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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. >>> Hmmmm....don't they have to reserve CFMWS for hotplug? >> >> I have to admit I'm a bit lost with hotplugging and CXL needs, and not real experience with hot plug devices at all, but in my mind hotplugging allows a great flexibility which is counterintuitive if you need to know before hand which devices are going to be hot plugged ..., regarding CXL, how a CFMWS can be created without having that information? Maybe there is something in the specs about this and I did not pay attention. >> > I believe BIOS needs to set aside a range of HPA via CFMWS to allow for hot plugged devices. > >> What do you think about my other concern below? >> >> >>> DJ >>> >>>> >>>>> 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. >>>> > The BIOS does not touch device when hot plugged I believe. It's all OS native handled. If this is the kernel expectation and I'm the only one concerned with BIOSes respecting it, nothing more to be discussed. Thanks, Alejandro. >>>> 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. >>>> >>>>