LXR linux/include/linux/processor.h

   1/* SPDX-License-Identifier: GPL-2.0 */
   2/* Misc low level processor primitives */
   3#ifndef _LINUX_PROCESSOR_H
   4#define _LINUX_PROCESSOR_H
   5
   6#include <asm/processor.h>
   7
   8/*
   9 * spin_begin is used before beginning a busy-wait loop, and must be paired
  10 * with spin_end when the loop is exited. spin_cpu_relax must be called
  11 * within the loop.
  12 *
  13 * The loop body should be as small and fast as possible, on the order of
  14 * tens of instructions/cycles as a guide. It should and avoid calling
  15 * cpu_relax, or any "spin" or sleep type of primitive including nested uses
  16 * of these primitives. It should not lock or take any other resource.
  17 * Violations of these guidelies will not cause a bug, but may cause sub
  18 * optimal performance.
  19 *
  20 * These loops are optimized to be used where wait times are expected to be
  21 * less than the cost of a context switch (and associated overhead).
  22 *
  23 * Detection of resource owner and decision to spin or sleep or guest-yield
  24 * (e.g., spin lock holder vcpu preempted, or mutex owner not on CPU) can be
  25 * tested within the loop body.
  26 */
  27#ifndef spin_begin
  28#define spin_begin()
  29#endif
  30
  31#ifndef spin_cpu_relax
  32#define spin_cpu_relax() cpu_relax()
  33#endif
  34
  35/*
  36 * spin_cpu_yield may be called to yield (undirected) to the hypervisor if
  37 * necessary. This should be used if the wait is expected to take longer
  38 * than context switch overhead, but we can't sleep or do a directed yield.
  39 */
  40#ifndef spin_cpu_yield
  41#define spin_cpu_yield() cpu_relax_yield()
  42#endif
  43
  44#ifndef spin_end
  45#define spin_end()
  46#endif
  47
  48/*
  49 * spin_until_cond can be used to wait for a condition to become true. It
  50 * may be expected that the first iteration will true in the common case
  51 * (no spinning), so that callers should not require a first "likely" test
  52 * for the uncontended case before using this primitive.
  53 *
  54 * Usage and implementation guidelines are the same as for the spin_begin
  55 * primitives, above.
  56 */
  57#ifndef spin_until_cond
  58#define spin_until_cond(cond)                                   \
  59do {                                                            \
  60        if (unlikely(!(cond))) {                                \
  61                spin_begin();                                   \
  62                do {                                            \
  63                        spin_cpu_relax();                       \
  64                } while (!(cond));                              \
  65                spin_end();                                     \
  66        }                                                       \
  67} while (0)
  68
  69#endif
  70
  71#endif /* _LINUX_PROCESSOR_H */
  72