Cache Bypass Mechanism for Trace Collection Visibility
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Solution Overview
Problem
Debugging failures in modern processors with multiple cores and complex cache hierarchies is challenging due to limited visibility of cache transactions, as many events causing failures occur before detection and are not captured by existing trace capture buffers.
Innovation Solution
A mechanism to selectively bypass the cache, allowing selected transactions to bypass the cache and be logged in a trace capture buffer, providing visibility of core requests to the cache interface for debugging purposes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If cache memory completes core requests autonomously, then processing speed is improved, but visibility of transactions for debugging is lost
Solution Approach 1:
The patent introduces a trace capture buffer (TCB) as an intermediary mechanism between the cache memory and the memory system. The TCB intercepts and records transactions that would otherwise be invisible, allowing debugging visibility without interfering with the normal high-speed cache operation. The TCB acts as a mediator that captures information from the cache interface without blocking the cache's autonomous request completion.
Solution Approach 2:
The patent extracts transaction information from the cache operation flow by capturing transactions at the cache interface before they are completely processed. This extraction allows the transaction data to be separated from the main processing flow, enabling debugging analysis while maintaining the performance benefits of autonomous cache operation.
2Loss of information
If trace capture buffer records all transactions, then debugging information is improved, but buffer capacity is exceeded faster
Solution Approach 1:
The patent applies partial action by selectively capturing only specific transactions that meet certain criteria (such as transactions that would otherwise be invisible due to cache autonomous completion). Rather than recording all transactions, the system captures a targeted subset that provides essential debugging information, thereby extending the effective buffer capacity for useful data.
3Loss of information
If cache is bypassed for trace collection, then visibility of transactions is improved, but processing performance is degraded
Solution Approach 1:
The trace capture buffer serves as an intermediary that enables transaction visibility without requiring a complete bypass of the cache. The TCB captures transactions at the cache interface, allowing the cache to continue operating efficiently while still providing debugging visibility through the captured trace information.
Data Source
AI summary
A method and apparatus for selectively bypassing a cache in a processor of a computing device are disclosed. A mechanism to provide visibility to transactions on the core to a cache interface (e.g., an L3 cache interface) in a trace controller buffer (TCB) for debugging purposes, by causing selected transactions, which would otherwise be satisfied by the cache, to bypass the cache and be presented to the memory system where they may be logged in the TCB is described. In an embodiment of the invention, there is provided a method for providing processing core request visibility comprising bypassing a higher level cache in response to a processing core request, capturing the processing core request in a TCB, providing a mask to filter the processing core request, and returning a transaction response to a requesting processing core.