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     ssize_t off = 0;
 115     while (i < iov_cnt) {
 116         ssize_t r = do_send
 117             ? send(sockfd, iov[i].iov_base + off, iov[i].iov_len - off, 0)
 118             : recv(sockfd, iov[i].iov_base + off, iov[i].iov_len - off, 0);
 119         if (r > 0) {
 120             ret += r;
 121             off += r;
 122             if (off < iov[i].iov_len) {
 123                 continue;
 124             }
 125         } else if (!r) {
 126             break;
 127         } else if (errno == EINTR) {
 128             continue;
 129         } else {
 130             /* else it is some "other" error,
 131              * only return if there was no data processed. */
 132             if (ret == 0) {
 133                 ret = -1;
 134             }
 135             break;
 136         }
 137         off = 0;
 138         i++;
 139     }
 140     return ret;
 141 #endif
 142 }
 143
 144 ssize_t iov_send_recv(int sockfd, const struct iovec *_iov, unsigned iov_cnt,
 145                       size_t offset, size_t bytes,
 146                       bool do_send)
 147 {
 148     ssize_t total = 0;
 149     ssize_t ret;
 150     size_t orig_len, tail;
 151     unsigned niov;
 152     struct iovec *local_iov, *iov;
 153
 154     if (bytes <= 0) {
 155         return 0;
 156     }
 157
 158     local_iov = g_new0(struct iovec, iov_cnt);
 159     iov_copy(local_iov, iov_cnt, _iov, iov_cnt, offset, bytes);
 160     offset = 0;
 161     iov = local_iov;
 162
 163     while (bytes > 0) {
 164         /* Find the start position, skipping `offset' bytes:
 165          * first, skip all full-sized vector elements, */
 166         for (niov = 0; niov < iov_cnt && offset >= iov[niov].iov_len; ++niov) {
 167             offset -= iov[niov].iov_len;
 168         }
 169
 170         /* niov == iov_cnt would only be valid if bytes == 0, which
 171          * we already ruled out in the loop condition.  */
 172         assert(niov < iov_cnt);
 173         iov += niov;
 174         iov_cnt -= niov;
 175
 176         if (offset) {
 177             /* second, skip `offset' bytes from the (now) first element,
 178              * undo it on exit */
 179             iov[0].iov_base += offset;
 180             iov[0].iov_len -= offset;
 181         }
 182         /* Find the end position skipping `bytes' bytes: */
 183         /* first, skip all full-sized elements */
 184         tail = bytes;
 185         for (niov = 0; niov < iov_cnt && iov[niov].iov_len <= tail; ++niov) {
 186             tail -= iov[niov].iov_len;
 187         }
 188         if (tail) {
 189             /* second, fixup the last element, and remember the original
 190              * length */
 191             assert(niov < iov_cnt);
 192             assert(iov[niov].iov_len > tail);
 193             orig_len = iov[niov].iov_len;
 194             iov[niov++].iov_len = tail;
 195             ret = do_send_recv(sockfd, iov, niov, do_send);
 196             /* Undo the changes above before checking for errors */
 197             iov[niov-1].iov_len = orig_len;
 198         } else {
 199             ret = do_send_recv(sockfd, iov, niov, do_send);
 200         }
 201         if (offset) {
 202             iov[0].iov_base -= offset;
 203             iov[0].iov_len += offset;
 204         }
 205
 206         if (ret < 0) {
 207             assert(errno != EINTR);
 208             g_free(local_iov);
 209             if (errno == EAGAIN && total > 0) {
 210                 return total;
 211             }
 212             return -1;
 213         }
 214
 215         if (ret == 0 && !do_send) {
 216             /* recv returns 0 when the peer has performed an orderly
 217              * shutdown. */
 218             break;
 219         }
 220
 221         /* Prepare for the next iteration */
 222         offset += ret;
 223         total += ret;
 224         bytes -= ret;
 225     }
 226
 227     g_free(local_iov);
 228     return total;
 229 }
 230
 231
 232 void iov_hexdump(const struct iovec *iov, const unsigned int iov_cnt,
 233                  FILE *fp, const char *prefix, size_t limit)
 234 {
 235     int v;
 236     size_t size = 0;
 237     char *buf;
 238
 239     for (v = 0; v < iov_cnt; v++) {
 240         size += iov[v].iov_len;
 241     }
