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  * Copyright (c) 2024 Seagate Technology LLC and/or its Affiliates
   7  *
   8  * Author(s):
   9  *  Anthony Liguori <aliguori@us.ibm.com>
  10  *  Amit Shah <amit.shah@redhat.com>
  11  *  Michael Tokarev <mjt@tls.msk.ru>
  12  *
  13  * This work is licensed under the terms of the GNU GPL, version 2.  See
  14  * the COPYING file in the top-level directory.
  15  *
  16  * Contributions after 2012-01-13 are licensed under the terms of the
  17  * GNU GPL, version 2 or (at your option) any later version.
  18  */
  19
  20 #include "qemu/osdep.h"
  21 #include "qemu/iov.h"
  22 #include "qemu/sockets.h"
  23 #include "qemu/cutils.h"
  24
  25 size_t iov_from_buf_full(const struct iovec *iov, unsigned int iov_cnt,
  26                          size_t offset, const void *buf, size_t bytes)
  27 {
  28     size_t done;
  29     unsigned int i;
  30     for (i = 0, done = 0; (offset || done < bytes) && i < iov_cnt; i++) {
  31         if (offset < iov[i].iov_len) {
  32             size_t len = MIN(iov[i].iov_len - offset, bytes - done);
  33             memcpy(iov[i].iov_base + offset, buf + done, len);
  34             done += len;
  35             offset = 0;
  36         } else {
  37             offset -= iov[i].iov_len;
  38         }
  39     }
  40     assert(offset == 0);
  41     return done;
  42 }
  43
  44 size_t iov_to_buf_full(const struct iovec *iov, const unsigned int iov_cnt,
  45                        size_t offset, void *buf, size_t bytes)
  46 {
  47     size_t done;
  48     unsigned int i;
  49     for (i = 0, done = 0; (offset || done < bytes) && i < iov_cnt; i++) {
  50         if (offset < iov[i].iov_len) {
  51             size_t len = MIN(iov[i].iov_len - offset, bytes - done);
  52             memcpy(buf + done, iov[i].iov_base + offset, len);
  53             done += len;
  54             offset = 0;
  55         } else {
  56             offset -= iov[i].iov_len;
  57         }
  58     }
  59     assert(offset == 0);
  60     return done;
  61 }
  62
  63 size_t iov_memset(const struct iovec *iov, const unsigned int iov_cnt,
  64                   size_t offset, int fillc, size_t bytes)
  65 {
  66     size_t done;
  67     unsigned int i;
  68     for (i = 0, done = 0; (offset || done < bytes) && i < iov_cnt; i++) {
  69         if (offset < iov[i].iov_len) {
  70             size_t len = MIN(iov[i].iov_len - offset, bytes - done);
  71             memset(iov[i].iov_base + offset, fillc, len);
  72             done += len;
  73             offset = 0;
  74         } else {
  75             offset -= iov[i].iov_len;
  76         }
  77     }
  78     assert(offset == 0);
  79     return done;
  80 }
  81
  82 size_t iov_size(const struct iovec *iov, const unsigned int iov_cnt)
  83 {
  84     size_t len;
  85     unsigned int i;
  86
  87     len = 0;
  88     for (i = 0; i < iov_cnt; i++) {
  89         len += iov[i].iov_len;
  90     }
  91     return len;
  92 }
  93
  94 /* helper function for iov_send_recv() */
  95 static ssize_t
  96 do_send_recv(int sockfd, int flags, struct iovec *iov, unsigned iov_cnt,
  97              bool do_send)
  98 {
  99 #ifdef CONFIG_POSIX
 100     ssize_t ret;
 101     struct msghdr msg;
 102     memset(&msg, 0, sizeof(msg));
 103     msg.msg_iov = iov;
 104     msg.msg_iovlen = iov_cnt;
 105     do {
 106         ret = do_send
 107             ? sendmsg(sockfd, &msg, flags)
 108             : recvmsg(sockfd, &msg, flags);
 109     } while (ret < 0 && errno == EINTR);
 110     return ret;
 111 #else
 112     /* else send piece-by-piece */
