cuda.core: expose mem-range attributes on ManagedBuffer (last_prefetch_location and read-side getters) #2109

Background

PR #1775 lands ManagedBuffer with a property-style advice API (buf.read_mostly = ..., buf.preferred_location = ..., buf.accessed_by.add(...)) for the write side of managed-memory advice. The corresponding read side is uneven:

• buf.preferred_location — exists, returns Device | Host | None.
• buf.read_mostly — exists as a getter (queries CU_MEM_RANGE_ATTRIBUTE_READ_MOSTLY).
• buf.accessed_by — exists as AccessedBySetProxy.
• last_prefetch_location — missing.

Because the last-prefetch query isn't exposed, the test suite reaches into cuda.bindings.driver directly:

last = _get_int_attr(buf, driver.CUmem_range_attribute.CU_MEM_RANGE_ATTRIBUTE_LAST_PREFETCH_LOCATION)

As @leofang noted on PR #1775 (#1775 (comment)):

The fact that these are needed at test time rings a bell. cuda.core tries hard to not leak the abstraction. This highlights a problem that we do not expose enough mem-range attributes for ManagedBuffer.

PR #1775 currently works around this with a private _last_prefetch_location(buf) helper in tests/memory/test_managed_ops.py carrying a TODO that points at this issue.

Proposal

Add ManagedBuffer.last_prefetch_location mirroring preferred_location's shape:

@property
def last_prefetch_location(self) -> Device | Host | None:
    """Location of the most recent prefetch on this range, or ``None``
    if no prefetch has been issued.
    """

Returns:

• Device(i) for i >= 0
• Host() for the legacy -1 ordinal
• None for the "no prefetch yet" sentinel

On CUDA 13, verify whether CU_MEM_RANGE_ATTRIBUTE_LAST_PREFETCH_LOCATION_TYPE / _ID exist; if they do, layer a v2 path the same way preferred_location does so Host(numa_id=N) round-trips. Otherwise document the legacy-attribute caveat consistently with preferred_location.

Follow-on cleanup

Once this lands, in cuda_core/tests/memory/test_managed_ops.py:

• Drop the private _last_prefetch_location(buf) helper.
• Replace last == _HOST_LOCATION_ID / last == device.device_id assertions with buf.last_prefetch_location == Host() / ... == device.
• Drop the from cuda.core._memory._managed_buffer import _get_int_attr import and most driver.CUmem_range_attribute.* references.

Scope notes

• Out of scope: the rest of CUmem_range_attribute (e.g., PAGE_ENGINE_LAST_GPU_USED and friends) — file separately if needed.
• This is the minimum needed to close PR Add managed-memory advise, prefetch, and discard-prefetch free functions #1775's abstraction-leak comment; everything else can come in follow-ups.