 242     size = size > limit ? limit : size;
 243     buf = g_malloc(size);
 244     iov_to_buf(iov, iov_cnt, 0, buf, size);
 245     qemu_hexdump(fp, prefix, buf, size);
 246     g_free(buf);
 247 }
 248
 249 unsigned iov_copy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
 250                  const struct iovec *iov, unsigned int iov_cnt,
 251                  size_t offset, size_t bytes)
 252 {
 253     size_t len;
 254     unsigned int i, j;
 255     for (i = 0, j = 0;
 256          i < iov_cnt && j < dst_iov_cnt && (offset || bytes); i++) {
 257         if (offset >= iov[i].iov_len) {
 258             offset -= iov[i].iov_len;
 259             continue;
 260         }
 261         len = MIN(bytes, iov[i].iov_len - offset);
 262
 263         dst_iov[j].iov_base = iov[i].iov_base + offset;
 264         dst_iov[j].iov_len = len;
 265         j++;
 266         bytes -= len;
 267         offset = 0;
 268     }
 269     assert(offset == 0);
 270     return j;
 271 }
 272
 273 /* io vectors */
 274
 275 void qemu_iovec_init(QEMUIOVector *qiov, int alloc_hint)
 276 {
 277     qiov->iov = g_new(struct iovec, alloc_hint);
 278     qiov->niov = 0;
 279     qiov->nalloc = alloc_hint;
 280     qiov->size = 0;
 281 }
 282
 283 void qemu_iovec_init_external(QEMUIOVector *qiov, struct iovec *iov, int niov)
 284 {
 285     int i;
 286
 287     qiov->iov = iov;
 288     qiov->niov = niov;
 289     qiov->nalloc = -1;
 290     qiov->size = 0;
 291     for (i = 0; i < niov; i++)
 292         qiov->size += iov[i].iov_len;
 293 }
 294
 295 void qemu_iovec_add(QEMUIOVector *qiov, void *base, size_t len)
 296 {
 297     assert(qiov->nalloc != -1);
 298
 299     if (qiov->niov == qiov->nalloc) {
 300         qiov->nalloc = 2 * qiov->nalloc + 1;
 301         qiov->iov = g_renew(struct iovec, qiov->iov, qiov->nalloc);
 302     }
 303     qiov->iov[qiov->niov].iov_base = base;
 304     qiov->iov[qiov->niov].iov_len = len;
 305     qiov->size += len;
 306     ++qiov->niov;
 307 }
 308
 309 /*
 310  * Concatenates (partial) iovecs from src_iov to the end of dst.
 311  * It starts copying after skipping `soffset' bytes at the
 312  * beginning of src and adds individual vectors from src to
 313  * dst copies up to `sbytes' bytes total, or up to the end
 314  * of src_iov if it comes first.  This way, it is okay to specify
 315  * very large value for `sbytes' to indicate "up to the end
 316  * of src".
 317  * Only vector pointers are processed, not the actual data buffers.
 318  */
 319 size_t qemu_iovec_concat_iov(QEMUIOVector *dst,
 320                              struct iovec *src_iov, unsigned int src_cnt,
 321                              size_t soffset, size_t sbytes)
 322 {
 323     int i;
 324     size_t done;
 325
 326     if (!sbytes) {
 327         return 0;
 328     }
 329     assert(dst->nalloc != -1);
 330     for (i = 0, done = 0; done < sbytes && i < src_cnt; i++) {
 331         if (soffset < src_iov[i].iov_len) {
 332             size_t len = MIN(src_iov[i].iov_len - soffset, sbytes - done);
 333             qemu_iovec_add(dst, src_iov[i].iov_base + soffset, len);
 334             done += len;
 335             soffset = 0;
 336         } else {
 337             soffset -= src_iov[i].iov_len;
 338         }
 339     }
 340     assert(soffset == 0); /* offset beyond end of src */
 341
 342     return done;
 343 }
 344
 345 /*
 346  * Concatenates (partial) iovecs from src to the end of dst.
 347  * It starts copying after skipping `soffset' bytes at the
 348  * beginning of src and adds individual vectors from src to
 349  * dst copies up to `sbytes' bytes total, or up to the end
 350  * of src if it comes first.  This way, it is okay to specify
 351  * very large value for `sbytes' to indicate "up to the end
 352  * of src".
 353  * Only vector pointers are processed, not the actual data buffers.
 354  */
 355 void qemu_iovec_concat(QEMUIOVector *dst,
 356                        QEMUIOVector *src, size_t soffset, size_t sbytes)
 357 {
 358     qemu_iovec_concat_iov(dst, src->iov, src->niov, soffset, sbytes);
 359 }
 360
 361 /*
 362  * qiov_find_iov
 363  *
 364  * Return pointer to iovec structure, where byte at @offset in original vector
 365  * @iov exactly is.
 366  * Set @remaining_offset to be offset inside that iovec to the same byte.
