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Software security techniques rely on correct execution by the hardware. Securing hardware components has been challenging due to their complexity and the proportionate attack surface they present during their design, manufacture, deployment, and operation. Recognizing that external communication represents one of the greatest threats to a system's security, this paper introduces the TrustGuard containment architecture. TrustGuard contains malicious and erroneous behavior using a relatively simple and pluggable gatekeeping hardware component called the Sentry. The Sentry bridges a physical gap between the untrusted system and its external interfaces. TrustGuard allows only communication that results from the correct execution of trusted software, thereby preventing the ill effects of actions by malicious hardware or software from leaving the system. The simplicity and pluggability of the Sentry, which is implemented in less than half the lines of code of a simple in-order processor, enables additional measures to secure this root of trust, including formal verification, supervised manufacture, and supply chain diversification with less than a 15% impact on performance. CCS Concepts • Security and privacy → Hardware security implementation.

Speculation with transactional memory systems helps programmers and compilers produce profitable thread-level parallel programs. Prior work shows that supporting transactions that can span multiple threads, rather than requiring transactions be contained within a single thread, enables new types of speculative parallelization techniques for both programmers and parallelizing compilers. Unfortunately, software support for multi-threaded transactions (MTXs) comes with significant additional inter-thread communication overhead for speculation validation. This overhead can make otherwise good parallelization unprofitable for programs with sizeable read and write sets. Some programs using these prior software MTXs overcame this problem through significant efforts by expert programmers to minimize these sets and optimize communication, capabilities which compiler technology has been unable to equivalently achieve. Instead, this paper makes speculative parallelization less laborious and more feasible through low-overhead speculation validation, presenting the first complete design, implementation, and evaluation of hardware MTXs. Even with maximal speculation validation of every load and store inside transactions of tens to hundreds of millions of instructions, profitable parallelization of complex programs can be achieved. Across 8 benchmarks, this system achieves a geomean speedup of 99% over sequential execution on a multicore machine with 4 cores. CCS Concepts • Computer systems organization ! Multicore architectures; • Software and its engineering ! Multithreading

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