Linux/RISC-V

This is a port of Linux kernel for the RISC-V instruction set architecture. Development is currently based on the 4.6 longterm branch.

Checking Out

The kernel history is quite large, you may want to limit the branches and commit history that you checkout:

    $ git clone --depth 50 --single-branch https://github.com/riscv/riscv-linux.git

Obtaining the Toolchain

You will need the RISC-V tools in order to build linux. In addition to following the RISC-V tools README, you must build the linux version of the gnu toolchain:

$ cd path/to/riscv-gnu-toolchain
$ make linux

Building the kernel image

1. Create kernel configuration based on architecture defaults:

    $ make ARCH=riscv defconfig
   

2. Optionally edit the configuration via an ncurses interface:

    $ make ARCH=riscv menuconfig
   

3. Build the uncompressed kernel image:

    $ make -j4 ARCH=riscv vmlinux
   

4. Link the kernel to the Berkeley Boot Loader (bbl)

    $ cd /path/to/risv-tools/riscv-pk/build
    $ ../configure --with-payload=/path/to/riscv-linux/vmlinux --host=riscv64-unknown-linux-gnu
    $ make bbl
   

5. Boot the kernel in the functional simulator.

    $ spike bbl
   

Booting from Initramfs

Block devices are not currently supported. If you want a useful linux, you will need to configure it to boot from an initramfs.

1. Enable initramfs in the linux config:

    "General setup -> Initial RAM Filesystem..." (CONFIG_BLK_DEV_INITRD=y)
   

2. Use a text file to specify contents of the initramfs:

    "General setup -> Initramfs source files = initramfs.txt" (CONFIG_INITRAMFS_SRC=initramfs.txt)
   

The file riscv-linux/initramfs.txt should contain a list of files to include. Here is a basic one to start with:

    dir /dev 755 0 0
    nod /dev/console 644 0 0 c 5 1
    nod /dev/null 644 0 0 c 1 3
    dir /proc 755 0 0
    slink /init /bin/busybox 755 0 0
    dir /bin 755 0 0
    file /bin/busybox /path/to/busybox 755 0 0
    dir /sbin 755 0 0
    dir /usr 755 0 0
    dir /usr/bin 755 0 0
    dir /usr/sbin 755 0 0
    dir /etc 755 0 0
    file /etc/inittab inittab 644 0 0
    dir /lib 755 0 0

Be sure to change "/path/to/busybox..." to point to your busybox binary (the RISC-V Tools README has instructions on building busybox). You will also need a riscv-linux/inittab file:

    ::sysinit:/bin/busybox --install
    ::sysinit:/bin/mount -t devtmpfs devtmpfs /dev
    ::sysinit:/bin/mount -t proc proc /proc
    /dev/console::sysinit:-/bin/ash

Now when you build and run linux, it should boot from the internal initramfs.

Exporting kernel headers

The riscv-gnu-toolchain repository includes a copy of the kernel header files. If the userspace API has changed, export the updated headers to the riscv-gnu-toolchain source directory:

$ make ARCH=riscv headers_check
$ make ARCH=riscv INSTALL_HDR_PATH=path/to/riscv-gnu-toolchain/linux-headers headers_install

Rebuild riscv64-unknown-linux-gnu-gcc with the linux target:

$ cd path/to/riscv-gnu-toolchain
$ make linux