Context sensitive barriers with an implicit access ordering constraint for a victim context

A method of data processing, the method comprising the steps of:

• executing data processing instructions to perform data processing operations, wherein the data processing operations comprise accessing a memory system, and the data processing instructions are executed in a current context of a plurality of contexts;
• in response to execution of a barrier instruction in the current context of the plurality of contexts, enforcing an access ordering constraint,
• buffering pending accesses in a store buffer and tagging each pending access with an identifier indicative of the context from which that pending access was issued;
• limiting enforcement of the access ordering constraint to accesses initiated when executing data processing instructions in an identified context;
• storing an entry for the current context in one of multiple storage locations of a context tracking storage when the current context has initiated accesses since the access ordering constraint was last enforced for the current context, and
• when all of the multiple storage locations in the context tracking storage are occupied and when the store buffer does not have an occupied entry for the current context, enforcing an implicit access ordering constraint for pending accesses initiated by a victim context other than the current context which has a corresponding victim entry, and clearing the victim entry for a selected context, wherein the implicit access ordering constraint does not require execution of a corresponding barrier instruction.

Apparatus for data processing comprising:

• means for executing data processing instructions to perform data processing operations, wherein the data processing operations comprise accessing a memory system, and the data processing instructions are executed in a current context of a plurality of contexts;
• and
• means for providing an interface between the means for executing data processing instructions and the memory system,
• wherein the means for providing an interface is capable of, in response to the means for executing data processing instructions executing a barrier instruction in the current context of the plurality of contexts, enforcing an access ordering constraint,
• wherein the means for providing an interface comprises a circuit for buffering pending accesses capable of tagging each pending access with an identifier indicative of the context from which that pending access was issued,
• wherein the means for executing data processing instructions is capable of limiting enforcement of the access ordering constraint to accesses initiated by the means for executing data processing instructions when operating in an identified context,
• wherein the circuit for buffering pending accesses comprises a circuit for context tracking with multiple storage locations, and wherein the circuit for buffering pending accesses is capable of storing an entry in one of the multiple storage locations for the current context when the current context has initiated accesses since the access ordering constraint was last enforced for the current context, and
• wherein when all of the multiple storage locations in the circuit for context tracking are occupied and the circuit for buffering pending accesses does not have an occupied entry for the current context, the circuit for buffering pending accesses is capable of enforcing an implicit access ordering constraint for pending accesses initiated by a victim context other than the current context which has a corresponding victim entry, and clearing the victim entry for a selected context, wherein the implicit access ordering constraint does not require the means for executing data processing instructions to execute a corresponding barrier instruction.