block/cow.c

   1 /*
   2  * Block driver for the COW format
   3  *
   4  * Copyright (c) 2004 Fabrice Bellard
   5  *
   6  * Permission is hereby granted, free of charge, to any person obtaining a copy
   7  * of this software and associated documentation files (the "Software"), to deal
   8  * in the Software without restriction, including without limitation the rights
   9  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  10  * copies of the Software, and to permit persons to whom the Software is
  11  * furnished to do so, subject to the following conditions:
  12  *
  13  * The above copyright notice and this permission notice shall be included in
  14  * all copies or substantial portions of the Software.
  15  *
  16  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  17  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  18  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
  19  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  20  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  21  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  22  * THE SOFTWARE.
  23  */
  24 #include "qemu-common.h"
  25 #include "block_int.h"
  26 #include "module.h"
  27
  28 /**************************************************************/
  29 /* COW block driver using file system holes */
  30
  31 /* user mode linux compatible COW file */
  32 #define COW_MAGIC 0x4f4f4f4d  /* MOOO */
  33 #define COW_VERSION 2
  34
  35 struct cow_header_v2 {
  36     uint32_t magic;
  37     uint32_t version;
  38     char backing_file[1024];
  39     int32_t mtime;
  40     uint64_t size;
  41     uint32_t sectorsize;
  42 };
  43
  44 typedef struct BDRVCowState {
  45     int64_t cow_sectors_offset;
  46 } BDRVCowState;
  47
  48 static int cow_probe(const uint8_t *buf, int buf_size, const char *filename)
  49 {
  50     const struct cow_header_v2 *cow_header = (const void *)buf;
  51
  52     if (buf_size >= sizeof(struct cow_header_v2) &&
  53         be32_to_cpu(cow_header->magic) == COW_MAGIC &&
  54         be32_to_cpu(cow_header->version) == COW_VERSION)
  55         return 100;
  56     else
  57         return 0;
  58 }
  59
  60 static int cow_open(BlockDriverState *bs, int flags)
  61 {
  62     BDRVCowState *s = bs->opaque;
  63     struct cow_header_v2 cow_header;
  64     int bitmap_size;
  65     int64_t size;
  66
  67     /* see if it is a cow image */
  68     if (bdrv_pread(bs->file, 0, &cow_header, sizeof(cow_header)) !=
  69             sizeof(cow_header)) {
  70         goto fail;
  71     }
  72
  73     if (be32_to_cpu(cow_header.magic) != COW_MAGIC ||
  74         be32_to_cpu(cow_header.version) != COW_VERSION) {
  75         goto fail;
  76     }
  77
  78     /* cow image found */
  79     size = be64_to_cpu(cow_header.size);
  80     bs->total_sectors = size / 512;
  81
  82     pstrcpy(bs->backing_file, sizeof(bs->backing_file),
  83             cow_header.backing_file);
  84
  85     bitmap_size = ((bs->total_sectors + 7) >> 3) + sizeof(cow_header);
  86     s->cow_sectors_offset = (bitmap_size + 511) & ~511;
  87     return 0;
  88  fail:
  89     return -1;
  90 }
  91
  92 /*
  93  * XXX(hch): right now these functions are extremly ineffcient.
  94  * We should just read the whole bitmap we'll need in one go instead.
  95  */
  96 static inline int cow_set_bit(BlockDriverState *bs, int64_t bitnum)
  97 {
  98     uint64_t offset = sizeof(struct cow_header_v2) + bitnum / 8;
  99     uint8_t bitmap;
 100     int ret;
 101
 102     ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));
 103     if (ret < 0) {
 104        return ret;
 105     }
 106
 107     bitmap |= (1 << (bitnum % 8));
 108
 109     ret = bdrv_pwrite_sync(bs->file, offset, &bitmap, sizeof(bitmap));
 110     if (ret < 0) {
 111        return ret;
 112     }
 113     return 0;
 114 }
 115
 116 static inline int is_bit_set(BlockDriverState *bs, int64_t bitnum)
 117 {
 118     uint64_t offset = sizeof(struct cow_header_v2) + bitnum / 8;
 119     uint8_t bitmap;
 120     int ret;
 121
 122     ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));
 123     if (ret < 0) {
 124        return ret;
 125     }
 126
 127     return !!(bitmap & (1 << (bitnum % 8)));
 128 }
 129
 130 /* Return true if first block has been changed (ie. current version is
 131  * in COW file).  Set the number of continuous blocks for which that
 132  * is true. */
 133 static int cow_is_allocated(BlockDriverState *bs, int64_t sector_num,
 134         int nb_sectors, int *num_same)
 135 {
 136     int changed;
 137
 138     if (nb_sectors == 0) {
 139         *num_same = nb_sectors;
 140         return 0;
 141     }
 142
 143     changed = is_bit_set(bs, sector_num);
 144     if (changed < 0) {
 145         return 0; /* XXX: how to return I/O errors? */
 146     }
 147
 148     for (*num_same = 1; *num_same < nb_sectors; (*num_same)++) {
 149         if (is_bit_set(bs, sector_num + *num_same) != changed)
 150             break;
 151     }
 152
 153     return changed;
 154 }
 155
 156 static int cow_update_bitmap(BlockDriverState *bs, int64_t sector_num,
 157         int nb_sectors)