 367  */
 368 static struct iovec *iov_skip_offset(struct iovec *iov, size_t offset,
 369                                      size_t *remaining_offset)
 370 {
 371     while (offset > 0 && offset >= iov->iov_len) {
 372         offset -= iov->iov_len;
 373         iov++;
 374     }
 375     *remaining_offset = offset;
 376
 377     return iov;
 378 }
 379
 380 /*
 381  * qiov_slice
 382  *
 383  * Find subarray of iovec's, containing requested range. @head would
 384  * be offset in first iov (returned by the function), @tail would be
 385  * count of extra bytes in last iovec (returned iov + @niov - 1).
 386  */
 387 static struct iovec *qiov_slice(QEMUIOVector *qiov,
 388                                 size_t offset, size_t len,
 389                                 size_t *head, size_t *tail, int *niov)
 390 {
 391     struct iovec *iov, *end_iov;
 392
 393     assert(offset + len <= qiov->size);
 394
 395     iov = iov_skip_offset(qiov->iov, offset, head);
 396     end_iov = iov_skip_offset(iov, *head + len, tail);
 397
 398     if (*tail > 0) {
 399         assert(*tail < end_iov->iov_len);
 400         *tail = end_iov->iov_len - *tail;
 401         end_iov++;
 402     }
 403
 404     *niov = end_iov - iov;
 405
 406     return iov;
 407 }
 408
 409 int qemu_iovec_subvec_niov(QEMUIOVector *qiov, size_t offset, size_t len)
 410 {
 411     size_t head, tail;
 412     int niov;
 413
 414     qiov_slice(qiov, offset, len, &head, &tail, &niov);
 415
 416     return niov;
 417 }
 418
 419 /*
 420  * Compile new iovec, combining @head_buf buffer, sub-qiov of @mid_qiov,
 421  * and @tail_buf buffer into new qiov.
 422  */
 423 int qemu_iovec_init_extended(
 424         QEMUIOVector *qiov,
 425         void *head_buf, size_t head_len,
 426         QEMUIOVector *mid_qiov, size_t mid_offset, size_t mid_len,
 427         void *tail_buf, size_t tail_len)
 428 {
 429     size_t mid_head, mid_tail;
 430     int total_niov, mid_niov = 0;
 431     struct iovec *p, *mid_iov = NULL;
 432
 433     assert(mid_qiov->niov <= IOV_MAX);
 434
 435     if (SIZE_MAX - head_len < mid_len ||
 436         SIZE_MAX - head_len - mid_len < tail_len)
 437     {
 438         return -EINVAL;
 439     }
 440
 441     if (mid_len) {
 442         mid_iov = qiov_slice(mid_qiov, mid_offset, mid_len,
 443                              &mid_head, &mid_tail, &mid_niov);
 444     }
 445
 446     total_niov = !!head_len + mid_niov + !!tail_len;
 447     if (total_niov > IOV_MAX) {
 448         return -EINVAL;
 449     }
 450
 451     if (total_niov == 1) {
 452         qemu_iovec_init_buf(qiov, NULL, 0);
 453         p = &qiov->local_iov;
 454     } else {
 455         qiov->niov = qiov->nalloc = total_niov;
 456         qiov->size = head_len + mid_len + tail_len;
 457         p = qiov->iov = g_new(struct iovec, qiov->niov);
 458     }
 459
 460     if (head_len) {
 461         p->iov_base = head_buf;
 462         p->iov_len = head_len;
 463         p++;
 464     }
 465
 466     assert(!mid_niov == !mid_len);
 467     if (mid_niov) {
 468         memcpy(p, mid_iov, mid_niov * sizeof(*p));
 469         p[0].iov_base = (uint8_t *)p[0].iov_base + mid_head;
 470         p[0].iov_len -= mid_head;
 471         p[mid_niov - 1].iov_len -= mid_tail;
 472         p += mid_niov;
 473     }
 474
 475     if (tail_len) {
 476         p->iov_base = tail_buf;
 477         p->iov_len = tail_len;
 478     }
 479
 480     return 0;
 481 }
 482
 483 /*
 484  * Check if the contents of subrange of qiov data is all zeroes.
