xref: /openbmc/qemu/util/buffer.c (revision 764a6ee9)
1 /*
2  * QEMU generic buffers
3  *
4  * Copyright (c) 2015 Red Hat, Inc.
5  *
6  * This library is free software; you can redistribute it and/or
7  * modify it under the terms of the GNU Lesser General Public
8  * License as published by the Free Software Foundation; either
9  * version 2.1 of the License, or (at your option) any later version.
10  *
11  * This library is distributed in the hope that it will be useful,
12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
14  * Lesser General Public License for more details.
15  *
16  * You should have received a copy of the GNU Lesser General Public
17  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
18  *
19  */
20 
21 #include "qemu/osdep.h"
22 #include "qemu/host-utils.h"
23 #include "qemu/buffer.h"
24 #include "trace.h"
25 
26 #define BUFFER_MIN_INIT_SIZE     4096
27 #define BUFFER_MIN_SHRINK_SIZE  65536
28 
29 /* define the factor alpha for the exponential smoothing
30  * that is used in the average size calculation. a shift
31  * of 7 results in an alpha of 1/2^7. */
32 #define BUFFER_AVG_SIZE_SHIFT       7
33 
34 static size_t buffer_req_size(Buffer *buffer, size_t len)
35 {
36     return MAX(BUFFER_MIN_INIT_SIZE,
37                pow2ceil(buffer->offset + len));
38 }
39 
40 static void buffer_adj_size(Buffer *buffer, size_t len)
41 {
42     size_t old = buffer->capacity;
43     buffer->capacity = buffer_req_size(buffer, len);
44     buffer->buffer = g_realloc(buffer->buffer, buffer->capacity);
45     trace_buffer_resize(buffer->name ?: "unnamed",
46                         old, buffer->capacity);
47 
48     /* make it even harder for the buffer to shrink, reset average size
49      * to current capacity if it is larger than the average. */
50     buffer->avg_size = MAX(buffer->avg_size,
51                            buffer->capacity << BUFFER_AVG_SIZE_SHIFT);
52 }
53 
54 void buffer_init(Buffer *buffer, const char *name, ...)
55 {
56     va_list ap;
57 
58     va_start(ap, name);
59     buffer->name = g_strdup_vprintf(name, ap);
60     va_end(ap);
61 }
62 
63 static uint64_t buffer_get_avg_size(Buffer *buffer)
64 {
65     return buffer->avg_size >> BUFFER_AVG_SIZE_SHIFT;
66 }
67 
68 void buffer_shrink(Buffer *buffer)
69 {
70     size_t new;
71 
72     /* Calculate the average size of the buffer as
73      * avg_size = avg_size * ( 1 - a ) + required_size * a
74      * where a is 1 / 2 ^ BUFFER_AVG_SIZE_SHIFT. */
75     buffer->avg_size *= (1 << BUFFER_AVG_SIZE_SHIFT) - 1;
76     buffer->avg_size >>= BUFFER_AVG_SIZE_SHIFT;
77     buffer->avg_size += buffer_req_size(buffer, 0);
78 
79     /* And then only shrink if the average size of the buffer is much
80      * too big, to avoid bumping up & down the buffers all the time.
81      * realloc() isn't exactly cheap ...  */
82     new = buffer_req_size(buffer, buffer_get_avg_size(buffer));
83     if (new < buffer->capacity >> 3 &&
84         new >= BUFFER_MIN_SHRINK_SIZE) {
85         buffer_adj_size(buffer, buffer_get_avg_size(buffer));
86     }
87 
88     buffer_adj_size(buffer, 0);
89 }
90 
91 void buffer_reserve(Buffer *buffer, size_t len)
92 {
93     if ((buffer->capacity - buffer->offset) < len) {
94         buffer_adj_size(buffer, len);
95     }
96 }
97 
98 gboolean buffer_empty(Buffer *buffer)
99 {
100     return buffer->offset == 0;
101 }
102 
103 uint8_t *buffer_end(Buffer *buffer)
104 {
105     return buffer->buffer + buffer->offset;
106 }
107 
108 void buffer_reset(Buffer *buffer)
109 {
110     buffer->offset = 0;
111     buffer_shrink(buffer);
112 }
113 
114 void buffer_free(Buffer *buffer)
115 {
116     trace_buffer_free(buffer->name ?: "unnamed", buffer->capacity);
117     g_free(buffer->buffer);
118     g_free(buffer->name);
119     buffer->offset = 0;
120     buffer->capacity = 0;
121     buffer->buffer = NULL;
122     buffer->name = NULL;
123 }
124 
125 void buffer_append(Buffer *buffer, const void *data, size_t len)
126 {
127     memcpy(buffer->buffer + buffer->offset, data, len);
128     buffer->offset += len;
129 }
130 
131 void buffer_advance(Buffer *buffer, size_t len)
132 {
133     memmove(buffer->buffer, buffer->buffer + len,
134             (buffer->offset - len));
135     buffer->offset -= len;
136     buffer_shrink(buffer);
137 }
138 
139 void buffer_move_empty(Buffer *to, Buffer *from)
140 {
141     trace_buffer_move_empty(to->name ?: "unnamed",
142                             from->offset,
143                             from->name ?: "unnamed");
144     assert(to->offset == 0);
145 
146     g_free(to->buffer);
147     to->offset = from->offset;
148     to->capacity = from->capacity;
149     to->buffer = from->buffer;
150 
151     from->offset = 0;
152     from->capacity = 0;
153     from->buffer = NULL;
154 }
155 
156 void buffer_move(Buffer *to, Buffer *from)
157 {
158     if (to->offset == 0) {
159         buffer_move_empty(to, from);
160         return;
161     }
162 
163     trace_buffer_move(to->name ?: "unnamed",
164                       from->offset,
165                       from->name ?: "unnamed");
166     buffer_reserve(to, from->offset);
167     buffer_append(to, from->buffer, from->offset);
168 
169     g_free(from->buffer);
170     from->offset = 0;
171     from->capacity = 0;
172     from->buffer = NULL;
173 }
174