util/iov.c

   1 /*
   2  * Helpers for getting linearized buffers from iov / filling buffers into iovs
   3  *
   4  * Copyright IBM, Corp. 2007, 2008
   5  * Copyright (C) 2010 Red Hat, Inc.
   6  *
   7  * Author(s):
   8  *  Anthony Liguori <aliguori@us.ibm.com>
   9  *  Amit Shah <amit.shah@redhat.com>
  10  *  Michael Tokarev <mjt@tls.msk.ru>
  11  *
  12  * This work is licensed under the terms of the GNU GPL, version 2.  See
  13  * the COPYING file in the top-level directory.
  14  *
  15  * Contributions after 2012-01-13 are licensed under the terms of the
  16  * GNU GPL, version 2 or (at your option) any later version.
  17  */
  18
  19 #include "qemu/osdep.h"
  20 #include "qemu/iov.h"
  21 #include "qemu/sockets.h"
  22 #include "qemu/cutils.h"
  23
  24 size_t iov_from_buf_full(const struct iovec *iov, unsigned int iov_cnt,
  25                          size_t offset, const void *buf, size_t bytes)
  26 {
  27     size_t done;
  28     unsigned int i;
  29     for (i = 0, done = 0; (offset || done < bytes) && i < iov_cnt; i++) {
  30         if (offset < iov[i].iov_len) {
  31             size_t len = MIN(iov[i].iov_len - offset, bytes - done);
  32             memcpy(iov[i].iov_base + offset, buf + done, len);
  33             done += len;
  34             offset = 0;
  35         } else {
  36             offset -= iov[i].iov_len;
  37         }
  38     }
  39     assert(offset == 0);
  40     return done;
  41 }
  42
  43 size_t iov_to_buf_full(const struct iovec *iov, const unsigned int iov_cnt,
  44                        size_t offset, void *buf, size_t bytes)
  45 {
  46     size_t done;
  47     unsigned int i;
  48     for (i = 0, done = 0; (offset || done < bytes) && i < iov_cnt; i++) {
  49         if (offset < iov[i].iov_len) {
  50             size_t len = MIN(iov[i].iov_len - offset, bytes - done);
  51             memcpy(buf + done, iov[i].iov_base + offset, len);
  52             done += len;
  53             offset = 0;
  54         } else {
  55             offset -= iov[i].iov_len;
  56         }
  57     }
  58     assert(offset == 0);
  59     return done;
  60 }
  61
  62 size_t iov_memset(const struct iovec *iov, const unsigned int iov_cnt,
  63                   size_t offset, int fillc, size_t bytes)
  64 {
  65     size_t done;
  66     unsigned int i;
  67     for (i = 0, done = 0; (offset || done < bytes) && i < iov_cnt; i++) {
  68         if (offset < iov[i].iov_len) {
  69             size_t len = MIN(iov[i].iov_len - offset, bytes - done);
  70             memset(iov[i].iov_base + offset, fillc, len);
  71             done += len;
  72             offset = 0;
  73         } else {
  74             offset -= iov[i].iov_len;
  75         }
  76     }
  77     assert(offset == 0);
  78     return done;
  79 }
  80
  81 size_t iov_size(const struct iovec *iov, const unsigned int iov_cnt)
  82 {
  83     size_t len;
  84     unsigned int i;
  85
  86     len = 0;
  87     for (i = 0; i < iov_cnt; i++) {
  88         len += iov[i].iov_len;
  89     }
  90     return len;
  91 }
  92
  93 /* helper function for iov_send_recv() */
  94 static ssize_t
  95 do_send_recv(int sockfd, struct iovec *iov, unsigned iov_cnt, bool do_send)
  96 {
  97 #ifdef CONFIG_POSIX
  98     ssize_t ret;
  99     struct msghdr msg;
 100     memset(&msg, 0, sizeof(msg));
 101     msg.msg_iov = iov;
 102     msg.msg_iovlen = iov_cnt;
 103     do {
 104         ret = do_send
