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 #ifndef spin_end
36 #define spin_end()
37 #endif
38
39 /*
40 * spin_until_cond can be used to wait for a condition to become true. It
41 * may be expected that the first iteration will true in the common case
42 * (no spinning), so that callers should not require a first "likely" test
43 * for the uncontended case before using this primitive.
44 *
45 * Usage and implementation guidelines are the same as for the spin_begin
46 * primitives, above.
47 */
48 #ifndef spin_until_cond
49 #define spin_until_cond(cond) \
50 do { \
51 if (unlikely(!(cond))) { \
52 spin_begin(); \
53 do { \
54 spin_cpu_relax(); \
55 } while (!(cond)); \
56 spin_end(); \
57 } \
58 } while (0)
59
60 #endif
61
62 #endif /* _LINUX_PROCESSOR_H */