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# Building RISC-V Target with Cmake #
> **Warning**
> Runtime rvv detection (using `hwcap`) requires linux kernel 6.5 or newer.
>
> When running on older kernels, we fall back to compile-time detection, potentially this can cause crashes if rvv is enabled at compile but not supported by the target cpu.
> Therefore if older kernel support is needed, rvv should be disabled if the target cpu does not support it.
## Prerequisite: Build RISC-V Clang Toolchain and QEMU ##
If you don't have prebuilt clang and riscv64 qemu, you can refer to the [script](https://github.com/sifive/prepare-riscv-toolchain-qemu/blob/main/prepare_riscv_toolchain_qemu.sh) to get the source. Copy the script to the zlib-ng root directory, and run it to download the source and build them. Modify the content according to your conditions (e.g., toolchain version).
```bash
./prepare_riscv_toolchain_qemu.sh
```
After running script, clang & qemu are built in `build-toolchain-qemu/riscv-clang/` & `build-toolchain-qemu/riscv-qemu/`.
`build-toolchain-qemu/riscv-clang/` is your `TOOLCHAIN_PATH`.
`build-toolchain-qemu/riscv-qemu/bin/qemu-riscv64` is your `QEMU_PATH`.
You can also download the prebuilt toolchain & qemu from [the release page](https://github.com/sifive/prepare-riscv-toolchain-qemu/releases), and enjoy using them.
## Cross-Compile for RISC-V Target ##
```bash
cmake -G Ninja -B ./build-riscv \
-D CMAKE_TOOLCHAIN_FILE=./cmake/toolchain-riscv.cmake \
-D CMAKE_INSTALL_PREFIX=./build-riscv/install \
-D TOOLCHAIN_PATH={TOOLCHAIN_PATH} \
-D QEMU_PATH={QEMU_PATH} \
.
cmake --build ./build-riscv
```
Disable the option if there is no RVV support:
```
-D WITH_RVV=OFF
```
## Run Unittests on User Mode QEMU ##
```bash
cd ./build-riscv && ctest --verbose
```
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