Cache coherency between CPU cache hierarchies

A first processor 101, such as a central processing unit (CPU), is in a cache coherency domain with a first set of coherency rules 109. Another processor 105, such as a graphics processing unit (GPU), is in a different coherency domain 111 with a second set of coherency rules. The first processor has a level 1 processor cache 103 (L1 cache) and a lower level processor cache 107. The second processor has a level 1 graphics cache 104 (L1 cache) and a lower level cache 108. The first processor uses the first set of coherency rules with the lower level graphics cache. The second processor uses the second set of coherency rules with the lower level graphics cache. The lower level graphics cache may be a mid-level or last level cache. The first processor may snoop the lower level graphics cache.