the standard reference time;

obtaining the relative frequency of the processor timer; and

obtaining the error correction factor.

6. The method of claim 1, wherein calculating the synchronized time value further comprises calculating, by the guest operating system, the synchronized time value.

7. The method of claim 1, further comprising assigning the guest operating system to the processor.

8. The method of claim 7, further comprising synchronizing the guest operating system with the synchronized time value after the guest operating system is assigned to the processor.

9. The method of claim 1, wherein storing the synchronized time value further comprises storing the synchronized time value in an accessible location comprising one of a shared memory, a virtual memory, a shared hard drive, a virtual hard drive, a cd-rom, and a virtual cd-rom.

10. The method of claim 1, wherein the guest operating system may be reassigned to different processors.

11. A system for synchronizing timers on multiple hypervisor controlled processors to a standard reference time, the system comprising:

a processor comprising a timer, the processor communicating with a guest operating system via a hypervisor;

a platform timer communicatively coupled to the hypervisor;

a computing system comprising the processor and the platform timer;

the hypervisor calculating a calibration constant using a relative frequency of a standard reference time, a relative frequency of the processor timer, and an error correction factor; and

a synchronization program executing on the computing system to;

obtain synchronization values,

calculate a synchronized time value using the calibration constant, and

store the synchronized time value in an accessible location.

12. The system of claim 11, wherein the hypervisor calculates the error correction factor by counter-balancing an absolute error of the standard reference time.

13. The system of claim 11, wherein the hypervisor calculates the standard reference time using an extrapolated reference time of the platform timer.

14. The system of claim 11, wherein the guest operating system synchronizes with the synchronized time value.

15. The system of claim 11, wherein the synchronization program obtains synchronization values by:

obtaining the relative frequency of the standard reference time;

obtaining the relative frequency of the processor timer; and

obtaining the error correction factor.

16. The system of claim 11, wherein the hypervisor assigns the guest operating system to the processor.

17. The system of claim 16, wherein the hypervisor synchronizes the guest operating system with the synchronized time value after assigning the guest operating system to the processor.

18. The system of claim 11, wherein the accessible location comprises one of a shared memory, a virtual memory, a shared hard drive, a virtual hard drive, a cd-rom, and a virtual cd-rom.

19. The system of claim 11, wherein the guest operating system may be reassigned to different processors.