Introduction of the RISC-V cross-compilation tools

A number of cross-compilation tools are provided in the $TOP/riscv-tools directory:

  • riscv-fesvr: The front-end server that serves system calls on the host machine.
  • riscv-gnu-toolchain: The GNU GCC cross-compiler for RISC-V ISA.
  • riscv-isa-sim: The RISC-V ISA simulator (Spike)
  • riscv-pk: The proxy kernel that serves system calls on target machine.

There are four ways to test a program:

  • Behavioural simulation: run the program in the RISC-V ISA simulator (Spike).
    The syscall() interface provided in the original RISC-V proxy kernel is not compatible with the I/O interfaces provided in this release. Spike can be used to run programs that do not access I/O devices or user mode programs that run inside the RISC-V Linux.
  • RTL simulation: simulate the program in Verilator ($Top/vsim/).
    No I/O devices is available in RTL simulation.
  • FPGA simulation: simulate the program using Xilinx ISim ($Top/fpga/board/$FPGA_BOARD/).
    Behavioural modules for I/O devices are provided by Xilinx IPs; however, host end modules (UART terminal and SD card) are not available.
  • FPGA run: actually run the program on an FPGA board ($Top/fpga/board/$FPGA_BOARD/).
    Full I/O support (UART and SD).

Programs can be compiled and run in three different modes:

  • Bare metal mode: supervisor programs with no I/O accesses.
    Behavioural simulation and RTL Simulation
    Programs run in this mode have no peripheral support. This mode is used only for ISA and cache regression tests. The return value of a program indicates the result of an ISA test case. 0 is success while none-zero indentifies the No. of the failing case. Programs compiled in this mode would run silently on FPGAs or in FPGA simulations.
  • Newlib mode: supervisor programs with access to I/O devices.
    FPGA simulation and FPGA run
    Programs run in this mode have the full control (supervisor priority) of peripherals (limited in simulation) but are single-threaded. Bootloaders are run in this mode.
  • Linux mode: user programs with Linux support.
    Behavioural simulation and FPGA run
    Programs runs in the RISC-V Linux. They get multi-thread and peripheral support from the Linux kernel.

Compiling and simulating programs in different modes depends on different tool sets.

  • Bare metal mode
    • Behavioural simulation:
      riscv-gnu-toolchain(newlib); riscv-isa-simriscv-fesvr.
    • RTL simulation:
      riscv-gnu-toolchain(newlib); verilator (built-in)
  • Newlib (supervisor) mode:
    • FPGA simulation:
      riscv-gnu-toolchain(newlib); vivado.
    • FPGA run:
      riscv-gnu-toolchain(newlib); vivado.
  • Linux (user) mode
    • Behavioural simulation:
      riscv-gnu-toolchain(newlib+linux); riscv-isa-simriscv-fesvrriscv-pkvmlinuxroot.bin.
    • FPGA run:
      riscv-gnu-toolchain(newlib+linux); vivadovmlinuxroot.bin.

Building the RISC-V cross-compilation tools

A build script is provided to build most of the cross-compilation tools and Spike:

# set up the RISCV environment variables
cd $TOP/riscv-tools

After the compilation, the Spike and the newlib GCC binaries should be available:

which spike
# the newlib gcc
which riscv64-unknown-elf-gcc

The RISC-V GCC/Newlib Toolchain Installation Manual can be found here.

Building the Linux GCC

The build script above provides a GCC build using the Newlib libc but not the GNU libc, which is needed for compiling programs to run in user mode on Linux. To build a Linux GCC compiler:

# set up the RISCV environment variables
cd $TOP/riscv-tools/riscv-gnu-toolchain
# ignore if build already exist
mkdir build
cd build
../configure --prefix=$RISCV
make -j$(nproc) linux

After the compilation, the Linux GCC binaries should be available:

which riscv64-unknown-linux-gnu-gcc