Creating Memory Objects from Offset SVM Pointers

Found while considering memory objects created from SVM allocations for the unified SVM extension:

When a memory object is created from an SVM allocation and CL_MEM_USE_HOST_PTR, must the passed-in host_ptr be the exact pointer returned by clSVMAlloc, or may the passed-in host pointer be offset from the pointer returned by clSVMAlloc?

Note, unlike creating a sub-buffer, there are no documented alignment requirements when creating a memory object from an SVM allocation.

There are also no documented alignment requirements when creating a memory object from an ordinary host allocation with CL_MEM_USE_HOST_PTR, though in this case the implementation may make a copy of the data and only keep the host pointer up-to-date when the memory object is mapped. It is not clear that a copy is supported similarly when the memory object is created from an SVM allocation.

If the host_ptr must be the exact pointer returned by clSVMAlloc, what should the behavior be if the passed-in host pointer points into an SVM allocation but is offset from a the pointer returned by clSVMAlloc? Is this an error, or undefined behavior, or something else?

The only CTS tests that I found that create a memory object from an SVM allocation create the memory object from the exact pointer returned by clSVMAlloc, and not a pointer offset from the pointer returned by clSVMAlloc.

Start by reviewing the OpenCL specification language for memory objects created from SVM allocations with CL_MEM_USE_HOST_PTR, then compare the existing CTS tests that use the exact clSVMAlloc pointer. Done means resolving the offset-pointer, alignment, and error or undefined-behavior semantics and documenting the decision in the relevant specification text.