reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/44d4d9e727e7ebec1aaeb896756a1671016bbd52
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommu_pt_vtd
        git checkout 44d4d9e727e7ebec1aaeb896756a1671016bbd52
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash