The pain here is that AMD64 defines a 64-bit virtual address space; Opteron and Athlon64 only have a 48-bit virtual address space (40-bits physical), and define their own mappings from 48-bits to 64-bits. The choice is therefore to break the offset semantics and keep to AMD's documentation, or to define a new mapping for 48-bit to 63-bit, and have to rework it for a later processor with a 64-bit virtual address space.
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