Finding a CPU Design Bug in the Xbox 360
7 days ago
- #Debugging
- #Speculative Execution
- #CPU Design
- The author discovered a design bug in the Xbox 360 CPU related to the xdcbt instruction, which bypassed the L2 cache, leading to memory coherency issues.
- The bug caused crashes when heap corruption occurred due to incoherent memory views between cores, exacerbated by speculative execution via the branch predictor.
- The xdcbt instruction was deemed too dangerous to use, as speculative execution could trigger it unpredictably, making it impossible to guarantee safe usage.
- The issue shares similarities with Meltdown and Spectre vulnerabilities, highlighting the risks of speculative execution in CPU design.
- Debugging involved replacing xdcbt instructions with breakpoints, proving speculative execution was the root cause of crashes.
- The author reflects on the broader implications of such bugs, relating them to logical contradictions in system design and experimental philosophy.