and Peter Sewell’s scientific contributions

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Publications (1)


Fig. 10. GIC automaton, for each PE and each INTID, based on Figure 4-3 "Interrupt handling state machine" from Arm [10, §4.1.2], specialised to edge-triggered behaviour.
Relaxed exception semantics for Arm-A (extended version)
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December 2024

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5 Reads

Ben Simner

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Alasdair Armstrong

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Thomas Bauereiss

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[...]

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and Peter Sewell

To manage exceptions, software relies on a key architectural guarantee, precision: that exceptions appear to execute between instructions. However, this definition, dating back over 60 years, fundamentally assumes a sequential programmers model. Modern architectures such as Arm-A with programmer-observable relaxed behaviour make such a naive definition inadequate, and it is unclear exactly what guarantees programmers have on exception entry and exit. In this paper, we clarify the concepts needed to discuss exceptions in the relaxed-memory setting -- a key aspect of precisely specifying the architectural interface between hardware and software. We explore the basic relaxed behaviour across exception boundaries, and the semantics of external aborts, using Arm-A as a representative modern architecture. We identify an important problem, present yet unexplored for decades: pinning down what it means for exceptions to be precise in a relaxed setting. We describe key phenomena that any definition should account for. We develop an axiomatic model for Arm-A precise exceptions, tooling for axiomatic model execution, and a library of tests. Finally we explore the relaxed semantics of software-generated interrupts, as used in sophisticated programming patterns, and sketch how they too could be modelled.

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