| Message ID | 1376897504-24672-1-git-send-email-ard.biesheuvel@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
No objections apparently, so pushed to Russell's patch tracker [7825/1] Regards, Ard. On 19 August 2013 09:31, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote: > Reviewed-by: Nicolas Pitre <nico@linaro.org> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> > --- > v2: > updated the NEON intrinsics section to reflect that the type ambiguity issue has > been addressed by patch 'ARM: add workaround for ambiguous C99 stdint.h types' > > Documentation/arm/kernel_mode_neon.txt | 121 +++++++++++++++++++++++++++++++++ > 1 file changed, 121 insertions(+) > create mode 100644 Documentation/arm/kernel_mode_neon.txt > > diff --git a/Documentation/arm/kernel_mode_neon.txt b/Documentation/arm/kernel_mode_neon.txt > new file mode 100644 > index 0000000..5254527 > --- /dev/null > +++ b/Documentation/arm/kernel_mode_neon.txt > @@ -0,0 +1,121 @@ > +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> > -- > 1.8.1.2 >
diff --git a/Documentation/arm/kernel_mode_neon.txt b/Documentation/arm/kernel_mode_neon.txt new file mode 100644 index 0000000..5254527 --- /dev/null +++ b/Documentation/arm/kernel_mode_neon.txt @@ -0,0 +1,121 @@ +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>