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 */
63