 158 {
 159     int error = 0;
 160     int i;
 161
 162     for (i = 0; i < nb_sectors; i++) {
 163         error = cow_set_bit(bs, sector_num + i);
 164         if (error) {
 165             break;
 166         }
 167     }
 168
 169     return error;
 170 }
 171
 172 static int cow_read(BlockDriverState *bs, int64_t sector_num,
 173                     uint8_t *buf, int nb_sectors)
 174 {
 175     BDRVCowState *s = bs->opaque;
 176     int ret, n;
 177
 178     while (nb_sectors > 0) {
 179         if (cow_is_allocated(bs, sector_num, nb_sectors, &n)) {
 180             ret = bdrv_pread(bs->file,
 181                         s->cow_sectors_offset + sector_num * 512,
 182                         buf, n * 512);
 183             if (ret != n * 512)
 184                 return -1;
 185         } else {
 186             if (bs->backing_hd) {
 187                 /* read from the base image */
 188                 ret = bdrv_read(bs->backing_hd, sector_num, buf, n);
 189                 if (ret < 0)
 190                     return -1;
 191             } else {
 192             memset(buf, 0, n * 512);
 193         }
 194         }
 195         nb_sectors -= n;
 196         sector_num += n;
 197         buf += n * 512;
 198     }
 199     return 0;
 200 }
 201
 202 static int cow_write(BlockDriverState *bs, int64_t sector_num,
 203                      const uint8_t *buf, int nb_sectors)
 204 {
 205     BDRVCowState *s = bs->opaque;
 206     int ret;
 207
 208     ret = bdrv_pwrite(bs->file, s->cow_sectors_offset + sector_num * 512,
 209                       buf, nb_sectors * 512);
 210     if (ret != nb_sectors * 512)
 211         return -1;
 212
 213     return cow_update_bitmap(bs, sector_num, nb_sectors);
 214 }
 215
 216 static void cow_close(BlockDriverState *bs)
 217 {
 218 }
 219
 220 static int cow_create(const char *filename, QEMUOptionParameter *options)
 221 {
 222     int fd, cow_fd;
 223     struct cow_header_v2 cow_header;
 224     struct stat st;
 225     int64_t image_sectors = 0;
 226     const char *image_filename = NULL;
 227     int ret;
 228
 229     /* Read out options */
 230     while (options && options->name) {
 231         if (!strcmp(options->name, BLOCK_OPT_SIZE)) {
 232             image_sectors = options->value.n / 512;
 233         } else if (!strcmp(options->name, BLOCK_OPT_BACKING_FILE)) {
 234             image_filename = options->value.s;
 235         }
 236         options++;
 237     }
 238
 239     cow_fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY,
 240               0644);
 241     if (cow_fd < 0)
 242         return -errno;
 243     memset(&cow_header, 0, sizeof(cow_header));
 244     cow_header.magic = cpu_to_be32(COW_MAGIC);
 245     cow_header.version = cpu_to_be32(COW_VERSION);
 246     if (image_filename) {
 247         /* Note: if no file, we put a dummy mtime */
 248         cow_header.mtime = cpu_to_be32(0);
 249
 250         fd = open(image_filename, O_RDONLY | O_BINARY);
 251         if (fd < 0) {
 252             close(cow_fd);
 253             goto mtime_fail;
 254         }
 255         if (fstat(fd, &st) != 0) {
 256             close(fd);
 257             goto mtime_fail;
 258         }
 259         close(fd);
 260         cow_header.mtime = cpu_to_be32(st.st_mtime);
 261     mtime_fail:
 262         pstrcpy(cow_header.backing_file, sizeof(cow_header.backing_file),
 263                 image_filename);
 264     }
 265     cow_header.sectorsize = cpu_to_be32(512);
 266     cow_header.size = cpu_to_be64(image_sectors * 512);
 267     ret = qemu_write_full(cow_fd, &cow_header, sizeof(cow_header));
 268     if (ret != sizeof(cow_header)) {
 269         ret = -errno;
 270         goto exit;
 271     }
 272
 273     /* resize to include at least all the bitmap */
 274     ret = ftruncate(cow_fd, sizeof(cow_header) + ((image_sectors + 7) >> 3));
 275     if (ret) {
 276         ret = -errno;
 277         goto exit;
 278     }
 279
 280 exit:
 281     close(cow_fd);
 282     return ret;
 283 }
 284
 285 static int cow_flush(BlockDriverState *bs)
 286 {
 287     return bdrv_flush(bs->file);
 288 }
 289
 290 static QEMUOptionParameter cow_create_options[] = {
 291     {
 292         .name = BLOCK_OPT_SIZE,
 293         .type = OPT_SIZE,
 294         .help = "Virtual disk size"
 295     },
 296     {
 297         .name = BLOCK_OPT_BACKING_FILE,
 298         .type = OPT_STRING,
 299         .help = "File name of a base image"
 300     },
 301     { NULL }
 302 };
 303
 304 static BlockDriver bdrv_cow = {
 305     .format_name        = "cow",
 306     .instance_size      = sizeof(BDRVCowState),
 307     .bdrv_probe         = cow_probe,
 308     .bdrv_open          = cow_open,
 309     .bdrv_read          = cow_read,
 310     .bdrv_write         = cow_write,
 311     .bdrv_close         = cow_close,
 312     .bdrv_create        = cow_create,
 313     .bdrv_flush         = cow_flush,
 314     .bdrv_is_allocated  = cow_is_allocated,
 315
 316     .create_options = cow_create_options,
 317 };
 318
 319 static void bdrv_cow_init(void)
 320 {
 321     bdrv_register(&bdrv_cow);
 322 }
 323
 324 block_init(bdrv_cow_init);