 113     /*XXX Note: windows has WSASend() and WSARecv() */
 114     unsigned i = 0;
 115     ssize_t ret = 0;
 116     ssize_t off = 0;
 117     while (i < iov_cnt) {
 118         ssize_t r = do_send
 119             ? send(sockfd, iov[i].iov_base + off, iov[i].iov_len - off, flags)
 120             : recv(sockfd, iov[i].iov_base + off, iov[i].iov_len - off, flags);
 121         if (r > 0) {
 122             ret += r;
 123             off += r;
 124             if (off < iov[i].iov_len) {
 125                 continue;
 126             }
 127         } else if (!r) {
 128             break;
 129         } else if (errno == EINTR) {
 130             continue;
 131         } else {
 132             /* else it is some "other" error,
 133              * only return if there was no data processed. */
 134             if (ret == 0) {
 135                 ret = -1;
 136             }
 137             break;
 138         }
 139         off = 0;
 140         i++;
 141     }
 142     return ret;
 143 #endif
 144 }
 145
 146 ssize_t iov_send_recv(int sockfd, const struct iovec *_iov, unsigned iov_cnt,
 147                       size_t offset, size_t bytes,
 148                       bool do_send)
 149 {
 150     return iov_send_recv_with_flags(sockfd, 0, _iov, iov_cnt, offset, bytes,
 151                                     do_send);
 152 }
 153
 154 ssize_t iov_send_recv_with_flags(int sockfd, int sockflags,
 155                                  const struct iovec *_iov,
 156                                  unsigned iov_cnt, size_t offset,
 157                                  size_t bytes, bool do_send)
 158 {
 159     ssize_t total = 0;
 160     ssize_t ret;
 161     size_t orig_len, tail;
 162     unsigned niov;
 163     struct iovec *local_iov, *iov;
 164
 165     if (bytes <= 0) {
 166         return 0;
 167     }
 168
 169     local_iov = g_new0(struct iovec, iov_cnt);
 170     iov_copy(local_iov, iov_cnt, _iov, iov_cnt, offset, bytes);
 171     offset = 0;
 172     iov = local_iov;
 173
 174     while (bytes > 0) {
 175         /* Find the start position, skipping `offset' bytes:
 176          * first, skip all full-sized vector elements, */
 177         for (niov = 0; niov < iov_cnt && offset >= iov[niov].iov_len; ++niov) {
 178             offset -= iov[niov].iov_len;
 179         }
 180
 181         /* niov == iov_cnt would only be valid if bytes == 0, which
 182          * we already ruled out in the loop condition.  */
 183         assert(niov < iov_cnt);
 184         iov += niov;
 185         iov_cnt -= niov;
 186
 187         if (offset) {
 188             /* second, skip `offset' bytes from the (now) first element,
 189              * undo it on exit */
 190             iov[0].iov_base += offset;
 191             iov[0].iov_len -= offset;
 192         }
 193         /* Find the end position skipping `bytes' bytes: */
 194         /* first, skip all full-sized elements */
 195         tail = bytes;
 196         for (niov = 0; niov < iov_cnt && iov[niov].iov_len <= tail; ++niov) {
 197             tail -= iov[niov].iov_len;
 198         }
 199         if (tail) {
 200             /* second, fixup the last element, and remember the original
 201              * length */
 202             assert(niov < iov_cnt);
 203             assert(iov[niov].iov_len > tail);
 204             orig_len = iov[niov].iov_len;
 205             iov[niov++].iov_len = tail;
 206             ret = do_send_recv(sockfd, sockflags, iov, niov, do_send);
 207             /* Undo the changes above before checking for errors */
 208             iov[niov-1].iov_len = orig_len;
 209         } else {
