We currently don't really support physical MMIO memory above 4G.
This is because we check that all physical page indices are below m_physical_page_entries_count in MemoryManager::get_physical_page_entry(), this variable gets set to the highest physical address in the multiboot memory map.
We have this limit as we currently store info about all physical pages into a contiguous array called m_physical_page_entries.

SeaBIOS started using BAR values above 4G when >= 4GB RAM is available, this will cause us to panic, as we currently rely on BIOS-provided BAR values.

Note: We panic at a different place until #22710 is merged, because we currently truncate 64 bit BARs to 32 bit by accident.