Fast and correct load-link/store-conditional instruction handling in DBT systems
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Dynamic Binary Translation (DBT) requires the implementation of load-link/store-conditional (LL/SC) primitives for guest systems that rely on this form of synchronization. When targeting e.g. x86 host systems, LL/SC guest instructions are typically emulated using atomic Compare-and-Swap (CAS) instructions on the host. Whilst this direct mapping is efficient, this approach is problematic due to subtle differences between LL/SC and CAS semantics. In this paper, we demonstrate that this is a real problem, and we provide code examples that fail to execute correctly on QEMU and a commercial DBT system, which both use the CAS approach to LL/SC emulation. We then develop two novel and provably correct LL/SC emulation schemes: (1) A purely software based scheme, which uses the DBT system’s page translation cache for correctly selecting between fast, but unsynchronized, and slow, but fully synchronized memory accesses, and (2) a hardware accelerated scheme that leverages hardware transactional memory (HTM) provided by the host. We have implemented these two schemes in the Synopsys DesignWare® ARC® nSIM DBT system, and we evaluate our implementations against full applications, and targeted micro-benchmarks. We demonstrate that our novel schemes are not only correct, but also deliver competitive performance on-par or better than the widely used, but broken CAS scheme.
Kristien , M , Spink , T , Campbell , B , Sarkar , S , Stark , I , Franke , B , Böhm , I & Topham , N 2020 , Fast and correct load-link/store-conditional instruction handling in DBT systems . in CASES '20: Proceedings of the International Conference on Compilers, Architectures and Synthesis for Embedded Systems . vol. Early Access , IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems , IEEE Computer Society , International Conference on Compilers, Architecture, and Synthesis for Embedded Systems (CASES '20) , 20/09/20 . https://doi.org/10.1109/TCAD.2020.3013048conference
CASES '20: Proceedings of the International Conference on Compilers, Architectures and Synthesis for Embedded Systems
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