Correct binding for WHV_UINT128

[benpye]

Hey,

I'm currently working on binding the Windows Hypervisor Platform APIs (doc).

There is a type WHV_UINT128 which is defined as a struct, aligned on a 16 byte boundary,

typedef union DECLSPEC_ALIGN(16) WHV_UINT128
{
    struct
    {
        UINT64  Low64;
        UINT64  High64;
    };

    UINT32  Dword[4];
} WHV_UINT128;

The difficulty here is that this type is used in a union with a bitfield hence my initial attempt did not work, see below:

STRUCT!{struct WHV_UINT128_s {
    Low64: UINT64,
    High64: UINT64,
}}
UNION!{union WHV_UINT128 {
    [u128; 1],
    s s_mut: WHV_UINT128_s,
    Dword Dword_mut: [UINT32; 4],
}}
STRUCT!{struct WHV_X64_FP_REGISTER {
    AsUINT128: WHV_UINT128,
}}
BITFIELD!{WHV_X64_FP_REGISTER AsUINT128: WHV_UINT128 [
    Mantissa set_Mantissa[0..64],
    BiasedExponent set_BiasedExponent[64..79],
    Sign set_Sign[79..80],
    Reserved set_Reserved[80..128],
]}

My naïve solution for this was to define WHV_UINT128 as u128 - this seemed to make the most logical sense, and results in the bitfield working.

Is there a preferred way forward here?