ACPI HMAT and CXL soft-reserved memory DAX driver
Exposes "soft-reserved" memory regions — heterogeneous memory described by ACPI HMAT or sitting behind CXL Type-3 memory-expansion devices — as direct-access (DAX) devices that software can map and use. It is the fallback that handles ranges firmware sets aside at boot but which aren't claimed by a more specific CXL region driver, mainly on modern servers with CXL memory expanders.
recommendation
It should stay in the kernel because the code is actively maintained — patches were still landing in early 2026 to fix real bugs and refine deferred soft-reserved memory registration — and it backs a current, documented boot flow for CXL Type-3 and HMAT memory. Deployments are niche (datacenter and high-end servers), but CXL memory-expansion hardware such as Astera Labs Leo is still sold new in 2025, and no other upstream driver fully covers the soft-reserved fallback role that cxl_dax_region leaves uncovered.
repository signals
sources
- lore.kernel.org
As of 2026-03-27 the directory is still receiving functional fixes, not just treewide churn ('Fix singleton confusion between dax_hmem_work and hmem devices').
- lore.kernel.org
As of 2026-01-13 there was active development around deferred Soft Reserved registration for hmem rather than removal discussion.
- docs.kernel.org
Kernel documentation says Specific Purpose / Soft Reserved memory is deferred to drivers to manage during boot, which is the operating context for hmem-managed ranges.
- docs.kernel.org
Current kernel CXL documentation describes CXL Type-3 memory exposure through DAX flows and explicitly shows an 'hmem' mode where the dax device is left as daxdev.
- asteralabs.com
CXL memory-expansion hardware remained an actively marketed product family in 2025-era deployments, indicating the underlying use case is not obsolete.
codex reasoning notes (technical)
Not an early-exit case: local code inspection via exec_command shows real platform drivers in hmem.c/device.c. lore_file_timeline on drivers/dax/hmem/device.c showed heavy 2025-2026 activity and no sampled removal thread; cited lore URLs came from the lore_file_timeline tool. Kernel-doc URLs were obtained via web search/open and show hmem/CXL soft-reserved memory remains a current documented flow. Astera Labs product URL was obtained via web search and is deployment evidence that CXL memory expansion hardware is still sold; however this remains a niche server/datacenter path, so deployments_today is low. No single upstream replacement cleanly covers the same fallback role because cxl_dax_region handles configured CXL regions while hmem still covers soft-reserved ranges not claimed there.