 210             ret = do_send_recv(sockfd, sockflags, iov, niov, do_send);
 211         }
 212         if (offset) {
 213             iov[0].iov_base -= offset;
 214             iov[0].iov_len += offset;
 215         }
 216
 217         if (ret < 0) {
 218             assert(errno != EINTR);
 219             g_free(local_iov);
 220             if (errno == EAGAIN && total > 0) {
 221                 return total;
 222             }
 223             return -1;
 224         }
 225
 226         if (ret == 0 && !do_send) {
 227             /* recv returns 0 when the peer has performed an orderly
 228              * shutdown. */
 229             break;
 230         }
 231
 232         /* Prepare for the next iteration */
 233         offset += ret;
 234         total += ret;
 235         bytes -= ret;
 236     }
 237
 238     g_free(local_iov);
 239     return total;
 240 }
 241
 242
 243 void iov_hexdump(const struct iovec *iov, const unsigned int iov_cnt,
 244                  FILE *fp, const char *prefix, size_t limit)
 245 {
 246     int v;
 247     size_t size = 0;
 248     char *buf;
 249
 250     for (v = 0; v < iov_cnt; v++) {
 251         size += iov[v].iov_len;
 252     }
 253     size = size > limit ? limit : size;
 254     buf = g_malloc(size);
 255     iov_to_buf(iov, iov_cnt, 0, buf, size);
 256     qemu_hexdump(fp, prefix, buf, size);
 257     g_free(buf);
 258 }
 259
 260 unsigned iov_copy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
 261                  const struct iovec *iov, unsigned int iov_cnt,
 262                  size_t offset, size_t bytes)
 263 {
 264     size_t len;
 265     unsigned int i, j;
 266     for (i = 0, j = 0;
 267          i < iov_cnt && j < dst_iov_cnt && (offset || bytes); i++) {
 268         if (offset >= iov[i].iov_len) {
 269             offset -= iov[i].iov_len;
 270             continue;
 271         }
 272         len = MIN(bytes, iov[i].iov_len - offset);
 273
 274         dst_iov[j].iov_base = iov[i].iov_base + offset;
 275         dst_iov[j].iov_len = len;
 276         j++;
 277         bytes -= len;
 278         offset = 0;
 279     }
 280     assert(offset == 0);
 281     return j;
 282 }
 283
 284 /* io vectors */
 285
 286 void qemu_iovec_init(QEMUIOVector *qiov, int alloc_hint)
 287 {
 288     qiov->iov = g_new(struct iovec, alloc_hint);
 289     qiov->niov = 0;
 290     qiov->nalloc = alloc_hint;
 291     qiov->size = 0;
 292 }
 293
 294 void qemu_iovec_init_external(QEMUIOVector *qiov, struct iovec *iov, int niov)
 295 {
 296     int i;
 297
 298     qiov->iov = iov;
 299     qiov->niov = niov;
 300     qiov->nalloc = -1;
 301     qiov->size = 0;
 302     for (i = 0; i < niov; i++)
 303         qiov->size += iov[i].iov_len;
 304 }
 305
 306 void qemu_iovec_add(QEMUIOVector *qiov, void *base, size_t len)
 307 {
 308     assert(qiov->nalloc != -1);
 309
 310     if (qiov->niov == qiov->nalloc) {
 311         qiov->nalloc = 2 * qiov->nalloc + 1;
 312         qiov->iov = g_renew(struct iovec, qiov->iov, qiov->nalloc);
 313     }
 314     qiov->iov[qiov->niov].iov_base = base;
 315     qiov->iov[qiov->niov].iov_len = len;
 316     qiov->size += len;
 317     ++qiov->niov;
 318 }
 319
 320 /*
 321  * Concatenates (partial) iovecs from src_iov to the end of dst.
 322  * It starts copying after skipping `soffset' bytes at the
 323  * beginning of src and adds individual vectors from src to
 324  * dst copies up to `sbytes' bytes total, or up to the end
 325  * of src_iov if it comes first.  This way, it is okay to specify
 326  * very large value for `sbytes' to indicate "up to the end
 327  * of src".
