Over the past couple of weeks I’ve been wrestling with building vllm (with CUDA support) under Fedora 42. Here’s the short version of what went wrong:-

1. Python version confusion
   • My virtualenv was pointing at Python 3.11 but CMake kept complaining it couldn’t find “python3.11.”
   • Fix: explicitly passed -DPYTHON_EXECUTABLE=$(which python) to CMake, which got past the Python lookup errors.
2. CUDA toolkit headers/libs not found
   • Although Fedora’s CUDA 12.9 RPMs were installed, CMake couldn’t locate CUDA_INCLUDE_DIRS or CUDA_CUDART_LIBRARY.
   • Fix: set CUDA_HOME=/usr/local/cuda-12.9 and passed -DCUDA_TOOLKIT_ROOT_DIR & -DCUDA_SDK_ROOT_DIR to CMake.
3. cuDNN import errors
   • Pip’s PyTorch import of libcudnn.so.9 failed during the vllm build.
   • Fix: reinstalled torch/cu121 via the official PyTorch Cu121 wheel index so that all the nvidia-cudnn-cu12 wheels were in place.
4. GCC / Clang version mismatches
   • CUDA 12.9’s nvcc choked on GCC 15 (“unsupported GNU version”) and later on Clang 20.
   • Tried installing gcc-14 and symlinking it into PATH, exporting CC=/usr/bin/gcc-14 / CXX=/usr/bin/g++-14, and even passing -DCMAKE_CUDA_HOST_COMPILER, but CMake’s CUDA‐ID test was still failing on the Fedora header mismatch.
   • Ultimately we switched to Clang 20 with --allow-unsupported-compiler, which let us get past the version “block.”
5. Math header noexcept conflicts
   • CMake’s nvcc identification build then ran into four “expected a declaration” errors in CUDA’s math_functions.h, caused by mismatched noexcept(true) on sinpi/cospi vs system headers.
   • I patched those lines (removing or adding, I forget which, the trailing noexcept(true)) so cudafe++ could preprocess happily.
6. Missing NVToolsExt library
   • After all that, CMake could find CUDA and compile—but hit kotlinCopyEditThe link interface of target "torch::nvtoolsext" contains: CUDA::nvToolsExt but the target was not found.
   • Looking under /usr/local/cuda-12.9, there was no libnvToolsExt.so* at all—only the NVTX‐3 interop helper (libnvtx3interop.so*) lived under the extracted toolkit tree.

Current hurdle
I still don’t have the core NVTX library (libnvToolsExt.so.*) in /usr/local/cuda-12.9/…/lib, so the CMake target CUDA::nvToolsExt remains unavailable. This library appears to be missing from the both the Fedora cuda-nvtx and the NVIDIA nvtx toolkit download/runfile. This appears to be a known issue with recent versions.

Work continues and a full process will be documented, once successful.