MIDAS

MIDAS — Modeling Infrastructure for Debugging and Simulation — is a heterogeneous-host platform framework for fast, cycle accurate-simulation of digital systems ranging from single-chip multiprocessors and accelerators to models of warehouse-scale computers such as FireBox.

MIDAS builds off the work of the RAMP and DIABLO projects in its aims to support simulation of arbitrarily large digital systems (ranging from a single-chip multi-cores and accelerators to a warehouse-scale computers like FireBox) on a host platform consisting of an mixture of FPGAs and multi-core CPUs. We refer to the design being simulated as the target, and the platform performing the simulation as the host.

Project Goals

Three major goals of the MIDAS project are:

  1. Enable simulation of arbitrarily large digital systems at speeds reaching those of direct FPGA emulation. We want to run the target workload and complete benchmarks on implemented designs.
  2. Reduce the engineering effort required to host a simulation. RAMP and DIABLO, in additional to inheriting many of the challenges of using FPGAs for emulation, required hand written models that are difficult to implement.
  3. Provide a standard debug API, that abstracts the underlying host-platform architecture. Users should be able to write against an API without knowing a priori how it is being hosted.

To achieve the goal of fast cycle-accurate simulation it is necessary to support designs that span multiple FPGAs and CPUs. FAME describes a number of abstractions that ameliorate many of the traditional challenges of directly emulating a design and will an integral part of MIDAS.

While RAMP and DIABLO were fast, they were difficult to implement as they required manual descriptions of these FAME models which are often more difficult to write than conventional RTL. MIDAS aims to reduce this effort by firstly, and like RAMP, formalizing how components of the design communicate on the host-platform. This will permit interoperability between C models running on an multi-core and RTL models running on FPGAs. Secondly, MIDAS will provided mechanisms to automatically generate models from an RTL implementation. Notably, MIDAS will provide FIRRTL passes to take RTL emit host-decoupled (FAME-xx1) and/or multi-threaded (FAME-1xx) RTL models.

Finally, MIDAS will define a standard debug API that every model presents to the framework regardless of how it is being hosted. This will enable system level visibility over the state of the target design and allow the reconfiguration of the simulation without lengthy recompilations. Here MIDAS can again use FIRRTL to inject debugging structures, such as scan-chains, and add instrumentation like performance counters, automatically. Through this API MIDAS will support other simulation tools being developed by the group, like STROBER.

How it works, from ten thousand feet.

Users give a description of the their system that divides it into pieces that will be mapped to different components of the host platform. The framework emits the required interfacing logic to support simulation channels between the regions of the design. Portions of the design can be described abstractly with C models or concretely with a RTL implementation. This allows the designer to concretize only that which is necessary to get a sufficient simulation fidelity without sacrificing performance. For instance, the pipeline and inner-most caches could modeled at the RTL level, but the outer-most cache and off-chip memory system, could be replaced with a soft-core manage model that uses the FPGA's memory system (~2-8GB) as a cache for a larger backing store managed by a multi-core machine (size of disk!).

Custom FIRRTL passes map the target design, performing platform specific optimizations, and injecting additional debug and instrumenting logic.

Contributors