 105             ? sendmsg(sockfd, &msg, 0)
 106             : recvmsg(sockfd, &msg, 0);
 107     } while (ret < 0 && errno == EINTR);
 108     return ret;
 109 #else
 110     /* else send piece-by-piece */
 111     /*XXX Note: windows has WSASend() and WSARecv() */
 112     unsigned i = 0;
 113     ssize_t ret = 0;
 114     while (i < iov_cnt) {
 115         ssize_t r = do_send
 116             ? send(sockfd, iov[i].iov_base, iov[i].iov_len, 0)
 117             : recv(sockfd, iov[i].iov_base, iov[i].iov_len, 0);
 118         if (r > 0) {
 119             ret += r;
 120         } else if (!r) {
 121             break;
 122         } else if (errno == EINTR) {
 123             continue;
 124         } else {
 125             /* else it is some "other" error,
 126              * only return if there was no data processed. */
 127             if (ret == 0) {
 128                 ret = -1;
 129             }
 130             break;
 131         }
 132         i++;
 133     }
 134     return ret;
 135 #endif
 136 }
 137
 138 ssize_t iov_send_recv(int sockfd, const struct iovec *_iov, unsigned iov_cnt,
 139                       size_t offset, size_t bytes,
 140                       bool do_send)
 141 {
 142     ssize_t total = 0;
 143     ssize_t ret;
 144     size_t orig_len, tail;
 145     unsigned niov;
 146     struct iovec *local_iov, *iov;
 147
 148     if (bytes <= 0) {
 149         return 0;
 150     }
 151
 152     local_iov = g_new0(struct iovec, iov_cnt);
 153     iov_copy(local_iov, iov_cnt, _iov, iov_cnt, offset, bytes);
 154     offset = 0;
 155     iov = local_iov;
 156
 157     while (bytes > 0) {
 158         /* Find the start position, skipping `offset' bytes:
 159          * first, skip all full-sized vector elements, */
 160         for (niov = 0; niov < iov_cnt && offset >= iov[niov].iov_len; ++niov) {
 161             offset -= iov[niov].iov_len;
 162         }
 163
 164         /* niov == iov_cnt would only be valid if bytes == 0, which
 165          * we already ruled out in the loop condition.  */
 166         assert(niov < iov_cnt);
 167         iov += niov;
 168         iov_cnt -= niov;
 169
 170         if (offset) {
 171             /* second, skip `offset' bytes from the (now) first element,
 172              * undo it on exit */
 173             iov[0].iov_base += offset;
 174             iov[0].iov_len -= offset;
 175         }
 176         /* Find the end position skipping `bytes' bytes: */
 177         /* first, skip all full-sized elements */
 178         tail = bytes;
 179         for (niov = 0; niov < iov_cnt && iov[niov].iov_len <= tail; ++niov) {
 180             tail -= iov[niov].iov_len;
 181         }
 182         if (tail) {
 183             /* second, fixup the last element, and remember the original
 184              * length */
 185             assert(niov < iov_cnt);
 186             assert(iov[niov].iov_len > tail);
 187             orig_len = iov[niov].iov_len;
 188             iov[niov++].iov_len = tail;
 189             ret = do_send_recv(sockfd, iov, niov, do_send);
 190             /* Undo the changes above before checking for errors */
 191             iov[niov-1].iov_len = orig_len;
 192         } else {
 193             ret = do_send_recv(sockfd, iov, niov, do_send);
 194         }
 195         if (offset) {
 196             iov[0].iov_base -= offset;
 197             iov[0].iov_len += offset;
 198         }
 199
 200         if (ret < 0) {