 485  */
 486 bool qemu_iovec_is_zero(QEMUIOVector *qiov, size_t offset, size_t bytes)
 487 {
 488     struct iovec *iov;
 489     size_t current_offset;
 490
 491     assert(offset + bytes <= qiov->size);
 492
 493     iov = iov_skip_offset(qiov->iov, offset, &current_offset);
 494
 495     while (bytes) {
 496         uint8_t *base = (uint8_t *)iov->iov_base + current_offset;
 497         size_t len = MIN(iov->iov_len - current_offset, bytes);
 498
 499         if (!buffer_is_zero(base, len)) {
 500             return false;
 501         }
 502
 503         current_offset = 0;
 504         bytes -= len;
 505         iov++;
 506     }
 507
 508     return true;
 509 }
 510
 511 void qemu_iovec_init_slice(QEMUIOVector *qiov, QEMUIOVector *source,
 512                            size_t offset, size_t len)
 513 {
 514     int ret;
 515
 516     assert(source->size >= len);
 517     assert(source->size - len >= offset);
 518
 519     /* We shrink the request, so we can't overflow neither size_t nor MAX_IOV */
 520     ret = qemu_iovec_init_extended(qiov, NULL, 0, source, offset, len, NULL, 0);
 521     assert(ret == 0);
 522 }
 523
 524 void qemu_iovec_destroy(QEMUIOVector *qiov)
 525 {
 526     if (qiov->nalloc != -1) {
 527         g_free(qiov->iov);
 528     }
 529
 530     memset(qiov, 0, sizeof(*qiov));
 531 }
 532
 533 void qemu_iovec_reset(QEMUIOVector *qiov)
 534 {
 535     assert(qiov->nalloc != -1);
 536
 537     qiov->niov = 0;
 538     qiov->size = 0;
 539 }
 540
 541 size_t qemu_iovec_to_buf(QEMUIOVector *qiov, size_t offset,
 542                          void *buf, size_t bytes)
 543 {
 544     return iov_to_buf(qiov->iov, qiov->niov, offset, buf, bytes);
 545 }
 546
 547 size_t qemu_iovec_from_buf(QEMUIOVector *qiov, size_t offset,
 548                            const void *buf, size_t bytes)
 549 {
 550     return iov_from_buf(qiov->iov, qiov->niov, offset, buf, bytes);
 551 }
 552
 553 size_t qemu_iovec_memset(QEMUIOVector *qiov, size_t offset,
 554                          int fillc, size_t bytes)
 555 {
 556     return iov_memset(qiov->iov, qiov->niov, offset, fillc, bytes);
 557 }
 558
 559 /**
 560  * Check that I/O vector contents are identical
 561  *
 562  * The IO vectors must have the same structure (same length of all parts).
 563  * A typical usage is to compare vectors created with qemu_iovec_clone().
 564  *
 565  * @a:          I/O vector
 566  * @b:          I/O vector
 567  * @ret:        Offset to first mismatching byte or -1 if match
 568  */
 569 ssize_t qemu_iovec_compare(QEMUIOVector *a, QEMUIOVector *b)
 570 {
 571     int i;
 572     ssize_t offset = 0;
 573
 574     assert(a->niov == b->niov);
 575     for (i = 0; i < a->niov; i++) {
 576         size_t len = 0;
 577         uint8_t *p = (uint8_t *)a->iov[i].iov_base;
 578         uint8_t *q = (uint8_t *)b->iov[i].iov_base;
 579
 580         assert(a->iov[i].iov_len == b->iov[i].iov_len);
 581         while (len < a->iov[i].iov_len && *p++ == *q++) {
 582             len++;
 583         }
 584
 585         offset += len;
 586
 587         if (len != a->iov[i].iov_len) {
 588             return offset;
 589         }
 590     }
 591     return -1;
 592 }
 593
 594 typedef struct {
 595     int src_index;
 596     struct iovec *src_iov;
 597     void *dest_base;
 598 } IOVectorSortElem;
 599
 600 static int sortelem_cmp_src_base(const void *a, const void *b)
 601 {
 602     const IOVectorSortElem *elem_a = a;
 603     const IOVectorSortElem *elem_b = b;
 604
 605     /* Don't overflow */
 606     if (elem_a->src_iov->iov_base < elem_b->src_iov->iov_base) {
 607         return -1;
 608     } else if (elem_a->src_iov->iov_base > elem_b->src_iov->iov_base) {
 609         return 1;
 610     } else {
 611         return 0;
 612     }
 613 }
 614
 615 static int sortelem_cmp_src_index(const void *a, const void *b)
 616 {
 617     const IOVectorSortElem *elem_a = a;
 618     const IOVectorSortElem *elem_b = b;
 619
 620     return elem_a->src_index - elem_b->src_index;
 621 }
 622
 623 /**
 624  * Copy contents of I/O vector
 625  *
 626  * The relative relationships of overlapping iovecs are preserved.  This is
 627  * necessary to ensure identical semantics in the cloned I/O vector.