 328  * Only vector pointers are processed, not the actual data buffers.
 329  */
 330 size_t qemu_iovec_concat_iov(QEMUIOVector *dst,
 331                              struct iovec *src_iov, unsigned int src_cnt,
 332                              size_t soffset, size_t sbytes)
 333 {
 334     int i;
 335     size_t done;
 336
 337     if (!sbytes) {
 338         return 0;
 339     }
 340     assert(dst->nalloc != -1);
 341     for (i = 0, done = 0; done < sbytes && i < src_cnt; i++) {
 342         if (soffset < src_iov[i].iov_len) {
 343             size_t len = MIN(src_iov[i].iov_len - soffset, sbytes - done);
 344             qemu_iovec_add(dst, src_iov[i].iov_base + soffset, len);
 345             done += len;
 346             soffset = 0;
 347         } else {
 348             soffset -= src_iov[i].iov_len;
 349         }
 350     }
 351     assert(soffset == 0); /* offset beyond end of src */
 352
 353     return done;
 354 }
 355
 356 /*
 357  * Concatenates (partial) iovecs from src to the end of dst.
 358  * It starts copying after skipping `soffset' bytes at the
 359  * beginning of src and adds individual vectors from src to
 360  * dst copies up to `sbytes' bytes total, or up to the end
 361  * of src if it comes first.  This way, it is okay to specify
 362  * very large value for `sbytes' to indicate "up to the end
 363  * of src".
 364  * Only vector pointers are processed, not the actual data buffers.
 365  */
 366 void qemu_iovec_concat(QEMUIOVector *dst,
 367                        QEMUIOVector *src, size_t soffset, size_t sbytes)
 368 {
 369     qemu_iovec_concat_iov(dst, src->iov, src->niov, soffset, sbytes);
 370 }
 371
 372 /*
 373  * qiov_find_iov
 374  *
 375  * Return pointer to iovec structure, where byte at @offset in original vector
 376  * @iov exactly is.
 377  * Set @remaining_offset to be offset inside that iovec to the same byte.
 378  */
 379 static struct iovec *iov_skip_offset(struct iovec *iov, size_t offset,
 380                                      size_t *remaining_offset)
 381 {
 382     while (offset > 0 && offset >= iov->iov_len) {
 383         offset -= iov->iov_len;
 384         iov++;
 385     }
 386     *remaining_offset = offset;
 387
 388     return iov;
 389 }
 390
 391 /*
 392  * qemu_iovec_slice
 393  *
 394  * Find subarray of iovec's, containing requested range. @head would
 395  * be offset in first iov (returned by the function), @tail would be
 396  * count of extra bytes in last iovec (returned iov + @niov - 1).
 397  */
 398 struct iovec *qemu_iovec_slice(QEMUIOVector *qiov,
 399                                size_t offset, size_t len,
 400                                size_t *head, size_t *tail, int *niov)
 401 {
 402     struct iovec *iov, *end_iov;
 403
 404     assert(offset + len <= qiov->size);
 405
 406     iov = iov_skip_offset(qiov->iov, offset, head);
 407     end_iov = iov_skip_offset(iov, *head + len, tail);
 408
 409     if (*tail > 0) {
 410         assert(*tail < end_iov->iov_len);
 411         *tail = end_iov->iov_len - *tail;
 412         end_iov++;
 413     }
 414
 415     *niov = end_iov - iov;
 416
 417     return iov;
 418 }
 419
 420 int qemu_iovec_subvec_niov(QEMUIOVector *qiov, size_t offset, size_t len)
 421 {
 422     size_t head, tail;
 423     int niov;
 424
 425     qemu_iovec_slice(qiov, offset, len, &head, &tail, &niov);
 426
 427     return niov;
 428 }
 429
 430 /*
 431  * Check if the contents of subrange of qiov data is all zeroes.
