2.2. FPGA-Accelerated Simulation

2.2.1. FireSim

FireSim is an open-source cycle-accurate FPGA-accelerated full-system hardware simulation platform that runs on cloud FPGAs (Amazon EC2 F1). FireSim allows RTL-level simulation at orders-of-magnitude faster speeds than software RTL simulators. FireSim also provides additional device models to allow full-system simulation, including memory models and network models.

FireSim currently supports running only on Amazon EC2 F1 FPGA-enabled virtual instances. In order to simulate your Chipyard design using FireSim, if you have not already, follow the initial EC2 setup instructions as detailed in the FireSim documentation. Then clone Chipyard onto your FireSim manager instance, and setup your Chipyard repository as you would normally.

Next, initalize FireSim as a library in Chipyard by running:

# At the root of your chipyard repo
./scripts/firesim-setup.sh --fast

firesim-setup.sh initializes additional submodules and then invokes firesim’s build-setup.sh script adding --library to properly initialize FireSim as a library submodule in chipyard. You may run ./sims/firesim/build-setup.sh --help to see more options.

Finally, source the following environment at the root of the firesim directory:

cd sims/firesim
# (Recommended) The default manager environment (includes env.sh)
source sourceme-f1-manager.sh


Every time you want to use FireSim with a fresh shell, you must source this sourceme-f1-manager.sh

At this point you’re ready to use FireSim with Chipyard. If you’re not already familiar with FireSim, please return to the FireSim Docs, and proceed with the rest of the tutorial.

2.2.2. Running your Design in FireSim

Converting a Chipyard config (one in chipyard/src/main/scala to run in FireSim is simple, and can be done either through the traditional configuration system or through FireSim’s build-recipes scheme.

A FireSim simulation requires 3 additional config fragments:

  • WithFireSimConfigTweaks modifies your design to better fit the FireSim usage model. This is composed of multiple smaller config fragments. For example, the removal of clock-gating (using the WithoutClockGating config fragment) which is required for correct functioning of the compiler. This config fragment also includes other config fragments such as the inclusion of UART in the design, which although may technically be optional,is strongly recommended.
  • WithDefaultMemModel provides a default configuration for FASED memory models in the FireSim simulation. See the FireSim documentation for details. This config fragment is currently included by default within WithFireSimConfigTweaks, so it isn’t neccessary to add in separately, but it is required if you choose not to use WithFireSimConfigTweaks.
  • WithDefaultFireSimBridges sets the IOBinders key to use FireSim’s Bridge system, which can drive target IOs with software bridge models running on the simulation host. See the FireSim documentation for details.

The simplest method to add this config fragments to your custom Chipyard config is through FireSim’s build recipe scheme. After your FireSim environment is setup, you will define your custom build recipe in sims/firesim/deploy/deploy/config_build_recipes.ini. By prepending the FireSim config fragments (separated by _) to your Chipyard configuration, these config fragments will be added to your custom configuration as if they were listed in a custom Chisel config class definition. For example, if you would like to convert the Chipyard LargeBoomConfig to a FireSim simulation with a DDR3 memory model, the appropriate FireSim TARGET_CONFIG would be DDR3FRFCFSLLC4MB_WithDefaultFireSimBridges_WithFireSimConfigTweaks_chipyard.LargeBoomConfig. Note that the FireSim config fragments are part of the firesim.firesim scala package and therefore there do not need to be prefixed with the full package name as opposed to the Chipyard config fragments which need to be prefixed with the chipyard package name.

An alternative method to prepending the FireSim config fragments in the FireSim build recipe is to create a new “permanent” FireChip custom configuration, which includes the FireSim config fragments. We are using the same target (top) RTL, and only need to specify a new set of connection behaviors for the IOs of that module. Simply create a matching config within generators/firechip/src/main/scala/TargetConfigs that inherits your config defined in chipyard.

class FireSimRocketConfig extends Config(
  new WithDefaultFireSimBridges ++
  new WithDefaultMemModel ++
  new WithFireSimConfigTweaks ++
  new chipyard.RocketConfig)

While this option seems to require the maintenance of additional configuration code, it has the benefit of allowing for the inclusion of more complex config fragments which also accept custom arguments (for example, WithDefaultMemModel can take an optional argument``)