 201             assert(errno != EINTR);
 202             g_free(local_iov);
 203             if (errno == EAGAIN && total > 0) {
 204                 return total;
 205             }
 206             return -1;
 207         }
 208
 209         if (ret == 0 && !do_send) {
 210             /* recv returns 0 when the peer has performed an orderly
 211              * shutdown. */
 212             break;
 213         }
 214
 215         /* Prepare for the next iteration */
 216         offset += ret;
 217         total += ret;
 218         bytes -= ret;
 219     }
 220
 221     g_free(local_iov);
 222     return total;
 223 }
 224
 225
 226 void iov_hexdump(const struct iovec *iov, const unsigned int iov_cnt,
 227                  FILE *fp, const char *prefix, size_t limit)
 228 {
 229     int v;
 230     size_t size = 0;
 231     char *buf;
 232
 233     for (v = 0; v < iov_cnt; v++) {
 234         size += iov[v].iov_len;
 235     }
 236     size = size > limit ? limit : size;
 237     buf = g_malloc(size);
 238     iov_to_buf(iov, iov_cnt, 0, buf, size);
 239     qemu_hexdump(fp, prefix, buf, size);
 240     g_free(buf);
 241 }
 242
 243 unsigned iov_copy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
 244                  const struct iovec *iov, unsigned int iov_cnt,
 245                  size_t offset, size_t bytes)
 246 {
 247     size_t len;
 248     unsigned int i, j;
 249     for (i = 0, j = 0;
 250          i < iov_cnt && j < dst_iov_cnt && (offset || bytes); i++) {
 251         if (offset >= iov[i].iov_len) {
 252             offset -= iov[i].iov_len;
 253             continue;
 254         }
 255         len = MIN(bytes, iov[i].iov_len - offset);
 256
 257         dst_iov[j].iov_base = iov[i].iov_base + offset;
 258         dst_iov[j].iov_len = len;
 259         j++;
 260         bytes -= len;
 261         offset = 0;
 262     }
 263     assert(offset == 0);
 264     return j;
 265 }
 266
 267 /* io vectors */
 268
 269 void qemu_iovec_init(QEMUIOVector *qiov, int alloc_hint)
 270 {
 271     qiov->iov = g_new(struct iovec, alloc_hint);
 272     qiov->niov = 0;
 273     qiov->nalloc = alloc_hint;
 274     qiov->size = 0;
 275 }
 276
 277 void qemu_iovec_init_external(QEMUIOVector *qiov, struct iovec *iov, int niov)
 278 {
 279     int i;
 280
 281     qiov->iov = iov;
 282     qiov->niov = niov;
 283     qiov->nalloc = -1;
 284     qiov->size = 0;
 285     for (i = 0; i < niov; i++)
 286         qiov->size += iov[i].iov_len;
 287 }
 288
 289 void qemu_iovec_add(QEMUIOVector *qiov, void *base, size_t len)
 290 {
 291     assert(qiov->nalloc != -1);
 292
 293     if (qiov->niov == qiov->nalloc) {
 294         qiov->nalloc = 2 * qiov->nalloc + 1;
 295         qiov->iov = g_renew(struct iovec, qiov->iov, qiov->nalloc);
 296     }
 297     qiov->iov[qiov->niov].iov_base = base;
 298     qiov->iov[qiov->niov].iov_len = len;
 299     qiov->size += len;
 300     ++qiov->niov;
 301 }
 302
 303 /*
 304  * Concatenates (partial) iovecs from src_iov to the end of dst.
 305  * It starts copying after skipping `soffset' bytes at the
 306  * beginning of src and adds individual vectors from src to
 307  * dst copies up to `sbytes' bytes total, or up to the end
 308  * of src_iov if it comes first.  This way, it is okay to specify
 309  * very large value for `sbytes' to indicate "up to the end
 310  * of src".
 311  * Only vector pointers are processed, not the actual data buffers.
