PVOID
MmDbgReadCheck(
  IN  PVOID VirtualAddress
  );

Routine Description:

    This routine returns the 43-bit superpage address which corresponds
    to the specified virtual address if the specified virtual address
    is readable.

    N.B. This function should only be called at IRQL DISPATCH_LEVEL or
         above.

Arguments:

    VirtualAddress - Supplies the virtual address to check.

Return Value:

    If the specified virtual address is invalid or not readable, then a
    value of NULL is returned. Otherwise, the 43-bit superpage address
    which corresponds to the specified virtual address is returned.