RISC-V Bootflow in QEMU Playground

As a beginner in RISC-V, In this repo, I am learning the bootflow of RISC-V, and want to emulate it with QEMU.

NOTE: different from Direct Boot, in which to launch a kernel on QEMU without having to make a full bootable image and just passing a parameter -kernel to specify the kernel image. Then qemu-system-riscv64 will load it automatically. (This is helpful for kernel debugging, but hides many details of the bootflow)

In this playground.

As it has variant bootflows, here I just focus on two of them:

1. I boot it by using a payload firmware of OpenSBI.

   ./boot_payload: BootROM -> FSBL -> FW_Payload -> UBoot -> Linux Kernel

2. I change it to use a dynamic firmware of OpenSBI.

   ./boot_uboot-spl: BootROM -> UBoot-SPL -> FW_Dynamic + Uboot-Proper -> Linux Kernel

Jargon

1. PBL: Primary Boot Loader
2. SPL: Secondary Program Loader
3. ZSBL: Zero Stage Boot Loader
4. FSBL: First Stage Boot Loader
5. SSBL: Second Stage Boot Loader

Bootflow Summary

Stage 0. ZSBL

BootROM (PBL)

• Processor boots from BootROM
• Copies FSBL from memory device to SRAM and start executing FSBL.

Stage 1. FSBL

Specific FSBL / U-Boot-SPL

• Copies next stage of code into DRAM
• Further initialization
• Launches SSBL

Stage 2. SSBL

OpenSBI + UBoot-Proper

• OpenSBI: FW_Payload / FW_Dynamic / FW_Jump
• Flexibilities in boot sources.

(Stage 3. Kernel + RootFS)

Several ways to boot linux in RISC-V

(a) BootROM -> Specific FSBL -> OpenSBI -> Linux
(b) BootROM -> Specific FSBL -> OpenSBI -> U-Boot-Proper -> Linux
(c) BootROM -> U-Boot-SPL -> OpenSBI -> U-Boot-Proper -> Linux

For a), usually FPGA platforms use this approach.

For b) and c), mostly real world SoC tend to use these flows. We have bootloaders which provide advance booting methods. We can load kernel image from SD card OR Flash OR TFTP without changing firmware or bootloader.

The c) approach is most flexible because we can upgrade any image in the boot flow without changing other images.

Reference

1. RISC-V Summit_bootflow
2. Software Boot and PL Configuration (Xilinx)
3. RISC-V Bringup Unmatched
4. Which is the easiest way to boot linux with opensbi
5. RISC-V CPU加电执行流程
6. u-boot SPL启动流程
7. Don't let a lack of hardware stop you from messing with new hardware.
8. Running U-Boot & Linux Kernel in QEMU