 312  */
 313 size_t qemu_iovec_concat_iov(QEMUIOVector *dst,
 314                              struct iovec *src_iov, unsigned int src_cnt,
 315                              size_t soffset, size_t sbytes)
 316 {
 317     int i;
 318     size_t done;
 319
 320     if (!sbytes) {
 321         return 0;
 322     }
 323     assert(dst->nalloc != -1);
 324     for (i = 0, done = 0; done < sbytes && i < src_cnt; i++) {
 325         if (soffset < src_iov[i].iov_len) {
 326             size_t len = MIN(src_iov[i].iov_len - soffset, sbytes - done);
 327             qemu_iovec_add(dst, src_iov[i].iov_base + soffset, len);
 328             done += len;
 329             soffset = 0;
 330         } else {
 331             soffset -= src_iov[i].iov_len;
 332         }
 333     }
 334     assert(soffset == 0); /* offset beyond end of src */
 335
 336     return done;
 337 }
 338
 339 /*
 340  * Concatenates (partial) iovecs from src to the end of dst.
 341  * It starts copying after skipping `soffset' bytes at the
 342  * beginning of src and adds individual vectors from src to
 343  * dst copies up to `sbytes' bytes total, or up to the end
 344  * of src if it comes first.  This way, it is okay to specify
 345  * very large value for `sbytes' to indicate "up to the end
 346  * of src".
 347  * Only vector pointers are processed, not the actual data buffers.
 348  */
 349 void qemu_iovec_concat(QEMUIOVector *dst,
 350                        QEMUIOVector *src, size_t soffset, size_t sbytes)
 351 {
 352     qemu_iovec_concat_iov(dst, src->iov, src->niov, soffset, sbytes);
 353 }
 354
 355 /*
 356  * qiov_find_iov
 357  *
 358  * Return pointer to iovec structure, where byte at @offset in original vector
 359  * @iov exactly is.
 360  * Set @remaining_offset to be offset inside that iovec to the same byte.
 361  */
 362 static struct iovec *iov_skip_offset(struct iovec *iov, size_t offset,
 363                                      size_t *remaining_offset)
 364 {
 365     while (offset > 0 && offset >= iov->iov_len) {
 366         offset -= iov->iov_len;
 367         iov++;
 368     }
 369     *remaining_offset = offset;
 370
 371     return iov;
 372 }
 373
 374 /*
 375  * qiov_slice
 376  *
 377  * Find subarray of iovec's, containing requested range. @head would
 378  * be offset in first iov (returned by the function), @tail would be
 379  * count of extra bytes in last iovec (returned iov + @niov - 1).
 380  */
 381 static struct iovec *qiov_slice(QEMUIOVector *qiov,
 382                                 size_t offset, size_t len,
 383                                 size_t *head, size_t *tail, int *niov)
 384 {
 385     struct iovec *iov, *end_iov;
 386
 387     assert(offset + len <= qiov->size);
 388
 389     iov = iov_skip_offset(qiov->iov, offset, head);
 390     end_iov = iov_skip_offset(iov, *head + len, tail);
 391
 392     if (*tail > 0) {
 393         assert(*tail < end_iov->iov_len);
 394         *tail = end_iov->iov_len - *tail;
 395         end_iov++;
 396     }
 397
 398     *niov = end_iov - iov;
 399
 400     return iov;
 401 }
 402
 403 int qemu_iovec_subvec_niov(QEMUIOVector *qiov, size_t offset, size_t len)
 404 {
 405     size_t head, tail;
 406     int niov;
 407
 408     qiov_slice(qiov, offset, len, &head, &tail, &niov);
 409
 410     return niov;
 411 }
 412
 413 /*
 414  * Compile new iovec, combining @head_buf buffer, sub-qiov of @mid_qiov,
 415  * and @tail_buf buffer into new qiov.
