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ARM: 7825/1: document the use of NEON in kernel mode
Add a file to Documentation/arm explaining how kernel mode NEON is supposed to be used. Reviewed-by: Nicolas Pitre <nico@linaro.org> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
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Ard Biesheuvel
Russell King
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Documentation/arm/kernel_mode_neon.txt
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Documentation/arm/kernel_mode_neon.txt
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Kernel mode NEON
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================
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TL;DR summary
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-------------
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* Use only NEON instructions, or VFP instructions that don't rely on support
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code
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* Isolate your NEON code in a separate compilation unit, and compile it with
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'-mfpu=neon -mfloat-abi=softfp'
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* Put kernel_neon_begin() and kernel_neon_end() calls around the calls into your
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NEON code
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* Don't sleep in your NEON code, and be aware that it will be executed with
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preemption disabled
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Introduction
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------------
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It is possible to use NEON instructions (and in some cases, VFP instructions) in
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code that runs in kernel mode. However, for performance reasons, the NEON/VFP
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register file is not preserved and restored at every context switch or taken
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exception like the normal register file is, so some manual intervention is
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required. Furthermore, special care is required for code that may sleep [i.e.,
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may call schedule()], as NEON or VFP instructions will be executed in a
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non-preemptible section for reasons outlined below.
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Lazy preserve and restore
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-------------------------
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The NEON/VFP register file is managed using lazy preserve (on UP systems) and
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lazy restore (on both SMP and UP systems). This means that the register file is
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kept 'live', and is only preserved and restored when multiple tasks are
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contending for the NEON/VFP unit (or, in the SMP case, when a task migrates to
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another core). Lazy restore is implemented by disabling the NEON/VFP unit after
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every context switch, resulting in a trap when subsequently a NEON/VFP
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instruction is issued, allowing the kernel to step in and perform the restore if
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necessary.
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Any use of the NEON/VFP unit in kernel mode should not interfere with this, so
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it is required to do an 'eager' preserve of the NEON/VFP register file, and
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enable the NEON/VFP unit explicitly so no exceptions are generated on first
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subsequent use. This is handled by the function kernel_neon_begin(), which
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should be called before any kernel mode NEON or VFP instructions are issued.
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Likewise, the NEON/VFP unit should be disabled again after use to make sure user
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mode will hit the lazy restore trap upon next use. This is handled by the
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function kernel_neon_end().
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Interruptions in kernel mode
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----------------------------
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For reasons of performance and simplicity, it was decided that there shall be no
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preserve/restore mechanism for the kernel mode NEON/VFP register contents. This
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implies that interruptions of a kernel mode NEON section can only be allowed if
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they are guaranteed not to touch the NEON/VFP registers. For this reason, the
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following rules and restrictions apply in the kernel:
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* NEON/VFP code is not allowed in interrupt context;
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* NEON/VFP code is not allowed to sleep;
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* NEON/VFP code is executed with preemption disabled.
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If latency is a concern, it is possible to put back to back calls to
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kernel_neon_end() and kernel_neon_begin() in places in your code where none of
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the NEON registers are live. (Additional calls to kernel_neon_begin() should be
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reasonably cheap if no context switch occurred in the meantime)
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VFP and support code
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--------------------
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Earlier versions of VFP (prior to version 3) rely on software support for things
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like IEEE-754 compliant underflow handling etc. When the VFP unit needs such
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software assistance, it signals the kernel by raising an undefined instruction
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exception. The kernel responds by inspecting the VFP control registers and the
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current instruction and arguments, and emulates the instruction in software.
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Such software assistance is currently not implemented for VFP instructions
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executed in kernel mode. If such a condition is encountered, the kernel will
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fail and generate an OOPS.
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Separating NEON code from ordinary code
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---------------------------------------
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The compiler is not aware of the special significance of kernel_neon_begin() and
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kernel_neon_end(), i.e., that it is only allowed to issue NEON/VFP instructions
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between calls to these respective functions. Furthermore, GCC may generate NEON
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instructions of its own at -O3 level if -mfpu=neon is selected, and even if the
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kernel is currently compiled at -O2, future changes may result in NEON/VFP
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instructions appearing in unexpected places if no special care is taken.
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Therefore, the recommended and only supported way of using NEON/VFP in the
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kernel is by adhering to the following rules:
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* isolate the NEON code in a separate compilation unit and compile it with
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'-mfpu=neon -mfloat-abi=softfp';
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* issue the calls to kernel_neon_begin(), kernel_neon_end() as well as the calls
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into the unit containing the NEON code from a compilation unit which is *not*
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built with the GCC flag '-mfpu=neon' set.
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As the kernel is compiled with '-msoft-float', the above will guarantee that
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both NEON and VFP instructions will only ever appear in designated compilation
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units at any optimization level.
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NEON assembler
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--------------
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NEON assembler is supported with no additional caveats as long as the rules
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above are followed.
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NEON code generated by GCC
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--------------------------
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The GCC option -ftree-vectorize (implied by -O3) tries to exploit implicit
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parallelism, and generates NEON code from ordinary C source code. This is fully
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supported as long as the rules above are followed.
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NEON intrinsics
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---------------
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NEON intrinsics are also supported. However, as code using NEON intrinsics
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relies on the GCC header <arm_neon.h>, (which #includes <stdint.h>), you should
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observe the following in addition to the rules above:
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* Compile the unit containing the NEON intrinsics with '-ffreestanding' so GCC
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uses its builtin version of <stdint.h> (this is a C99 header which the kernel
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does not supply);
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* Include <arm_neon.h> last, or at least after <linux/types.h>
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