Linux Kernel Boot

Raptor boots Linux through an OpenSBI fw_payload.bin image:

reset/MROM -> OpenSBI (M-mode) -> Linux (S-mode) -> init/userspace

The top-level Makefile is the preferred entry point. It downloads prebuilt OpenSBI + Linux payloads from Kingfish404/linux-build and keeps NEMU/NPC configuration in sync. By default it fetches v6.18.51 assets from the rv-v6.18.51 release.

Quick Commands

# Download prebuilt RV32 + RV64 payloads
make linux-download

# RV32 Linux through NEMU / NPC
make linux-boot-nemu32 ARGS="-b -n"
make linux-boot-rv32 ARGS="-b -n"

# RV32 Linux on the KU15P FPGA through LiteX BIOS + MIG DDR4
make -C fpga/litex linux-fpga-rv32-e2e UART_PORT=/dev/ttyUSB0

# RV64 prebuilt payload and NPC paths
make linux-download-rv64
make linux-boot-rv64 ARGS="-b -n"
make linux-boot-nemu64-device

# Hard-float F/D Buildroot payloads (downloaded automatically)
make linux-boot-rv32gc
make linux-boot-rv64gc

# Complete RV32GC Buildroot payload used by the FPGA flow
make linux-download-rv32gc-fpga

Useful overrides:

make linux-boot-rv32 LINUX_BUILD_RELEASE=rv-v6.18.51 LINUX_BUILD_VERSION=v6.18.51 MAX_INST=100000000 ARGS="-b -n"
make linux-boot-rv32 LINUX_RV32_PAYLOAD=/path/to/fw_payload.bin ARGS="-b -n"

All NPC Linux boot targets use the matching RV32/RV64 NEMU difftest reference by default, including the hard-float rv32gc and rv64gc targets. For focused simulator diagnosis, pass DIFF_REF_SO= explicitly to disable comparison.

Supported Modes

The presence of an RV64 or hard-float target documents integration and build wiring, not a reproduced full boot result for every checkout. Use a dated boot log or the corresponding verification target when recording validated status.

See linux/README.md and app/tinyos/README.md for the lower-level payload and xv6/egos helpers.

Payload Paths

Prebuilt qemu/NEMU payloads live under linux/build/. Distro/sim kernels from make -C linux build-rv* land under linux/build/raptor/<version>/:

linux/build/linux-riscv-qemu-rv32-fast-<version>/fw_payload.bin
linux/build/linux-riscv-qemu-rv64-m-<version>/fw_payload.bin
linux/build/raptor/v6.18.51/rv32-buildroot-sim/
linux/build/raptor/v6.18.51/rv64-alpine-sim/

Use the linux/Makefile helpers when another flow needs the exact path:

make -C linux print-rv32-payload
make -C linux print-rv64-payload

Device Tree

NPC/NEMU Linux payloads use the platform device-tree data packaged with the payload and simulator ROM flow. The RTL-visible platform devices are:

• CLINT at 0x0200_0000 (standard 0x000c_0000 window)
• PLIC at 0x0c00_0000 (16 MB window, 31 sources, M/S contexts)
• UART/peripheral window at 0x1000_0000
• Memory at 0x8000_0000 and related platform windows documented in Ecosystem

RAPT_MTIME_FREQ_MHZ / RAPT_MTIME_DIV must match the device-tree timebase-frequency, because CSR time and CLINT mtime are paced together.

The KU15P Linux DT advertises zicbom and riscv,cbom-block-size using the selected RTL preset’s RAPT_CACHE_LINE_BYTES. LiteSDCard DMA is non-coherent; the BIOS calls the CPU cache-maintenance hook after DMA, and Raptor currently implements that hook as a conservative whole-L1D cbo.flush operation.

The NEMU RV32GC/RV64GC configurations inject a fresh 256-bit host-random seed into the external DTB /chosen/rng-seed property before each boot. This matches QEMU’s early-boot entropy handoff and works even when a pre-built guest kernel does not contain a hardware RNG driver. They also expose a virtio-mmio entropy device at 0x10003000 on PLIC source 3 for ongoing entropy. Guest kernels must enable CONFIG_HW_RANDOM=y and CONFIG_HW_RANDOM_VIRTIO=y to use that device; Buildroot then sees virtio_rng.0 as /dev/hwrng.

Build From Source

Get the Linux Kernel source code from The Linux Kernel Archives:

wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.18.51.tar.xz

tar -xf linux-6.18.51.tar.xz
cd linux-6.18.51

See more details in linux/README.md.

The repository also keeps source-build helpers in linux/Makefile:

make -C linux build_linux
make -C linux opensbi-with-kernel

For day-to-day RTL validation, prefer the top-level linux-boot-* targets so the simulator config, payload path, and difftest reference stay aligned.

References

• Build mini linux for your own RISC-V emulator!
• 给NEMU移植Linux Kernel!