 628  */
 629 void qemu_iovec_clone(QEMUIOVector *dest, const QEMUIOVector *src, void *buf)
 630 {
 631     IOVectorSortElem sortelems[src->niov];
 632     void *last_end;
 633     int i;
 634
 635     /* Sort by source iovecs by base address */
 636     for (i = 0; i < src->niov; i++) {
 637         sortelems[i].src_index = i;
 638         sortelems[i].src_iov = &src->iov[i];
 639     }
 640     qsort(sortelems, src->niov, sizeof(sortelems[0]), sortelem_cmp_src_base);
 641
 642     /* Allocate buffer space taking into account overlapping iovecs */
 643     last_end = NULL;
 644     for (i = 0; i < src->niov; i++) {
 645         struct iovec *cur = sortelems[i].src_iov;
 646         ptrdiff_t rewind = 0;
 647
 648         /* Detect overlap */
 649         if (last_end && last_end > cur->iov_base) {
 650             rewind = last_end - cur->iov_base;
 651         }
 652
 653         sortelems[i].dest_base = buf - rewind;
 654         buf += cur->iov_len - MIN(rewind, cur->iov_len);
 655         last_end = MAX(cur->iov_base + cur->iov_len, last_end);
 656     }
 657
 658     /* Sort by source iovec index and build destination iovec */
 659     qsort(sortelems, src->niov, sizeof(sortelems[0]), sortelem_cmp_src_index);
 660     for (i = 0; i < src->niov; i++) {
 661         qemu_iovec_add(dest, sortelems[i].dest_base, src->iov[i].iov_len);
 662     }
 663 }
 664
 665 void iov_discard_undo(IOVDiscardUndo *undo)
 666 {
 667     /* Restore original iovec if it was modified */
 668     if (undo->modified_iov) {
 669         *undo->modified_iov = undo->orig;
 670     }
 671 }
 672
 673 size_t iov_discard_front_undoable(struct iovec **iov,
 674                                   unsigned int *iov_cnt,
 675                                   size_t bytes,
 676                                   IOVDiscardUndo *undo)
 677 {
 678     size_t total = 0;
 679     struct iovec *cur;
 680
 681     if (undo) {
 682         undo->modified_iov = NULL;
 683     }
 684
 685     for (cur = *iov; *iov_cnt > 0; cur++) {
 686         if (cur->iov_len > bytes) {
 687             if (undo) {
 688                 undo->modified_iov = cur;
 689                 undo->orig = *cur;
 690             }
 691
 692             cur->iov_base += bytes;
 693             cur->iov_len -= bytes;
 694             total += bytes;
 695             break;
 696         }
 697
 698         bytes -= cur->iov_len;
 699         total += cur->iov_len;
 700         *iov_cnt -= 1;
 701     }
 702
 703     *iov = cur;
 704     return total;
 705 }
 706
 707 size_t iov_discard_front(struct iovec **iov, unsigned int *iov_cnt,
 708                          size_t bytes)
 709 {
 710     return iov_discard_front_undoable(iov, iov_cnt, bytes, NULL);
 711 }
 712
 713 size_t iov_discard_back_undoable(struct iovec *iov,
 714                                  unsigned int *iov_cnt,
 715                                  size_t bytes,
 716                                  IOVDiscardUndo *undo)
 717 {
 718     size_t total = 0;
 719     struct iovec *cur;
 720
 721     if (undo) {
 722         undo->modified_iov = NULL;
 723     }
 724
 725     if (*iov_cnt == 0) {
 726         return 0;
 727     }
 728
 729     cur = iov + (*iov_cnt - 1);
 730
 731     while (*iov_cnt > 0) {
 732         if (cur->iov_len > bytes) {
 733             if (undo) {
 734                 undo->modified_iov = cur;
 735                 undo->orig = *cur;
 736             }
 737
 738             cur->iov_len -= bytes;
 739             total += bytes;
 740             break;
 741         }
 742
 743         bytes -= cur->iov_len;
 744         total += cur->iov_len;
 745         cur--;
 746         *iov_cnt -= 1;
 747     }
 748
 749     return total;
 750 }
 751
 752 size_t iov_discard_back(struct iovec *iov, unsigned int *iov_cnt,
 753                         size_t bytes)
 754 {
 755     return iov_discard_back_undoable(iov, iov_cnt, bytes, NULL);
 756 }
 757
 758 void qemu_iovec_discard_back(QEMUIOVector *qiov, size_t bytes)
 759 {
 760     size_t total;
 761     unsigned int niov = qiov->niov;
 762
 763     assert(qiov->size >= bytes);
 764     total = iov_discard_back(qiov->iov, &niov, bytes);
 765     assert(total == bytes);
 766
 767     qiov->niov = niov;
 768     qiov->size -= bytes;
 769 }