 416  */
 417 int qemu_iovec_init_extended(
 418         QEMUIOVector *qiov,
 419         void *head_buf, size_t head_len,
 420         QEMUIOVector *mid_qiov, size_t mid_offset, size_t mid_len,
 421         void *tail_buf, size_t tail_len)
 422 {
 423     size_t mid_head, mid_tail;
 424     int total_niov, mid_niov = 0;
 425     struct iovec *p, *mid_iov = NULL;
 426
 427     assert(mid_qiov->niov <= IOV_MAX);
 428
 429     if (SIZE_MAX - head_len < mid_len ||
 430         SIZE_MAX - head_len - mid_len < tail_len)
 431     {
 432         return -EINVAL;
 433     }
 434
 435     if (mid_len) {
 436         mid_iov = qiov_slice(mid_qiov, mid_offset, mid_len,
 437                              &mid_head, &mid_tail, &mid_niov);
 438     }
 439
 440     total_niov = !!head_len + mid_niov + !!tail_len;
 441     if (total_niov > IOV_MAX) {
 442         return -EINVAL;
 443     }
 444
 445     if (total_niov == 1) {
 446         qemu_iovec_init_buf(qiov, NULL, 0);
 447         p = &qiov->local_iov;
 448     } else {
 449         qiov->niov = qiov->nalloc = total_niov;
 450         qiov->size = head_len + mid_len + tail_len;
 451         p = qiov->iov = g_new(struct iovec, qiov->niov);
 452     }
 453
 454     if (head_len) {
 455         p->iov_base = head_buf;
 456         p->iov_len = head_len;
 457         p++;
 458     }
 459
 460     assert(!mid_niov == !mid_len);
 461     if (mid_niov) {
 462         memcpy(p, mid_iov, mid_niov * sizeof(*p));
 463         p[0].iov_base = (uint8_t *)p[0].iov_base + mid_head;
 464         p[0].iov_len -= mid_head;
 465         p[mid_niov - 1].iov_len -= mid_tail;
 466         p += mid_niov;
 467     }
 468
 469     if (tail_len) {
 470         p->iov_base = tail_buf;
 471         p->iov_len = tail_len;
 472     }
 473
 474     return 0;
 475 }
 476
 477 /*
 478  * Check if the contents of subrange of qiov data is all zeroes.
 479  */
 480 bool qemu_iovec_is_zero(QEMUIOVector *qiov, size_t offset, size_t bytes)
 481 {
 482     struct iovec *iov;
 483     size_t current_offset;
 484
 485     assert(offset + bytes <= qiov->size);
 486
 487     iov = iov_skip_offset(qiov->iov, offset, &current_offset);
 488
 489     while (bytes) {
 490         uint8_t *base = (uint8_t *)iov->iov_base + current_offset;
 491         size_t len = MIN(iov->iov_len - current_offset, bytes);
 492
 493         if (!buffer_is_zero(base, len)) {
 494             return false;
 495         }
 496
 497         current_offset = 0;
 498         bytes -= len;
 499         iov++;
 500     }
 501
 502     return true;
 503 }
 504
 505 void qemu_iovec_init_slice(QEMUIOVector *qiov, QEMUIOVector *source,
 506                            size_t offset, size_t len)
 507 {
 508     int ret;
 509
 510     assert(source->size >= len);
 511     assert(source->size - len >= offset);
 512
 513     /* We shrink the request, so we can't overflow neither size_t nor MAX_IOV */
 514     ret = qemu_iovec_init_extended(qiov, NULL, 0, source, offset, len, NULL, 0);
 515     assert(ret == 0);
 516 }
 517
 518 void qemu_iovec_destroy(QEMUIOVector *qiov)
 519 {
 520     if (qiov->nalloc != -1) {
 521         g_free(qiov->iov);
 522     }
 523
 524     memset(qiov, 0, sizeof(*qiov));
 525 }
 526
 527 void qemu_iovec_reset(QEMUIOVector *qiov)
 528 {
 529     assert(qiov->nalloc != -1);
 530
 531     qiov->niov = 0;
 532     qiov->size = 0;
 533 }
 534
 535 size_t qemu_iovec_to_buf(QEMUIOVector *qiov, size_t offset,
 536                          void *buf, size_t bytes)
 537 {
 538     return iov_to_buf(qiov->iov, qiov->niov, offset, buf, bytes);
 539 }
 540
 541 size_t qemu_iovec_from_buf(QEMUIOVector *qiov, size_t offset,
 542                            const void *buf, size_t bytes)
 543 {
 544     return iov_from_buf(qiov->iov, qiov->niov, offset, buf, bytes);
 545 }
 546
 547 size_t qemu_iovec_memset(QEMUIOVector *qiov, size_t offset,
 548                          int fillc, size_t bytes)
 549 {
 550     return iov_memset(qiov->iov, qiov->niov, offset, fillc, bytes);
 551 }
 552
 553 /**
 554  * Check that I/O vector contents are identical
 555  *
 556  * The IO vectors must have the same structure (same length of all parts).
