Metal: An Open Architecture for Developing Processor Features

In recent years, an increasing number of hardware devices started providing programming interfaces to developers such as smart NICs. Processor vendors use microcode to extend processors' features such as Intel SGX and VT-x. This enables processor architects to quickly evolve processor designs and features. However, modern processors still lack general programmability as microcode is inaccessible to system developers. Developers still cannot define custom processor features. We argue that processors should expose this capability to developers, which enables new operating system and application designs. We propose Metal, a novel open architecture that enables system developers to define custom instructions with microcode level overhead. We implement a prototype of Metal on a 5-stage pipelined RISC processor with minimal additional hardware resources. We demonstrate Metal's capability by building a variety of architectural extensions such as user defined privilege levels. We also discuss other potential applications and future directions for Metal.