SPARC: Shared Contexts Support

The context mechanism, which is used by the Memory Management Unit (MMU) hardware to distinguish between the use of the same virtual address in different process address spaces, introduces some inefficiencies when shared memory is used. The inefficiencies in shared memory are because the data at a particular shared memory and the address in different processes might really be identical, but the context number associated with each process is different. Therefore, the MMU hardware cannot recognize a match. This inability to recognize a match results in mappings being unnecessarily evicted from the MMU translation cache and the Translation Lookaside Buffer (TLB), only to be replaced by identical mappings with a different context number.

The Niagara 2 system has an additional shared context, which is a hardware feature that can be used to prevent the inefficiency in handling shared memory. Searching the TLB for mapping a match on either the private or the shared context results in a TLB hit. The current software support for shared context activates the feature for processes that use the Dynamic Intimate Shared Memory (DISM). In this case, the process text segment and DISM segments mapped at the same virtual address with the same permissions for each process use the shared context.