 557  * A typical usage is to compare vectors created with qemu_iovec_clone().
 558  *
 559  * @a:          I/O vector
 560  * @b:          I/O vector
 561  * @ret:        Offset to first mismatching byte or -1 if match
 562  */
 563 ssize_t qemu_iovec_compare(QEMUIOVector *a, QEMUIOVector *b)
 564 {
 565     int i;
 566     ssize_t offset = 0;
 567
 568     assert(a->niov == b->niov);
 569     for (i = 0; i < a->niov; i++) {
 570         size_t len = 0;
 571         uint8_t *p = (uint8_t *)a->iov[i].iov_base;
 572         uint8_t *q = (uint8_t *)b->iov[i].iov_base;
 573
 574         assert(a->iov[i].iov_len == b->iov[i].iov_len);
 575         while (len < a->iov[i].iov_len && *p++ == *q++) {
 576             len++;
 577         }
 578
 579         offset += len;
 580
 581         if (len != a->iov[i].iov_len) {
 582             return offset;
 583         }
 584     }
 585     return -1;
 586 }
 587
 588 typedef struct {
 589     int src_index;
 590     struct iovec *src_iov;
 591     void *dest_base;
 592 } IOVectorSortElem;
 593
 594 static int sortelem_cmp_src_base(const void *a, const void *b)
 595 {
 596     const IOVectorSortElem *elem_a = a;
 597     const IOVectorSortElem *elem_b = b;
 598
 599     /* Don't overflow */
 600     if (elem_a->src_iov->iov_base < elem_b->src_iov->iov_base) {
 601         return -1;
 602     } else if (elem_a->src_iov->iov_base > elem_b->src_iov->iov_base) {
 603         return 1;
 604     } else {
 605         return 0;
 606     }
 607 }
 608
 609 static int sortelem_cmp_src_index(const void *a, const void *b)
 610 {
 611     const IOVectorSortElem *elem_a = a;
 612     const IOVectorSortElem *elem_b = b;
 613
 614     return elem_a->src_index - elem_b->src_index;
 615 }
 616
 617 /**
 618  * Copy contents of I/O vector
 619  *
 620  * The relative relationships of overlapping iovecs are preserved.  This is
 621  * necessary to ensure identical semantics in the cloned I/O vector.
