Explore RISC-V¶

Version History

Date	Description
Jun 17, 2021	Initial

Architecture¶

First of all, explore its architecture designs, such as ISA, virtual memory, virtualization support, devices, and so on. At the time of writing, RISC-V’s hypervisor extension has not been defined yet (hence not IOMMU as well).

ISA
VM
Virtualization
I/O, I/O MMU

Firmware/Bootloader¶

This section discuss the boot flow and the firmware status. RISC-V has been ported to most of the major firmware and bootloaders, e.g., coreboot (first-stage-bootloader, prepare RAM), UEFI, U-Boot.

RISC-V has a cleaner design compared to X86. Rather than relying on messed up ACPI interfaces, it relys on a layer defined by SBI (an open-source implementation called OpenSBI). The OpenSBI layer sits in Machine-Mode, i.e., the most priviledged mode. It directly manages hardware and exposes standard APIs to upperlayer OS. For example, it exposed send_IPI, reset APIs. Hence, in Linux, it could simply call SBI to send IPI rather than implementing on its own and concerns about hardware details. I like this separation of concerns.

OpenSBI¶

OpenSBI. This is the default firmware in QEMU for RISC-V. It replaces the old BBL. This one runs after ROM and coreboot (if any). This one will discover/probe hardware (I suppose). After that, it passes control to normal bootloaders like u-boot or GRUB, or just to linux kernel.

Code Study¶

firmware/fw_base.S is the entry point.
mostly doing the usual, setting up envionment for C functions. It will save some critical information into a struct passed to C.
In the end, it will call into C sbi_init(struct sbi_scratch *). Note the sbi_scratch structure is filled with crutial info by fw_base.S.
It seems every hart will do the same?
lib/sbi/sbi_init.c is the C entry point after assembly.

Reset: - See sbi_system.c and sbi_ipi.c. It looks like the reset is: send an IPI to target HART, which will then ran a sbi_ipi_process_halt handler to halt. - Is this warm or cold reboot? Or neither?

Scripts¶

make V=1 CROSS_COMPILE=riscv64-linux-gnu- PLATFORM=generic FW_PAYLOAD_PATH=../linux/arch/riscv/boot/Image

QEMU¶

Code in hw/riscv/, hw/intc/*_clint_*.