 432  */
 433 bool qemu_iovec_is_zero(QEMUIOVector *qiov, size_t offset, size_t bytes)
 434 {
 435     struct iovec *iov;
 436     size_t current_offset;
 437
 438     assert(offset + bytes <= qiov->size);
 439
 440     iov = iov_skip_offset(qiov->iov, offset, &current_offset);
 441
 442     while (bytes) {
 443         uint8_t *base = (uint8_t *)iov->iov_base + current_offset;
 444         size_t len = MIN(iov->iov_len - current_offset, bytes);
 445
 446         if (!buffer_is_zero(base, len)) {
 447             return false;
 448         }
 449
 450         current_offset = 0;
 451         bytes -= len;
 452         iov++;
 453     }
 454
 455     return true;
 456 }
 457
 458 void qemu_iovec_init_slice(QEMUIOVector *qiov, QEMUIOVector *source,
 459                            size_t offset, size_t len)
 460 {
 461     struct iovec *slice_iov;
 462     int slice_niov;
 463     size_t slice_head, slice_tail;
 464
 465     assert(source->size >= len);
 466     assert(source->size - len >= offset);
 467
 468     slice_iov = qemu_iovec_slice(source, offset, len,
 469                                  &slice_head, &slice_tail, &slice_niov);
 470     if (slice_niov == 1) {
 471         qemu_iovec_init_buf(qiov, slice_iov[0].iov_base + slice_head, len);
 472     } else {
 473         qemu_iovec_init(qiov, slice_niov);
 474         qemu_iovec_concat_iov(qiov, slice_iov, slice_niov, slice_head, len);
 475     }
 476 }
 477
 478 void qemu_iovec_destroy(QEMUIOVector *qiov)
 479 {
 480     if (qiov->nalloc != -1) {
 481         g_free(qiov->iov);
 482     }
 483
 484     memset(qiov, 0, sizeof(*qiov));
 485 }
 486
 487 void qemu_iovec_reset(QEMUIOVector *qiov)
 488 {
 489     assert(qiov->nalloc != -1);
 490
 491     qiov->niov = 0;
 492     qiov->size = 0;
 493 }
 494
 495 size_t qemu_iovec_to_buf(QEMUIOVector *qiov, size_t offset,
 496                          void *buf, size_t bytes)
 497 {
 498     return iov_to_buf(qiov->iov, qiov->niov, offset, buf, bytes);
 499 }
 500
 501 size_t qemu_iovec_from_buf(QEMUIOVector *qiov, size_t offset,
 502                            const void *buf, size_t bytes)
 503 {
 504     return iov_from_buf(qiov->iov, qiov->niov, offset, buf, bytes);
 505 }
 506
 507 size_t qemu_iovec_memset(QEMUIOVector *qiov, size_t offset,
 508                          int fillc, size_t bytes)
 509 {
 510     return iov_memset(qiov->iov, qiov->niov, offset, fillc, bytes);
 511 }
 512
 513 /**
 514  * Check that I/O vector contents are identical
 515  *
 516  * The IO vectors must have the same structure (same length of all parts).
 517  * A typical usage is to compare vectors created with qemu_iovec_clone().