 622  */
 623 void qemu_iovec_clone(QEMUIOVector *dest, const QEMUIOVector *src, void *buf)
 624 {
 625     IOVectorSortElem sortelems[src->niov];
 626     void *last_end;
 627     int i;
 628
 629     /* Sort by source iovecs by base address */
 630     for (i = 0; i < src->niov; i++) {
 631         sortelems[i].src_index = i;
 632         sortelems[i].src_iov = &src->iov[i];
 633     }
 634     qsort(sortelems, src->niov, sizeof(sortelems[0]), sortelem_cmp_src_base);
 635
 636     /* Allocate buffer space taking into account overlapping iovecs */
 637     last_end = NULL;
 638     for (i = 0; i < src->niov; i++) {
 639         struct iovec *cur = sortelems[i].src_iov;
 640         ptrdiff_t rewind = 0;
 641
 642         /* Detect overlap */
 643         if (last_end && last_end > cur->iov_base) {
 644             rewind = last_end - cur->iov_base;
 645         }
 646
 647         sortelems[i].dest_base = buf - rewind;
 648         buf += cur->iov_len - MIN(rewind, cur->iov_len);
 649         last_end = MAX(cur->iov_base + cur->iov_len, last_end);
 650     }
 651
 652     /* Sort by source iovec index and build destination iovec */
 653     qsort(sortelems, src->niov, sizeof(sortelems[0]), sortelem_cmp_src_index);
 654     for (i = 0; i < src->niov; i++) {
 655         qemu_iovec_add(dest, sortelems[i].dest_base, src->iov[i].iov_len);
 656     }
 657 }
 658
 659 void iov_discard_undo(IOVDiscardUndo *undo)
 660 {
 661     /* Restore original iovec if it was modified */
 662     if (undo->modified_iov) {
 663         *undo->modified_iov = undo->orig;
 664     }
 665 }
 666
 667 size_t iov_discard_front_undoable(struct iovec **iov,
 668                                   unsigned int *iov_cnt,
 669                                   size_t bytes,
 670                                   IOVDiscardUndo *undo)
 671 {
 672     size_t total = 0;
 673     struct iovec *cur;
 674
 675     if (undo) {
 676         undo->modified_iov = NULL;
 677     }
 678
 679     for (cur = *iov; *iov_cnt > 0; cur++) {
 680         if (cur->iov_len > bytes) {
 681             if (undo) {
 682                 undo->modified_iov = cur;
 683                 undo->orig = *cur;
 684             }
 685
 686             cur->iov_base += bytes;
 687             cur->iov_len -= bytes;
 688             total += bytes;
 689             break;
 690         }
 691
 692         bytes -= cur->iov_len;
 693         total += cur->iov_len;
 694         *iov_cnt -= 1;
 695     }
 696
 697     *iov = cur;
 698     return total;
 699 }
 700
 701 size_t iov_discard_front(struct iovec **iov, unsigned int *iov_cnt,
 702                          size_t bytes)
 703 {
 704     return iov_discard_front_undoable(iov, iov_cnt, bytes, NULL);
 705 }
 706
 707 size_t iov_discard_back_undoable(struct iovec *iov,
 708                                  unsigned int *iov_cnt,
 709                                  size_t bytes,
 710                                  IOVDiscardUndo *undo)
 711 {
 712     size_t total = 0;
 713     struct iovec *cur;
 714
 715     if (undo) {
 716         undo->modified_iov = NULL;
 717     }
 718
 719     if (*iov_cnt == 0) {
 720         return 0;
 721     }
 722
 723     cur = iov + (*iov_cnt - 1);
 724
 725     while (*iov_cnt > 0) {
 726         if (cur->iov_len > bytes) {
 727             if (undo) {
 728                 undo->modified_iov = cur;
 729                 undo->orig = *cur;
 730             }
 731
 732             cur->iov_len -= bytes;
 733             total += bytes;
 734             break;
 735         }
 736
 737         bytes -= cur->iov_len;
 738         total += cur->iov_len;
 739         cur--;
 740         *iov_cnt -= 1;
 741     }
 742
 743     return total;
 744 }
 745
 746 size_t iov_discard_back(struct iovec *iov, unsigned int *iov_cnt,
 747                         size_t bytes)
 748 {
 749     return iov_discard_back_undoable(iov, iov_cnt, bytes, NULL);
 750 }
 751
 752 void qemu_iovec_discard_back(QEMUIOVector *qiov, size_t bytes)
 753 {
 754     size_t total;
 755     unsigned int niov = qiov->niov;
 756
 757     assert(qiov->size >= bytes);
 758     total = iov_discard_back(qiov->iov, &niov, bytes);
 759     assert(total == bytes);
 760
 761     qiov->niov = niov;
 762     qiov->size -= bytes;
 763 }