xref: /openbmc/qemu/util/stats64.c (revision ed75658a)
1 /*
2  * Atomic operations on 64-bit quantities.
3  *
4  * Copyright (C) 2017 Red Hat, Inc.
5  *
6  * Author: Paolo Bonzini <pbonzini@redhat.com>
7  *
8  * This work is licensed under the terms of the GNU GPL, version 2 or later.
9  * See the COPYING file in the top-level directory.
10  */
11 
12 #include "qemu/osdep.h"
13 #include "qemu/atomic.h"
14 #include "qemu/stats64.h"
15 #include "qemu/processor.h"
16 
17 #ifndef CONFIG_ATOMIC64
18 static inline void stat64_rdlock(Stat64 *s)
19 {
20     /* Keep out incoming writers to avoid them starving us. */
21     qatomic_add(&s->lock, 2);
22 
23     /* If there is a concurrent writer, wait for it.  */
24     while (qatomic_read(&s->lock) & 1) {
25         cpu_relax();
26     }
27 }
28 
29 static inline void stat64_rdunlock(Stat64 *s)
30 {
31     qatomic_sub(&s->lock, 2);
32 }
33 
34 static inline bool stat64_wrtrylock(Stat64 *s)
35 {
36     return qatomic_cmpxchg(&s->lock, 0, 1) == 0;
37 }
38 
39 static inline void stat64_wrunlock(Stat64 *s)
40 {
41     qatomic_dec(&s->lock);
42 }
43 
44 uint64_t stat64_get(const Stat64 *s)
45 {
46     uint32_t high, low;
47 
48     stat64_rdlock((Stat64 *)s);
49 
50     /* 64-bit writes always take the lock, so we can read in
51      * any order.
52      */
53     high = qatomic_read(&s->high);
54     low = qatomic_read(&s->low);
55     stat64_rdunlock((Stat64 *)s);
56 
57     return ((uint64_t)high << 32) | low;
58 }
59 
60 void stat64_set(Stat64 *s, uint64_t val)
61 {
62     while (!stat64_wrtrylock(s)) {
63         cpu_relax();
64     }
65 
66     qatomic_set(&s->high, val >> 32);
67     qatomic_set(&s->low, val);
68     stat64_wrunlock(s);
69 }
70 
71 bool stat64_add32_carry(Stat64 *s, uint32_t low, uint32_t high)
72 {
73     uint32_t old;
74 
75     if (!stat64_wrtrylock(s)) {
76         cpu_relax();
77         return false;
78     }
79 
80     /* 64-bit reads always take the lock, so they don't care about the
81      * order of our update.  By updating s->low first, we can check
82      * whether we have to carry into s->high.
83      */
84     old = qatomic_fetch_add(&s->low, low);
85     high += (old + low) < old;
86     qatomic_add(&s->high, high);
87     stat64_wrunlock(s);
88     return true;
89 }
90 
91 bool stat64_min_slow(Stat64 *s, uint64_t value)
92 {
93     uint32_t high, low;
94     uint64_t orig;
95 
96     if (!stat64_wrtrylock(s)) {
97         cpu_relax();
98         return false;
99     }
100 
101     high = qatomic_read(&s->high);
102     low = qatomic_read(&s->low);
103 
104     orig = ((uint64_t)high << 32) | low;
105     if (value < orig) {
106         /* We have to set low before high, just like stat64_min reads
107          * high before low.  The value may become higher temporarily, but
108          * stat64_get does not notice (it takes the lock) and the only ill
109          * effect on stat64_min is that the slow path may be triggered
110          * unnecessarily.
111          */
112         qatomic_set(&s->low, (uint32_t)value);
113         smp_wmb();
114         qatomic_set(&s->high, value >> 32);
115     }
116     stat64_wrunlock(s);
117     return true;
118 }
119 
120 bool stat64_max_slow(Stat64 *s, uint64_t value)
121 {
122     uint32_t high, low;
123     uint64_t orig;
124 
125     if (!stat64_wrtrylock(s)) {
126         cpu_relax();
127         return false;
128     }
129 
130     high = qatomic_read(&s->high);
131     low = qatomic_read(&s->low);
132 
133     orig = ((uint64_t)high << 32) | low;
134     if (value > orig) {
135         /* We have to set low before high, just like stat64_max reads
136          * high before low.  The value may become lower temporarily, but
137          * stat64_get does not notice (it takes the lock) and the only ill
138          * effect on stat64_max is that the slow path may be triggered
139          * unnecessarily.
140          */
141         qatomic_set(&s->low, (uint32_t)value);
142         smp_wmb();
143         qatomic_set(&s->high, value >> 32);
144     }
145     stat64_wrunlock(s);
146     return true;
147 }
148 #endif
149