arch/arm64/include/asm/simd.h

   1 /*
   2  * Copyright (C) 2017 Linaro Ltd. <ard.biesheuvel@linaro.org>
   3  *
   4  * This program is free software; you can redistribute it and/or modify it
   5  * under the terms of the GNU General Public License version 2 as published
   6  * by the Free Software Foundation.
   7  */
   8
   9 #ifndef __ASM_SIMD_H
  10 #define __ASM_SIMD_H
  11
  12 #include <linux/compiler.h>
  13 #include <linux/irqflags.h>
  14 #include <linux/percpu.h>
  15 #include <linux/preempt.h>
  16 #include <linux/types.h>
  17
  18 #ifdef CONFIG_KERNEL_MODE_NEON
  19
  20 DECLARE_PER_CPU(bool, kernel_neon_busy);
  21
  22 /*
  23  * may_use_simd - whether it is allowable at this time to issue SIMD
  24  *                instructions or access the SIMD register file
  25  *
  26  * Callers must not assume that the result remains true beyond the next
  27  * preempt_enable() or return from softirq context.
  28  */
  29 static __must_check inline bool may_use_simd(void)
  30 {
  31         /*
  32          * The raw_cpu_read() is racy if called with preemption enabled.
  33          * This is not a bug: kernel_neon_busy is only set when
  34          * preemption is disabled, so we cannot migrate to another CPU
  35          * while it is set, nor can we migrate to a CPU where it is set.
  36          * So, if we find it clear on some CPU then we're guaranteed to
  37          * find it clear on any CPU we could migrate to.
  38          *
  39          * If we are in between kernel_neon_begin()...kernel_neon_end(),
  40          * the flag will be set, but preemption is also disabled, so we
  41          * can't migrate to another CPU and spuriously see it become
  42          * false.
  43          */
  44         return !in_irq() && !irqs_disabled() && !in_nmi() &&
  45                 !raw_cpu_read(kernel_neon_busy);
  46 }
  47
  48 #else /* ! CONFIG_KERNEL_MODE_NEON */
  49
  50 static __must_check inline bool may_use_simd(void) {
  51         return false;
  52 }
  53
  54 #endif /* ! CONFIG_KERNEL_MODE_NEON */
  55
  56 #endif