7.2. The RISC-V ISA Simulator (Spike)

Spike is the golden reference functional RISC-V ISA C++ sofware simulator. It provides full system emulation or proxied emulation with HTIF/FESVR. It serves as a starting point for running software on a RISC-V target. Here is a highlight of some of Spikes main features:

  • Multiple ISAs: RV32IMAFDQCV extensions

  • Multiple memory models: Weak Memory Ordering (WMO) and Total Store Ordering (TSO)

  • Privileged Spec: Machine, Supervisor, User modes (v1.11)

  • Debug Spec

  • Single-step debugging with support for viewing memory/register contents

  • Multiple CPU support

  • JTAG support

  • Highly extensible (add and test new instructions)

In most cases, software development for a Chipyard target will begin with functional simulation using Spike (usually with the addition of custom Spike models for custom accelerator functions), and only later move on to full cycle-accurate simulation using software RTL simulators or FireSim.

Spike comes pre-packaged in the RISC-V toolchain and is available on the path as spike. More information can be found in the Spike repository.

7.2.1. Spike-as-a-Tile

Chipyard contains experimental support for simulating a Spike processor model with the uncore, similar to a virtual-platform. In this configuration, Spike is cache-coherent, and communicates with the uncore through a C++ TileLink private cache model.

make CONFIG=SpikeConfig run-binary BINARY=hello.riscv

Spike-as-a-Tile also supports Tightly-Coupled-Memory (TCM) for the SpikeTile, in which the main system memory is entirely modeled within the Spike tile, allowing for very fast simulatoin performance.

make CONFIG=SpikeUltraFastConfig run-binary BINARY=hello.riscv

Spike-as-a-Tile can be configured with custom IPC, commit logging, and other behaviors. Spike-specific flags can be added as plusargs to EXTRA_SIM_FLAGS

make CONFIG=SpikeUltraFastConfig run-binary BINARY=hello.riscv EXTRA_SPIKE_FLAGS="+spike-ipc=10000 +spike-fast-clint +spike-debug" LOADMEM=1

  • +spike-ipc=: Sets the maximum number of instructions Spike can retire in a single “tick”, or cycle of the uncore simulation.

  • +spike-fast-clint: Enables fast-forwarding through WFI stalls by generating fake timer interrupts

  • +spike-debug: Enables debug Spike logging

  • +spike-verbose: Enables Spike commit-log generation