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