OpenCL 3.0 optional SVM support limits portable zero-copy sharing

A design goal is to define portable limits for zero-copy host-device buffer sharing across OpenCL 3.0 implementations without assuming optional features.

Device limits for allocation size and base address alignment are device-specific and must be queried at runtime. CL_MEM_USE_HOST_PTR and CL_MEM_ALLOC_HOST_PTR have implementation-defined requirements for pointer alignment, host memory lifetime and coherence. OpenCL 3.0 made Shared Virtual Memory optional, so SVM availability cannot be assumed across vendors.

What documented limits apply to portable use of CL_MEM_USE_HOST_PTR under OpenCL 3.0? What behavior is defined when an application assumes zero-copy sharing on an implementation without SVM support? What limits separate device-reported maximum allocation size from usable buffer size with host pointer flags?