reproduce (this is a W=1 build):
        git remote add hch-misc git://git.infradead.org/users/hch/misc.git
        git fetch --no-tags hch-misc dma-split-wip
        git checkout d35bb33120e529c39ab215445ba649c32985ab29
        # 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 drivers/iommu/