x86-64 VA Calculator 4-Level Paging

// Part of the Sequoia Hypervisor project. Works on both Intel and AMD (because why wouldn't it).
UINT64 Hv::HvmmComputeVA(
    IN UINT16 Pml4Index, 
    IN UINT16 PdpIndex,
    IN UINT16 PdIndex, 
    IN UINT16 PtIndex, 
    IN UINT16 PageOffset
)
{
    constexpr auto mask = [](UINT8 Bits) -> UINT16
        {
            return (UINT16(1) << Bits) - 1;
        };

    VIRTUAL_ADDRESS address = {
        .Fields =
        {
            .InpageIndex = static_cast<UINT16>(PageOffset & mask(12)),
            .PteIndex    = static_cast<UINT16>(PtIndex & mask(9)),
            .PdeIndex    = static_cast<UINT16>(PdIndex & mask(9)),
            .PdpeIndex   = static_cast<UINT16>(PdpIndex & mask(9)),
            .Pml4Index   = static_cast<UINT16>(Pml4Index & mask(9)),
            .Pml5Index   = 0
        }
    };

    // Handle sign extension if the address is in the higher half (Bit 47 / PML4 Bit 8 set)
    if (Pml4Index & 0x100)
        address.Contents |= 0xFFFF000000000000;

    return address.Contents;
}