Reducing Startup Time in Co-Designed Virtual Machines

A Co-Designed Virtual Machine allows designers to implement a processor via a combination of hardware and software. Dynamic binary translation converts code written for a conventional (legacy) ISA into optimized code for an underlying implementation-specific ISA. Because translation is done dynamically, an important consideration in such systems is the startup time for performing the initial translations. Beginning with a previously proposed co-designed VM that implements the x86 ISA, we study runtime binary translation overhead effects. The co-designed x86 virtual machine is based on an adaptive translation system that uses a basic block translator for initial emulation and a superblock translator for hotspot optimization. We analyze and model VM startup performance via simulation. We observe that non-hotspot emulation via basic block translation is the major part of the startup overhead. To reduce startup translation overhead, we follow the co-designed hardware / software philosophy and propose hardware assists to dramatically accelerate basic block translations. By combining hardware assists with balanced translation strategies, the co-designed translation system reduces runtime overhead significantly and demonstrates very competitive startup performance when compared with conventional processors running a set of Windows application benchmarks.