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Hello, UEFI uses a flat memory model where virtual equals physical memory. During normal UEFI application processing I use AllocatePage() and FreePage(), but at a certain time I call GetMemoryMap() and ExitBootServices(). Can I at that time 'movl %cr3, %eax' and than work with the page tables?

Yes, but if you want to use UEFI runtime services then before you change the mapping you'll need to call SetVirtualAddressMap() with your custom map.