13.3.1. Cacheable and shareable memory attributes

Regions of memory marked as Normal can be specified as either cached or non-cached. See Chapter 14 Multi-core processors for more information about cacheable memory. Memory caching can be separately controlled through inner and outer attributes, for multiple levels of cache. The division between inner and outer is implementation defined, but typically the set of inner attributes is used by caches that are integrated into the processor, whereas the outer attributes are exported from the processor to the external memory bus and are therefore potentially used by cache hardware external to the core or cluster.

The shareable attribute is used to define whether a location is shared with multiple cores. Marking a region as Non-shareable means that it is only used by this core, whereas marking it as inner shareable or outer shareable, or both, means that the location is shared with other observers, for example, a GPU or DMA device might be considered another observer. In the same way, the division between inner and outer is implementation defined. The architectural definition of these attributes is that they enable us to define sets of observers for which the shareability attributes make the data or unified caches transparent for data accesses. This means that the system provides hardware coherency management so that two cores in the inner shareable domain must see a coherent copy of locations marked as inner shareable. If a processor or other master in the system does not support coherency, then it must treat the shareable regions as non-cacheable.

Figure 13.4. Inner and outer shareable domains

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There is a certain overhead associated with cache coherency hardware. Data memory accesses can take longer and consume more power than they otherwise would do. This overhead can be minimized by maintaining coherency between smaller numbers of masters and ensuring that they are physically close together in silicon. For this reason, the architecture splits the system into domains, making it possible to limit the overhead to just those locations where the coherency is required.

The following shareability domain options are available:

Non-shareable

This represents memory accessible only by a single processor or other agent, so memory accesses never need to be synchronized with other processors. This domain is not typically used in SMP systems.

Inner shareable

This represents a shareability domain that can be shared by multiple processors, but not necessarily all of the agents in the system. A system might have multiple Inner Shareable domains. An operation that affects one Inner Shareable domain does not affect other Inner Shareable domains in the system. An example of such a domain might be a quad-core Cortex-A57 cluster.

Outer shareable

An outer shareable (OSH) domain re-orderis shared by multiple agents and can consist of one or more inner shareable domains. An operation that affects an outer shareable domain also implicitly affects all inner shareable domains inside it. However, it does not otherwise behave as an inner shareable operation.

Full system

An operation on the full system (SY) affects all observers in the system.

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