User-Level MSR Instructions for Register Access
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Solution Overview
Problem
Current solutions for allowing application software to access control and status registers in x86 processors are inefficient due to high latency system calls and the need for custom ISA extensions, which are inflexible, costly, and require extensive enabling processes.
Innovation Solution
Introduction of user-level MSR instructions (URDMSR and UWRMSR) that enable read and write access to control and status registers without changing privilege levels, using a user-level access control bitmap to manage access permissions, allowing OS-controlled high-performance access.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If system calls are used for application software to access control and status registers, then access control is maintained, but latency increases significantly
Solution Approach 1:
The patent introduces user-level MSR instructions as an intermediary mechanism between application software and control/status registers. These instructions act as a mediator that allows user-mode code to directly access registers without triggering expensive system calls, while the OS retains control through the access control bitmap mechanism. This resolves the contradiction by providing a fast direct access path while maintaining security through software-enforced access control.
Solution Approach 2:
The patent replaces the mechanical system of system calls (which involve context switches and privilege level changes) with a more efficient instruction-based mechanism. The user-level MSR instructions substitute the complex system call pathway with direct register access, eliminating the need for costly context switches while maintaining access control through the bitmap mechanism. This substitution dramatically reduces latency while preserving security.
2Adaptability or versatility
If custom ISA extensions are added for user access to control and status registers, then access capability is improved, but hardware complexity and development cost increase
Solution Approach 1:
The patent makes the existing RDMSR and WRMSR instructions universal by enabling them to work in user mode through the introduction of user-level access control bitmaps. Instead of creating new custom ISA extensions for each access scenario, the same instructions serve both kernel and user modes, with access controlled by the bitmap. This universality eliminates the need for multiple custom extensions and reduces hardware complexity.
Solution Approach 2:
The patent changes the parameter of privilege level accessibility by modifying the execution environment in which RDMSR and WRMSR instructions can run. Through the user-level access control bitmap mechanism, these instructions are enabled for execution in user mode (CPL 3) as well as kernel mode, without changing the instructions themselves. This parameter change allows existing instructions to serve multiple purposes, avoiding custom hardware extensions.
3Adaptability or versatility
If custom ISA extensions are implemented for user register access, then access functionality is added, but extensive enabling processes are required
Solution Approach 1:
The patent performs preliminary action by pre-configuring the user-level access control bitmap with OS-defined permissions before user code executes. The OS sets up the bitmap in advance, establishing which registers are accessible to users. This preliminary configuration eliminates the need for complex runtime enabling processes, as the access control is predetermined and simply checked during instruction execution. The system is manufactured with built-in access control logic that requires no additional enabling steps.
4Speed
If privileged instructions RDMSR and WRMSR are used, then direct register access is achieved, but application software cannot use them
Solution Approach 1:
The patent introduces user-level access control bitmaps as an intermediary layer that mediates between application software and the privileged RDMSR/WRMSR instructions. This intermediary allows user-mode code to execute these instructions by checking permissions against the bitmap, effectively making the privileged instructions accessible to application software while maintaining the speed advantage of direct register access. The mediator enables software accessibility without sacrificing performance.
Solution Approach 2:
The patent creates a virtual copy of the privileged instruction behavior for user mode. Instead of blocking user code, the system provides a user-level version of register access that mirrors the functionality of the privileged instructions but with OS-controlled permissions via the access control bitmap. This copying approach allows application software to use the same fast direct access mechanism while maintaining security through the bitmap-based permission system.
Data Source
AI summary
Techniques for allowing a control and/or status register to be read or written to in a user privilege level are described. An example of an instruction for user privilege read is to include one or more fields for an opcode, one or more fields for a source operand that is to store a control and/or status register address, and one or more fields for a destination register operand, wherein the opcode is to indicate that execution circuitry is to read data from the control and/or status register whose identity is stored in the source operand and write the data in the destination register operand responsive to access to the control and/or status register being allowed, wherein access to the control and/or status register is at least in part determined by data of an operating system controlled data structure indexed by the control and/or status register address.