An apparatus includes a CPU core and a L1 cache subsystem including a L1 main cache, a L1 victim cache, and a L1 controller. The apparatus includes a L2 cache subsystem coupled to the L1 cache subsystem by a transaction bus and a tag update bus. The L2 cache subsystem includes a L2 main cache, a shadow L1 main cache, a shadow L1 victim cache, and a L2 controller. The L2 controller receives a message from the L1 controller over the tag update bus, including a valid signal, an address, and a coherence state. In response to the valid signal being asserted, the L2 controller identifies an entry in the shadow L1 main cache or the shadow L1 victim cache having an address corresponding to the address of the message and updates a coherence state of the identified entry to be the coherence state of the message.