Speculative Tablewalk Promotion in Load-Store Units
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Solution Overview
Problem
Conventional processor-based devices face inefficiencies and poor performance when performing speculative tablewalks due to complex considerations in modern data processor architectures, often resulting in prohibited or abandoned speculative operations, especially after cache misses or non-cacheable page table entries.
Innovation Solution
A method and apparatus that allow speculative operations to be upgraded to non-speculative during a tablewalk, enabling the completion of address translations by determining the speculative status concurrently with the tablewalk and adapting the processing device to perform the necessary actions based on this status.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If speculative tablewalks are prohibited or abandoned, then reliability is improved by avoiding incorrect address translations, but productivity deteriorates due to lost performance opportunities
Solution Approach 1:
The patent applies dynamics by allowing the speculative tablewalk to transition from a speculative state to a non-speculative state during execution. The system dynamically adjusts the behavior of the tablewalk based on changing conditions - initially performing it speculatively for performance, then converting to non-speculative mode if needed for correctness, thereby resolving the contradiction between speed and reliability
Solution Approach 2:
The patent changes the parameter of operational mode from speculative to non-speculative during the tablewalk process. By modifying this parameter dynamically based on whether the operation can be safely completed speculatively, the system optimizes both performance and correctness without needing to choose one permanently
2Productivity
If speculative tablewalks are allowed to complete, then productivity is improved through faster address translation, but reliability worsens due to potential incorrect translations from speculative operations
Solution Approach 1:
The system performs preliminary speculative tablewalks to obtain address translations quickly, then verifies or corrects these translations if needed. This preliminary action allows the system to benefit from fast speculative computation while maintaining the ability to correct errors, thus improving both speed and reliability
Solution Approach 2:
The patent implements feedback mechanisms that monitor the speculative tablewalk process and can convert it to non-speculative mode if incorrect translations are detected. This feedback loop allows the system to maintain high performance through speculative operations while ensuring correctness through continuous verification and correction
3Reliability
If speculative operations are upgraded to non-speculative during tablewalk, then reliability is improved by ensuring correct address translations, but device complexity increases due to additional status monitoring and state management
Solution Approach 1:
The patent makes the tablewalk mechanism universal by enabling it to handle both speculative and non-speculative operations through a single unified structure. The same tablewalk hardware and logic can operate in both modes, eliminating the need for separate dedicated paths and reducing overall device complexity despite the added functionality
Data Source
AI summary
A method includes performing a speculative tablewalk. The method includes performing a tablewalk to determine an address translation for a speculative operation and determining whether the speculative operation has been upgraded to a non-speculative operation concurrently with performing the tablewalk. An apparatus is provided that includes a load-store unit to maintain execution operations. The load-store unit includes a tablewalker to perform a tablewalk and includes an input indicative of the operation being speculative or non-speculative as well as a state machine to determine actions performed during the tablewalk based on the input. The apparatus also includes a translation look-aside buffer. Computer readable storage devices for performing the methods and adapting a fabrication facility to manufacture the apparatus are provided.