Why prevent boot memory to be hot-removed on arm64?

[Huang, Ying]

Hi, Anshuman,

When I read memory hotplug code on arm64, I found that a memory notifier
(prevent_bootmem_remove_notifier()) is used to prevent the boot memory
to be hot-removed on arm64.  I understand that some memory (for example
the boot memory) shouldn't be hot-removed on some platforms.  However,
after checking the code, I found that the memory can only be hot-removed
via ACPI (acpi_memory_device_remove()), CXL/NVDIMM
(dev_dax_kmem_remove()), or virtio mem (virtio_mem_remove_memory).
IIUC, CXL/NVDIMM or virtio mem shouldn't be boot memory, and ACPI memory
hot-remove shouldn't be setup by the firmware if some memory ranges
shouldn't be hot-removed.  If so, the boot memory cannot be hot-removed
accidentally, prevent_bootmem_remove_notifier() just makes the boot
memory can not be offlined.

Or I missed something?

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Best Regards,
Huang, Ying