[coreboot] [GSoC] Better RISC-V support, wrap-up

本文详细介绍了如何在RISC-V架构上编译和运行coreboot及Linux操作系统。首先，编译RISC-V指令集级模拟器spike，然后更新coreboot的工具链以支持RISC-V，接着编译并启动coreboot。最后，编译Linux内核，并将其添加到coreboot中以实现部分启动。未来工作包括简化流程、支持更多RISC-V平台和修复Linux完全启动的问题。

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In less than an hour, Google Summer of Code 2016 will be over (at least for us students). In this blog post, I will describe how you can use coreboot on RISC-V.

You can find the complete list of commits that I made during GSoC with this gerrit query.

The details

Compiling spike, the RISC-V instruction-set-level simulator

Spike, also known as riscv-isa-sim, is the reference implementation of RISC-V, and the only RISC-V platform that is currently known to work with coreboot (QEMU is nominally also supported, but the corresponding coreboot code has not been updated in a while).

First, you need to build and install libfesvr:

Clone the libfesvr git repository
run ./configure --prefix="$HOME" && make && make install

Then, you can compile and install spike:

Clone the spike git repository.
Apply the patch in this pull request to make console output possible.
Open riscv/processor.cc in a text editor and find processor_t::get_csr. Add a line that reads case CSR_MTIME: return 0;.
run ./configure --prefix="$HOME" --with-fesvr="$HOME" && make && make install

Compiling coreboot for RISC-V

Clone the coreboot git repository, if you haven’t already
Apply Iru Cai’s patch that updates the toolchain to GCC 6.1. You will currently have to fix a merge conflict when you apply this patch, but it’s an easy one.
Run make crossgcc-riscv
Run make menuconfig to configure coreboot. Select Emulation and SPIKE usb riscv in the Mainboard menu.
Run make
Run util/riscvtools/make-spike-elf.sh build/coreboot.rom build/coreboot.elf
Start coreboot by running spike build/coreboot.elf. You should see a few pages of output, ending in Payload not loaded.

Compiling and running Linux

Clone the riscv-linux git repository and check out the priv-1.9 branch
Apply this patch that allows linux to be started in machine-mode.
Download a copy linux 4.6.x from kernel.org, and unpack it. I’ll assume version 4.6.2 is used.
cd into linux-4.6.2/arch and symlink the arch/riscv directory from riscv-linux
Back in linux-4.6.2, run make O=build ARCH=riscv defconfig; cd into the newly created build directory.
Run make ARCH=riscv menuconfig. In the “General Setup” menu of the linux menuconfig, enter path/to/coreboot/util/crossgcc/xgcc/bin/riscv64-elf- as the cross-compiler tool prefix.
Run make ARCH=riscv vmlinux to compile linux.
Open vmlinux in a hex editor, such as hexer. Change the 8-byte number at 0x18 to 00 00 00 90 00 00 00 00; Add 00 00 00 90 00 00 00 00 to the numbers at 0x58 and 0x90, to load linux at physical addresses within RAM. It’s unfortunate that I have to recommend this step, but I did not come up with a better fix yet.

Next, you need to add vmlinux to coreboot:

Return to the coreboot directory.
Apply the remaining coreboot patches that are tagged riscv.
In the Payload menu, select An ELF executable payload. Instead of payload.elf, select the vmlinux file.
Run make and util/riscvtools/make-spike-elf.sh build/coreboot.rom build/coreboot.elf again.
Run spike build/coreboot.elf. You should now see a Linux boot, at least partially.

Future work

Even though my GSoC is over, coreboot’s support for RISC-V can still be improved, and I intend to fix at least some of the following things:

As you can see above, running coreboot and linux on RISC-V currently isn’t straight-forward, but involves a few manual steps.
There are other RISC-V platforms that I’d like to see coreboot on, such as lowRISC.
Linux does not completely boot, i.e. into userspace. There are still some bugs to be ironed out.
Automatic testing could be used to detect regressions.
I only tested coreboot on RISC-V with Linux; support for other operating systems or payloads is welcome.