1================
  2Kernel mode NEON
  3================
  4
  5TL;DR summary
  6-------------
  7* Use only NEON instructions, or VFP instructions that don't rely on support
  8  code
  9* Isolate your NEON code in a separate compilation unit, and compile it with
 10  '-march=armv7-a -mfpu=neon -mfloat-abi=softfp'
 11* Put kernel_neon_begin() and kernel_neon_end() calls around the calls into your
 12  NEON code
 13* Don't sleep in your NEON code, and be aware that it will be executed with
 14  preemption disabled
 15
 16
 17Introduction
 18------------
 19It is possible to use NEON instructions (and in some cases, VFP instructions) in
 20code that runs in kernel mode. However, for performance reasons, the NEON/VFP
 21register file is not preserved and restored at every context switch or taken
 22exception like the normal register file is, so some manual intervention is
 23required. Furthermore, special care is required for code that may sleep [i.e.,
 24may call schedule()], as NEON or VFP instructions will be executed in a
 25non-preemptible section for reasons outlined below.
 26
 27
 28Lazy preserve and restore
 29-------------------------
 30The NEON/VFP register file is managed using lazy preserve (on UP systems) and
 31lazy restore (on both SMP and UP systems). This means that the register file is
 32kept 'live', and is only preserved and restored when multiple tasks are
 33contending for the NEON/VFP unit (or, in the SMP case, when a task migrates to
 34another core). Lazy restore is implemented by disabling the NEON/VFP unit after
 35every context switch, resulting in a trap when subsequently a NEON/VFP
 36instruction is issued, allowing the kernel to step in and perform the restore if
 37necessary.
 38
 39Any use of the NEON/VFP unit in kernel mode should not interfere with this, so
 40it is required to do an 'eager' preserve of the NEON/VFP register file, and
 41enable the NEON/VFP unit explicitly so no exceptions are generated on first
 42subsequent use. This is handled by the function kernel_neon_begin(), which
 43should be called before any kernel mode NEON or VFP instructions are issued.
 44Likewise, the NEON/VFP unit should be disabled again after use to make sure user
 45mode will hit the lazy restore trap upon next use. This is handled by the
 46function kernel_neon_end().
 47
 48
 49Interruptions in kernel mode
 50----------------------------
 51For reasons of performance and simplicity, it was decided that there shall be no
 52preserve/restore mechanism for the kernel mode NEON/VFP register contents. This
 53implies that interruptions of a kernel mode NEON section can only be allowed if
 54they are guaranteed not to touch the NEON/VFP registers. For this reason, the
 55following rules and restrictions apply in the kernel:
 56* NEON/VFP code is not allowed in interrupt context;
 57* NEON/VFP code is not allowed to sleep;
 58* NEON/VFP code is executed with preemption disabled.
 59
 60If latency is a concern, it is possible to put back to back calls to
 61kernel_neon_end() and kernel_neon_begin() in places in your code where none of
 62the NEON registers are live. (Additional calls to kernel_neon_begin() should be
 63reasonably cheap if no context switch occurred in the meantime)
 64
 65
 66VFP and support code
 67--------------------
 68Earlier versions of VFP (prior to version 3) rely on software support for things
 69like IEEE-754 compliant underflow handling etc. When the VFP unit needs such
 70software assistance, it signals the kernel by raising an undefined instruction
 71exception. The kernel responds by inspecting the VFP control registers and the
 72current instruction and arguments, and emulates the instruction in software.
 73
 74Such software assistance is currently not implemented for VFP instructions
 75executed in kernel mode. If such a condition is encountered, the kernel will
 76fail and generate an OOPS.
 77
 78
 79Separating NEON code from ordinary code
 80---------------------------------------
 81The compiler is not aware of the special significance of kernel_neon_begin() and
 82kernel_neon_end(), i.e., that it is only allowed to issue NEON/VFP instructions
 83between calls to these respective functions. Furthermore, GCC may generate NEON
 84instructions of its own at -O3 level if -mfpu=neon is selected, and even if the
 85kernel is currently compiled at -O2, future changes may result in NEON/VFP
 86instructions appearing in unexpected places if no special care is taken.
 87
 88Therefore, the recommended and only supported way of using NEON/VFP in the
 89kernel is by adhering to the following rules:
 90
 91* isolate the NEON code in a separate compilation unit and compile it with
 92  '-march=armv7-a -mfpu=neon -mfloat-abi=softfp';
 93* issue the calls to kernel_neon_begin(), kernel_neon_end() as well as the calls
 94  into the unit containing the NEON code from a compilation unit which is *not*
 95  built with the GCC flag '-mfpu=neon' set.
 96
 97As the kernel is compiled with '-msoft-float', the above will guarantee that
 98both NEON and VFP instructions will only ever appear in designated compilation
 99units at any optimization level.
100
101
102NEON assembler
103--------------
104NEON assembler is supported with no additional caveats as long as the rules
105above are followed.
106
107
108NEON code generated by GCC
109--------------------------
110The GCC option -ftree-vectorize (implied by -O3) tries to exploit implicit
111parallelism, and generates NEON code from ordinary C source code. This is fully
112supported as long as the rules above are followed.
113
114
115NEON intrinsics
116---------------
117NEON intrinsics are also supported. However, as code using NEON intrinsics
118relies on the GCC header <arm_neon.h>, (which #includes <stdint.h>), you should
119observe the following in addition to the rules above:
120
121* Compile the unit containing the NEON intrinsics with '-ffreestanding' so GCC
122  uses its builtin version of <stdint.h> (this is a C99 header which the kernel
123  does not supply);
124* Include <arm_neon.h> last, or at least after <linux/types.h>