mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-12-05 18:14:07 +08:00
122 lines
5.6 KiB
Plaintext
122 lines
5.6 KiB
Plaintext
|
Kernel mode NEON
|
||
|
================
|
||
|
|
||
|
TL;DR summary
|
||
|
-------------
|
||
|
* Use only NEON instructions, or VFP instructions that don't rely on support
|
||
|
code
|
||
|
* Isolate your NEON code in a separate compilation unit, and compile it with
|
||
|
'-mfpu=neon -mfloat-abi=softfp'
|
||
|
* Put kernel_neon_begin() and kernel_neon_end() calls around the calls into your
|
||
|
NEON code
|
||
|
* Don't sleep in your NEON code, and be aware that it will be executed with
|
||
|
preemption disabled
|
||
|
|
||
|
|
||
|
Introduction
|
||
|
------------
|
||
|
It is possible to use NEON instructions (and in some cases, VFP instructions) in
|
||
|
code that runs in kernel mode. However, for performance reasons, the NEON/VFP
|
||
|
register file is not preserved and restored at every context switch or taken
|
||
|
exception like the normal register file is, so some manual intervention is
|
||
|
required. Furthermore, special care is required for code that may sleep [i.e.,
|
||
|
may call schedule()], as NEON or VFP instructions will be executed in a
|
||
|
non-preemptible section for reasons outlined below.
|
||
|
|
||
|
|
||
|
Lazy preserve and restore
|
||
|
-------------------------
|
||
|
The NEON/VFP register file is managed using lazy preserve (on UP systems) and
|
||
|
lazy restore (on both SMP and UP systems). This means that the register file is
|
||
|
kept 'live', and is only preserved and restored when multiple tasks are
|
||
|
contending for the NEON/VFP unit (or, in the SMP case, when a task migrates to
|
||
|
another core). Lazy restore is implemented by disabling the NEON/VFP unit after
|
||
|
every context switch, resulting in a trap when subsequently a NEON/VFP
|
||
|
instruction is issued, allowing the kernel to step in and perform the restore if
|
||
|
necessary.
|
||
|
|
||
|
Any use of the NEON/VFP unit in kernel mode should not interfere with this, so
|
||
|
it is required to do an 'eager' preserve of the NEON/VFP register file, and
|
||
|
enable the NEON/VFP unit explicitly so no exceptions are generated on first
|
||
|
subsequent use. This is handled by the function kernel_neon_begin(), which
|
||
|
should be called before any kernel mode NEON or VFP instructions are issued.
|
||
|
Likewise, the NEON/VFP unit should be disabled again after use to make sure user
|
||
|
mode will hit the lazy restore trap upon next use. This is handled by the
|
||
|
function kernel_neon_end().
|
||
|
|
||
|
|
||
|
Interruptions in kernel mode
|
||
|
----------------------------
|
||
|
For reasons of performance and simplicity, it was decided that there shall be no
|
||
|
preserve/restore mechanism for the kernel mode NEON/VFP register contents. This
|
||
|
implies that interruptions of a kernel mode NEON section can only be allowed if
|
||
|
they are guaranteed not to touch the NEON/VFP registers. For this reason, the
|
||
|
following rules and restrictions apply in the kernel:
|
||
|
* NEON/VFP code is not allowed in interrupt context;
|
||
|
* NEON/VFP code is not allowed to sleep;
|
||
|
* NEON/VFP code is executed with preemption disabled.
|
||
|
|
||
|
If latency is a concern, it is possible to put back to back calls to
|
||
|
kernel_neon_end() and kernel_neon_begin() in places in your code where none of
|
||
|
the NEON registers are live. (Additional calls to kernel_neon_begin() should be
|
||
|
reasonably cheap if no context switch occurred in the meantime)
|
||
|
|
||
|
|
||
|
VFP and support code
|
||
|
--------------------
|
||
|
Earlier versions of VFP (prior to version 3) rely on software support for things
|
||
|
like IEEE-754 compliant underflow handling etc. When the VFP unit needs such
|
||
|
software assistance, it signals the kernel by raising an undefined instruction
|
||
|
exception. The kernel responds by inspecting the VFP control registers and the
|
||
|
current instruction and arguments, and emulates the instruction in software.
|
||
|
|
||
|
Such software assistance is currently not implemented for VFP instructions
|
||
|
executed in kernel mode. If such a condition is encountered, the kernel will
|
||
|
fail and generate an OOPS.
|
||
|
|
||
|
|
||
|
Separating NEON code from ordinary code
|
||
|
---------------------------------------
|
||
|
The compiler is not aware of the special significance of kernel_neon_begin() and
|
||
|
kernel_neon_end(), i.e., that it is only allowed to issue NEON/VFP instructions
|
||
|
between calls to these respective functions. Furthermore, GCC may generate NEON
|
||
|
instructions of its own at -O3 level if -mfpu=neon is selected, and even if the
|
||
|
kernel is currently compiled at -O2, future changes may result in NEON/VFP
|
||
|
instructions appearing in unexpected places if no special care is taken.
|
||
|
|
||
|
Therefore, the recommended and only supported way of using NEON/VFP in the
|
||
|
kernel is by adhering to the following rules:
|
||
|
* isolate the NEON code in a separate compilation unit and compile it with
|
||
|
'-mfpu=neon -mfloat-abi=softfp';
|
||
|
* issue the calls to kernel_neon_begin(), kernel_neon_end() as well as the calls
|
||
|
into the unit containing the NEON code from a compilation unit which is *not*
|
||
|
built with the GCC flag '-mfpu=neon' set.
|
||
|
|
||
|
As the kernel is compiled with '-msoft-float', the above will guarantee that
|
||
|
both NEON and VFP instructions will only ever appear in designated compilation
|
||
|
units at any optimization level.
|
||
|
|
||
|
|
||
|
NEON assembler
|
||
|
--------------
|
||
|
NEON assembler is supported with no additional caveats as long as the rules
|
||
|
above are followed.
|
||
|
|
||
|
|
||
|
NEON code generated by GCC
|
||
|
--------------------------
|
||
|
The GCC option -ftree-vectorize (implied by -O3) tries to exploit implicit
|
||
|
parallelism, and generates NEON code from ordinary C source code. This is fully
|
||
|
supported as long as the rules above are followed.
|
||
|
|
||
|
|
||
|
NEON intrinsics
|
||
|
---------------
|
||
|
NEON intrinsics are also supported. However, as code using NEON intrinsics
|
||
|
relies on the GCC header <arm_neon.h>, (which #includes <stdint.h>), you should
|
||
|
observe the following in addition to the rules above:
|
||
|
* Compile the unit containing the NEON intrinsics with '-ffreestanding' so GCC
|
||
|
uses its builtin version of <stdint.h> (this is a C99 header which the kernel
|
||
|
does not supply);
|
||
|
* Include <arm_neon.h> last, or at least after <linux/types.h>
|