Below is a (human-written) transcript of how to to get the Linux kernel building with `make defconfig`.
It works on Claude's the development machine but I have not tested it on any other machine.

cargo build --release

wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.9.tar.xz
xz -d linux-6.9.tar.xz
tar -xf linux-6.9.tar
cd linux-6.9/

make ARCH=riscv CC=/workspace/code/target/release/ccc-riscv HOSTCC=/workspace/code/target/release/ccc-x86 CROSS_COMPILE=riscv64-linux-gnu- defconfig
make ARCH=riscv CC=/workspace/code/target/release/ccc-riscv HOSTCC=/workspace/code/target/release/ccc-x86 CROSS_COMPILE=riscv64-linux-gnu- -j$(nproc) Image

# 4. Create the init script
cat > basic-init <<'EOF'
#!/bin/busybox sh
/bin/busybox mkdir -p /dev /etc /proc /sys /bin /sbin /usr/bin/games /usr/sbin
/bin/busybox mount -t proc proc /proc
/bin/busybox mount -t sysfs sys /sys
/bin/busybox mount -t devtmpfs dev /dev
/bin/busybox --install -s 2>/dev/null
export TERM=linux
export PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/bin/games
echo 1 > /proc/sys/kernel/printk
sleep 0.2
exec /bin/busybox setsid /bin/busybox cttyhack /bin/sh
EOF

wget https://busybox.net/downloads/busybox-1.36.1.tar.bz2
tar xf busybox-1.36.1.tar.bz2
cd busybox-1.36.1

# Configure
make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- defconfig
sed -i 's/# CONFIG_STATIC is not set/CONFIG_STATIC=y/' .config
sed -i 's/CONFIG_TC=y/# CONFIG_TC is not set/' .config

make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- -j$(nproc)

cd ..

# 5. Create the initramfs list
cat > initramfs.list <<'EOF'
dir /dev 0755 0 0
nod /dev/console 0600 0 0 c 5 1
nod /dev/tty 0666 0 0 c 5 0
nod /dev/null 0666 0 0 c 1 3
dir /proc 0755 0 0
dir /sys 0755 0 0
dir /root 0700 0 0
dir /bin 0755 0 0
dir /sbin 0755 0 0
dir /usr 0755 0 0
dir /usr/bin 0755 0 0
dir /usr/bin/games 0755 0 0
file /init ./basic-init 0755 0 0
file /bin/busybox busybox-1.36.1/busybox 0755 0 0
EOF


# 6. Generate the initramfs
usr/gen_init_cpio initramfs.list | gzip -9 > initramfs.cpio.gz

qemu-system-riscv64 -M virt -m 512 \
    -kernel arch/riscv/boot/Image \
    -initrd ./initramfs.cpio.gz \
    -append "console=ttyS0" \
    -nographic \
    -no-reboot

# dmesg | head -n 1
[    0.000000] Linux version 6.9.0 (appuser@3e854e88ac71) (ccc (Claude's C Compiler, GCC-compatible) 14.2.0, GNU ld (GNU Binutils for Ubuntu) 2.42) #1 SMP Thu Feb  5 16:59:16 UTC 2026