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This requires an existing committed region, > so registration fails when firmware has not configured one. > > Firmware may provide a suitable CFMWS window without programming a > region. Hi Richard, FWIW, I unsuccessfully  tried to explain this possibility to "BIOS people". The fact is, nowadays for AMD and I think for Intel as well, if there is no CXL device detected with CXL.mem enabled, the BIOS will neither set HDMs  nor there will be a CFMWS. > Hot-added devices can have the same requirement. A provider > needs its endpoint topology established before deciding when to > create a region and how much memory to map. Another unsuccessfully discussion was to avoid BIOS setting HDMs at hotplug time as this could race with host dynamic regions creation. I'm commenting these two things here for maybe having a community/kernel discussion with the goal of defining what BIOSes should do and mostly what they should not do if they want to be compatible with kernel expectations.