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 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/buffer.h" 23 #include "trace.h" 24 25 #define BUFFER_MIN_INIT_SIZE 4096 26 #define BUFFER_MIN_SHRINK_SIZE 65536 27 28 /* define the factor alpha for the expentional smoothing 29 * that is used in the average size calculation. a shift 30 * of 7 results in an alpha of 1/2^7. */ 31 #define BUFFER_AVG_SIZE_SHIFT 7 32 33 static size_t buffer_req_size(Buffer *buffer, size_t len) 34 { 35 return MAX(BUFFER_MIN_INIT_SIZE, 36 pow2ceil(buffer->offset + len)); 37 } 38 39 static void buffer_adj_size(Buffer *buffer, size_t len) 40 { 41 size_t old = buffer->capacity; 42 buffer->capacity = buffer_req_size(buffer, len); 43 buffer->buffer = g_realloc(buffer->buffer, buffer->capacity); 44 trace_buffer_resize(buffer->name ?: "unnamed", 45 old, buffer->capacity); 46 47 /* make it even harder for the buffer to shrink, reset average size 48 * to currenty capacity if it is larger than the average. */ 49 buffer->avg_size = MAX(buffer->avg_size, 50 buffer->capacity << BUFFER_AVG_SIZE_SHIFT); 51 } 52 53 void buffer_init(Buffer *buffer, const char *name, ...) 54 { 55 va_list ap; 56 57 va_start(ap, name); 58 buffer->name = g_strdup_vprintf(name, ap); 59 va_end(ap); 60 } 61 62 static uint64_t buffer_get_avg_size(Buffer *buffer) 63 { 64 return buffer->avg_size >> BUFFER_AVG_SIZE_SHIFT; 65 } 66 67 void buffer_shrink(Buffer *buffer) 68 { 69 size_t new; 70 71 /* Calculate the average size of the buffer as 72 * avg_size = avg_size * ( 1 - a ) + required_size * a 73 * where a is 1 / 2 ^ BUFFER_AVG_SIZE_SHIFT. */ 74 buffer->avg_size *= (1 << BUFFER_AVG_SIZE_SHIFT) - 1; 75 buffer->avg_size >>= BUFFER_AVG_SIZE_SHIFT; 76 buffer->avg_size += buffer_req_size(buffer, 0); 77 78 /* And then only shrink if the average size of the buffer is much 79 * too big, to avoid bumping up & down the buffers all the time. 80 * realloc() isn't exactly cheap ... */ 81 new = buffer_req_size(buffer, buffer_get_avg_size(buffer)); 82 if (new < buffer->capacity >> 3 && 83 new >= BUFFER_MIN_SHRINK_SIZE) { 84 buffer_adj_size(buffer, buffer_get_avg_size(buffer)); 85 } 86 87 buffer_adj_size(buffer, 0); 88 } 89 90 void buffer_reserve(Buffer *buffer, size_t len) 91 { 92 if ((buffer->capacity - buffer->offset) < len) { 93 buffer_adj_size(buffer, len); 94 } 95 } 96 97 gboolean buffer_empty(Buffer *buffer) 98 { 99 return buffer->offset == 0; 100 } 101 102 uint8_t *buffer_end(Buffer *buffer) 103 { 104 return buffer->buffer + buffer->offset; 105 } 106 107 void buffer_reset(Buffer *buffer) 108 { 109 buffer->offset = 0; 110 buffer_shrink(buffer); 111 } 112 113 void buffer_free(Buffer *buffer) 114 { 115 trace_buffer_free(buffer->name ?: "unnamed", buffer->capacity); 116 g_free(buffer->buffer); 117 g_free(buffer->name); 118 buffer->offset = 0; 119 buffer->capacity = 0; 120 buffer->buffer = NULL; 121 buffer->name = NULL; 122 } 123 124 void buffer_append(Buffer *buffer, const void *data, size_t len) 125 { 126 memcpy(buffer->buffer + buffer->offset, data, len); 127 buffer->offset += len; 128 } 129 130 void buffer_advance(Buffer *buffer, size_t len) 131 { 132 memmove(buffer->buffer, buffer->buffer + len, 133 (buffer->offset - len)); 134 buffer->offset -= len; 135 buffer_shrink(buffer); 136 } 137 138 void buffer_move_empty(Buffer *to, Buffer *from) 139 { 140 trace_buffer_move_empty(to->name ?: "unnamed", 141 from->offset, 142 from->name ?: "unnamed"); 143 assert(to->offset == 0); 144 145 g_free(to->buffer); 146 to->offset = from->offset; 147 to->capacity = from->capacity; 148 to->buffer = from->buffer; 149 150 from->offset = 0; 151 from->capacity = 0; 152 from->buffer = NULL; 153 } 154 155 void buffer_move(Buffer *to, Buffer *from) 156 { 157 if (to->offset == 0) { 158 buffer_move_empty(to, from); 159 return; 160 } 161 162 trace_buffer_move(to->name ?: "unnamed", 163 from->offset, 164 from->name ?: "unnamed"); 165 buffer_reserve(to, from->offset); 166 buffer_append(to, from->buffer, from->offset); 167 168 g_free(from->buffer); 169 from->offset = 0; 170 from->capacity = 0; 171 from->buffer = NULL; 172 } 173