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