Host Loop Pseudocode#

Constants:

ACCOUNT_ADDR_SIZE = 20
SLOT_ID_SIZE = 32

Data Encoding Functions:

write_byte_slice(arr):
  write_u32_le len(arr)
  write_bytes arr

read_slice():
  len := read_u32_le
  data := read_bytes(len)
  return data

Multi-Host#

GenVM supports multiple host connections. Each host method is routed to a specific host connection based on a method_hosts mapping in ExecutionData. The mapping is a byte array where each index corresponds to a method ID and the value is the host connection index. When the index is out of bounds or the array is empty, host 0 is used as the default.

The executor accepts multiple --host arguments. Each host connection independently runs the protocol described below.

In the manager deployment, host 0 is the node’s host loop and host 1 is a socketpair to the manager. The manager routes consume_result to itself (host 1) while all other methods go to host 0.

Protocol Loop#

The host processes requests in a loop. Each host connection runs independently, handling only the methods routed to it:

loop:
  method_id := read_byte
  match method_id
    json/methods/storage_read:
      read_type := read_byte as json/storage_type
      address := read_bytes(ACCOUNT_ADDR_SIZE)
      slot := read_bytes(SLOT_ID_SIZE)
      index := read_u32_le
      len := read_u32_le
      data, err := host_storage_read(read_type, address, slot, index, len)
      if err != json/errors/ok:
        write_byte err
      else:
        write_byte json/errors/ok
        write_bytes data # must be exactly len in size

    json/methods/consume_result:
      host_result := read_slice()
      # this is needed to ensure that genvm doesn't close socket before all data is read
      write_byte 0x00

    json/methods/consume_fuel:
      gas := read_u64_le
      host_consume_fuel(gas)
      # note: this method doesn't send any response

    json/methods/eth_call:
      address := read_bytes(ACCOUNT_ADDR_SIZE)
      calldata := read_slice()
      result, err := host_eth_call(address, calldata)
      if err != json/errors/ok:
        write_byte err
      else:
        write_byte json/errors/ok
        write_byte_slice result

    json/methods/get_balance:
      address := read_bytes(ACCOUNT_ADDR_SIZE)
      balance, err := host_get_balance(address)
      if err != json/errors/ok:
        write_byte err
      else:
        write_byte json/errors/ok
        write_bytes balance.to_le_bytes(32) # 256-bit integer

    json/methods/remaining_fuel_as_gen:
      fuel, err := host_remaining_fuel_as_gen()
      if err != json/errors/ok:
        write_byte err
      else:
        write_byte json/errors/ok
        write_bytes fuel.to_le_bytes(8) # 64-bit integer, must be safe integer (fits in double)

    json/methods/notify_nondet_disagreement:
      call_no := read_u32_le
      host_notify_nondet_disagreement(call_no)
      # note: this method doesn't send any response

    json/methods/notify_finished:
      # signals that all execution is complete
      write_byte 0x00
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