An apparatus has first processing circuitry and second processing circuity. The second processing circuitry has at least one hardware mechanism providing a greater level of fault protection or fault detection than is provided for the first processing circuitry. Coherency control circuitry controls access to data from at least part of a shared address space by the first and second processing circuitry according to an asymmetric coherency protocol in which a local-only update of data in a local cache of the first processing circuitry is restricted in comparison to a local-only update of data in a local cache of the second processing circuitry.