Apparatus and method for maintaining cache coherence data for memory blocks of different size granularities using a snoop filter storage comprising an n-way set associative storage structure

Abstract

An apparatus is provided for receiving requests from a plurality of processing units, at least some of which may have associated cache storage. A snoop unit implements a cache coherency protocol when a request received by the apparatus identifies a cacheable memory address. Snoop filter storage is provided comprising an N-way set associative storage structure with a plurality of entries. Each entry stores coherence data for an associated address range identifying a memory block, and the coherence data is used to determine which cache storages need to be subjected to a snoop operation when implementing the cache coherency protocol in response to a received request. The snoop filter storage stores coherence data for memory blocks of at least a plurality P of different size granularities, and is organised as a plurality of at least P banks that are accessible in parallel, where each bank has entries within each of the N-ways of the snoop filter storage. The snoop control circuitry controls access to the snoop filter storage, and is responsive to a received address to create a group of indexes, the group of indexes comprising an index for each different size granularity amongst the P different size granularities, and each index in the group being constrained so as to identify an entry in a different bank of the snoop filter storage. The snoop control circuitry uses the group of indexes to perform a lookup operation in parallel within the snoop filter storage in order to determine, taking into account each of the different size granularities, whether an entry stores coherence data for the received address.