Microprocessor Virtualization via Segmented Privilege Levels
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Solution Overview
Problem
Traditional microprocessor virtualization approaches with two levels of privilege are inefficient due to high hypervisor software processing operations, as they rely on de-privileging and trap-and-emulate systems that result in numerous exceptions, limiting performance.
Innovation Solution
The introduction of guest mode control registers and root mode control registers allows for simultaneous operation of guest and root contexts, enabling efficient virtualization of hardware resources by reducing the need for context switching and allowing immediate transitions between modes, thereby minimizing hypervisor intervention.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If de-privileging and trap-and-emulate systems are used for virtualization, then guest operating systems can run on hardware resources, but numerous exceptions occur resulting in high hypervisor software processing operations and limited performance
Solution Approach 1:
The processor privilege levels are segmented into three distinct levels: root mode (highest privilege), guest mode (intermediate privilege), and user mode (lowest privilege). This segmentation allows the guest operating system to run in its own privileged guest mode rather than being de-privileged to user mode, reducing the need for exceptions while maintaining virtualization capability and improving performance
Solution Approach 2:
The guest mode acts as an intermediary privilege level between root mode and user mode. This intermediate level serves as a mediator that allows guest operating systems to execute privileged instructions within their own context without trapping to the hypervisor, thereby reducing hypervisor software processing operations while maintaining security through the hierarchical privilege structure
2Adaptability or versatility
If de-privileging is used to run guest operating systems, then virtualization is achieved, but context switching overhead increases due to frequent mode transitions
Solution Approach 1:
The processor is pre-configured with guest mode control registers and root mode control registers that establish the guest mode privilege level before execution begins. This preliminary setup allows the guest operating system to operate continuously in guest mode without frequent context switching, as the privileged state is prepared in advance rather than requiring repeated transitions to and from user mode
3Ease of operation
If more privileged instructions are made accessible to guest mode, then guest operating system functionality is improved, but security and control by the hypervisor may be compromised
Solution Approach 1:
Different privilege levels are assigned to different operational contexts: root mode retains full control over critical hypervisor functions, while guest mode is granted selective privileged instruction access appropriate for operating system operations. This localized quality assignment allows guest OS functionality to be enhanced where needed while maintaining hypervisor control where security is paramount
Solution Approach 2:
The privilege level parameter is changed from a binary state (user/kernel) to a three-level hierarchy (root/guest/user). This parameter change enables the system to differentiate between hypervisor-privileged operations and guest-privileged operations, allowing guest operating systems to access necessary privileged instructions while the hypervisor maintains ultimate control through the root mode level
Data Source
AI summary
A processor includes guest mode control registers supporting guest mode operating behavior defined by guest context specified in the guest mode control registers. Root mode control registers support root mode operating behavior defined by root context specified in the root mode control registers. The guest context and the root context are simultaneously active to support virtualization of hardware resources such that multiple operating systems supporting multiple applications are executed by the hardware resources.