Disparate deployment models of cloud computing pose varying requirements on cloud infrastructure components such as networking, storage, provisioning, and security. Infrastructure providers need to study these and often create custom infrastructure components to satisfy these requirements. A major challenge in the research and development of these cloud infrastructure solutions, however, is the availability of customizable platforms for experimentation and trade-off analysis of the various hardware and software components. Most platforms are either general purpose or bespoke solutions created to assist a particular task, too rigid to allow meaningful customization. In this work, we present a 100G reconfigurable smartNIC prototyping platform called Janus that enables cloud infrastructure research and hardware-software co-design of infrastructure components such as hypervisor, secure boot, software defined networking and distributed storage. The platform provides a path to optimize the stack by offloading the functionalities from the host x86 to the embedded processor on the smartNIC and optimize performance by moving pieces to hardware using P4. Further, our platform provides hardware-enforced isolation of cloud network control plane, thereby securing the control plane from the tenants even for bare-metal deployments.