Circuit IR Compilers and Tools

ESI cosimulation model

Elastic Silicon Interfaces provides a feature called cosimulation. Cosim in general allows communication between the simulation and software. In the ESI case, it is typed and can be used to build an application and language specific API which is nearly identical to how the real hardware would interface. This allows users to simulate against the actual target software (or some simplification of it), enabling easier co-design.

ESI cosim uses Cap’nProto as a message format and RPC client/server. Capnp was chosen due to its relatively low-overhead encoding/decoding (as compared to Protocol Buffers and ilk) is a good fit for hardware. Using a standard messaging protocol allows users to work with a variety of languages: the Cap’nProto website lists C++, C#, Erlang, Go, Haskell, JavaScript, Ocaml, Python, and Rust as languages which support messages and RPC!

Status: prototype


To interface with RTL simulators, the DPI interface is used. ESI cosim builds and provides a set of SystemVerilog sources and a shared library which implement both DPI sides. The shared library (C++) starts a capnp RPC server for client(s) to connect to and interface with the simulations.

Generating a system-specific schema 

ESI has the capability to generate a Cap’nProto schema customized to an ESI system. Run command below on an MLIR assembly file with esi.cosim ops. It will find all of the cosim ops and output a capnp schema struct for each input and output type.

circt-translate <esi_system.mlir> -export-esi-capnp

Comments in the generated file indicate the type converted from. In cases where the ESI type is smaller than the capnp type (e.g. i5 vs UInt8), the ESI-generated conversion gasket will simply ignore the extra bits.

The struct IDs (0x<hexStructID>) will match the TypeIDs in the EsiDpiInterfaceDesc which dynamically describes each endpoint, described below.


ESI cosim works through a notion of endpoints – typed, bi-directional cosim bridges which are exposed over RPC. Endpoints are registered with the RPC interface.

On the RTL side, we provide a SystemVerilog module (Cosim_Endpoint) which provides a simple interface to the client. The modules instances take care of registering themselves. DataOut and DataIn carry the raw capnp messages with corresponding control signals. At present, we only support fixed-size messages.

module Cosim_Endpoint
   parameter int ENDPOINT_ID = -1,
   parameter longint ESI_TYPE_ID = -1,
   parameter int TYPE_SIZE_BITS = -1
   input  logic clk,
   input  logic rst,

   output logic DataOutValid,
   input  logic DataOutReady,
   output logic[TYPE_SIZE_BITS-1:0] DataOut,

   input  logic DataInValid,
   output logic DataInReady,
   input  logic [TYPE_SIZE_BITS-1:0] DataIn

The RPC interface allows clients to query all the registered endpoints, grab a reference to one, and send/receive messages and/or raw data. Once one client opens an Endpoint, it is locked until said client closes it.

interface CosimDpiServer {
    list @0 () -> (ifaces :List(EsiDpiInterfaceDesc));
    open @1 [S, T] (iface :EsiDpiInterfaceDesc) -> (iface :EsiDpiEndpoint(S, T));

struct EsiDpiInterfaceDesc {
  sendTypeID @0 :UInt64;
  recvTypeID @1 :UInt64;
  endpointID @2 :Int32;

interface EsiDpiEndpoint(SendMsgType, RecvMsgType) {
    send @0 (msg :SendMsgType);
    recv @1 (block :Bool = true) -> (hasData :Bool, resp :RecvMsgType); # If 'resp' null, no data

    close @2 ();

struct UntypedData {
    data @0 :Data;

This RPC interface can be used from any supported language. Here’s an example for Python:

import capnp

class LoopbackTester:
    def __init__(self, schemaPath):
        self.dpi = capnp.load(schemaPath)
        hostname = os.uname()[1]
        self.rpc_client = capnp.TwoPartyClient(f"{hostname}:1111")
        self.cosim = self.rpc_client.bootstrap().cast_as(self.dpi.CosimDpiServer)

    def openEP(self):
        ifaces = self.cosim.list().wait().ifaces
        openResp =[0]).wait()
        assert openResp.iface is not None
        return openResp.iface

    def write(self, ep):
        r = random.randrange(0, 2**24)
        data = r.to_bytes(3, 'big')
        print(f'Sending: {binascii.hexlify(data)}')
        return data

    def read(self, ep):
        while True:
            recvResp = ep.recv(False).wait()
            if recvResp.hasData:
        assert recvResp.resp is not None
        dataMsg = recvResp.resp.as_struct(self.dpi.UntypedData)
        data =
        return data

    def write_read(self):
        ep = self.openEP()
        print("Testing writes")
        dataSent = self.write(ep)
        print("Testing reads")
        dataRecv =
        assert dataSent == dataRecv

Implementation of the RPC server DPI plugin 

In short, an instance of Cosim_Endpoint registers itself. The first registration starts the RPC server (or it can be started via a direct dpi call). Starting the RPC server involves spining up a thread in which the RPC server runs. Communication between the simulator thread(s) and the RPC server thread is through per-endpoint, thread-safe queues. The DPI functions poll for incoming data or push outgoing data to/from said queues. There is no flow control yet so it is currently very easy to bloat the infinitely-sized queues. For the time being, flow-contol has be handled at a higher level.