14.1.7. Exclusive monitor system location

A typical multi-core system might include multiple exclusive monitors. Each core has its own local monitor and there are one or more global monitors. The shareable and cacheable attributes of the translation table entry relating to the location used for the exclusive load or store instruction determines which exclusive monitor is used.

In hardware, the core includes a device named the local monitor. This monitor observes the core. When the core performs an exclusive load access, it records that fact in the local monitor. When it performs an exclusive store, it checks that a previous exclusive load was performed and fails the exclusive store if this was not the case. The architecture enables individual implementations to determine the level of checking performed by the monitor. The core can only tag one Physical Address at a time.

The local exclusive monitor gets cleared on every exception return, that is, on execution of the ERET instruction. In the Linux kernel multiple tasks run in kernel context at EL1, and can be context-switched without an exception return. Only when we return to a userspace thread within the context of its associated kernel task do we perform the exception return. This is different to the ARMv7 architecture, where the kernel task scheduler must explicitly clear the exclusive access monitor on each task switch. It is implementation defined whether the resetting of the local exclusive monitor also resets the global exclusive monitor.

The local monitor is used when the location used for the exclusive access is marked as non-shareable, that is, threads running on the same core only. Local monitors can also handle the case where accesses are marked as inner shareable, for example, a mutex protecting a resource shared between SMP threads running on any core within the shareable domain. For threads running on different, non-coherent cores, the mutex location is marked as normal, non-cacheable and requires a global access monitor in the system.

A system might not include a global monitor, or a global monitor might only be available for certain address regions. It is implementation defined what happens if an exclusive access is performed to a location for which no suitable monitor exists in the system. The following are some of the permitted options:

The Exclusives Reservation Granule (ERG) is the granularity of the exclusive monitor. Its size is implementaion defined, but is typically one cache line. It gives the minimum spacing between addresses for the monitor to distinguish between them. Placing two mutexes within a single ERG can lead to false negatives, where executing a STXR instruction to either mutex clears the exclusive tag of both. This does not prevent architecturally-correct software from functioning correctly but it might be less efficient. The size of the ERG of the exclusive monitor on a specific core might be read from the Cache Type Register, CTR_EL0.

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