Kernel extensions in the 32-bit kernel run in 32-bit mode, while kernel extensions in the 64-bit kernel run in 64-bit mode. Kernel extensions can be programmed to support both 32- and 64-bit applications. A 32-bit kernel extension that supports 64-bit processes can also be loaded on a 32-bit system (where 64-bit programs cannot run at all).
System calls can be made available to 32- or 64-bit processes, selectively. If an application invokes a system call that is not exported to processes running in the current mode, the call will fail.
A 32-bit kernel extension that supports 64-bit applications on AIX 4.3 cannot be used to support 64-bit applications on AIX 5.1 and beyond, because of a potential incompatibility with data types. Therefore, one of the following three techniques must be used to indicate that a 32-bit kernel extension can be used with 64-bit applications:
If none of these techniques is used, a kernel extension will still load, but 64-bit programs with calls to one of the exported system calls will not execute.
Kernel extension support for 64-bit applications has two aspects:
The first aspect is the use of kernel services for working with the 64-bit user address space. The 64-bit services for examining and manipulating the 64-bit address space are as_att64, as_det64, as_geth64, as_puth64, as_seth64, and as_getsrval64. The services for copying data to or from 64-bit address spaces are copyin64, copyout64, copyinstr64, fubyte64, fuword64, subyte64, and suword64. The service for doing cross-memory attaches to memory in a 64-bit address space is xmattach64. The services for creating real memory mappings are rmmap_create64 and rmmap_remove64. The major difference between all these services and their 32-bit counterparts is that they use 64-bit user addresses rather than 32-bit user addresses.
The service for determining whether a process (and its address space) is 32-bit or 64-bit is IS64U.
The second aspect of supporting 64-bit applications on the 32-bit kernel is taking 64-bit user data pointers and using the pointers directly or transforming 64-bit pointers into 32-bit pointers which can be used in the kernel. If the types of the parameters passed to a system call are all 32 bits or smaller when compiled in 64-bit mode, no additional work is required. However, if 64-bit data, long or pointers, are passed to a system call, the function must reconstruct the full 64-bit values.
When a 64-bit process makes a system call in the 32-bit kernel, the system call handler saves the high-order 32 bits of each parameter and converts the parameters to 32-bit values. If the full 64-bit value is needed, the get64bitparm service should be called. This service converts a 32-bit parameter and a 0-based parameter number into a 64-bit long long value.
These 64-bit values can be manipulated directly by using services such as copyin64, or mapped to a 32-bit value, by calling as_remap64. In this way, much of the kernel does not have to deal with 64-bit addresses. Services such as copyin will correctly transform a 32-bit value back into a 64-bit value before referencing user space.
It is also possible to obtain the 64-bit value from a 32-bit pointer by calling as_unremap64. Both as_remap64 and as_unremap64 are prototyped in /usr/include/sys/remap.h.