 518  *
 519  * @a:          I/O vector
 520  * @b:          I/O vector
 521  * @ret:        Offset to first mismatching byte or -1 if match
 522  */
 523 ssize_t qemu_iovec_compare(QEMUIOVector *a, QEMUIOVector *b)
 524 {
 525     int i;
 526     ssize_t offset = 0;
 527
 528     assert(a->niov == b->niov);
 529     for (i = 0; i < a->niov; i++) {
 530         size_t len = 0;
 531         uint8_t *p = (uint8_t *)a->iov[i].iov_base;
 532         uint8_t *q = (uint8_t *)b->iov[i].iov_base;
 533
 534         assert(a->iov[i].iov_len == b->iov[i].iov_len);
 535         while (len < a->iov[i].iov_len && *p++ == *q++) {
 536             len++;
 537         }
 538
 539         offset += len;
 540
 541         if (len != a->iov[i].iov_len) {
 542             return offset;
 543         }
 544     }
 545     return -1;
 546 }
 547
 548 typedef struct {
 549     int src_index;
 550     struct iovec *src_iov;
 551     void *dest_base;
 552 } IOVectorSortElem;
 553
 554 static int sortelem_cmp_src_base(const void *a, const void *b)
 555 {
 556     const IOVectorSortElem *elem_a = a;
 557     const IOVectorSortElem *elem_b = b;
 558
 559     /* Don't overflow */
 560     if (elem_a->src_iov->iov_base < elem_b->src_iov->iov_base) {
 561         return -1;
 562     } else if (elem_a->src_iov->iov_base > elem_b->src_iov->iov_base) {
 563         return 1;
 564     } else {
 565         return 0;
 566     }
 567 }
 568
 569 static int sortelem_cmp_src_index(const void *a, const void *b)
 570 {
 571     const IOVectorSortElem *elem_a = a;
 572     const IOVectorSortElem *elem_b = b;
 573
 574     return elem_a->src_index - elem_b->src_index;
 575 }
 576
 577 /**
 578  * Copy contents of I/O vector
 579  *
 580  * The relative relationships of overlapping iovecs are preserved.  This is
 581  * necessary to ensure identical semantics in the cloned I/O vector.
 582  */
 583 void qemu_iovec_clone(QEMUIOVector *dest, const QEMUIOVector *src, void *buf)
 584 {
 585     g_autofree IOVectorSortElem *sortelems = g_new(IOVectorSortElem, src->niov);
 586     void *last_end;
 587     int i;
 588
 589     /* Sort by source iovecs by base address */
 590     for (i = 0; i < src->niov; i++) {
 591         sortelems[i].src_index = i;
 592         sortelems[i].src_iov = &src->iov[i];
 593     }
 594     qsort(sortelems, src->niov, sizeof(sortelems[0]), sortelem_cmp_src_base);
 595
 596     /* Allocate buffer space taking into account overlapping iovecs */
 597     last_end = NULL;
 598     for (i = 0; i < src->niov; i++) {
 599         struct iovec *cur = sortelems[i].src_iov;
 600         ptrdiff_t rewind = 0;
 601
 602         /* Detect overlap */
 603         if (last_end && last_end > cur->iov_base) {
 604             rewind = last_end - cur->iov_base;
 605         }
 606
 607         sortelems[i].dest_base = buf - rewind;
 608         buf += cur->iov_len - MIN(rewind, cur->iov_len);
 609         last_end = MAX(cur->iov_base + cur->iov_len, last_end);
 610     }
 611
 612     /* Sort by source iovec index and build destination iovec */
 613     qsort(sortelems, src->niov, sizeof(sortelems[0]), sortelem_cmp_src_index);
 614     for (i = 0; i < src->niov; i++) {
 615         qemu_iovec_add(dest, sortelems[i].dest_base, src->iov[i].iov_len);
 616     }
 617 }
 618
 619 void iov_discard_undo(IOVDiscardUndo *undo)
 620 {
 621     /* Restore original iovec if it was modified */
 622     if (undo->modified_iov) {
 623         *undo->modified_iov = undo->orig;
 624     }
 625 }
 626
 627 size_t iov_discard_front_undoable(struct iovec **iov,
 628                                   unsigned int *iov_cnt,
 629                                   size_t bytes,
 630                                   IOVDiscardUndo *undo)
 631 {
 632     size_t total = 0;
 633     struct iovec *cur;
 634
 635     if (undo) {
 636         undo->modified_iov = NULL;
 637     }
 638
 639     for (cur = *iov; *iov_cnt > 0; cur++) {
 640         if (cur->iov_len > bytes) {
 641             if (undo) {
 642                 undo->modified_iov = cur;
 643                 undo->orig = *cur;
 644             }
 645
 646             cur->iov_base += bytes;
 647             cur->iov_len -= bytes;
 648             total += bytes;
 649             break;
 650         }
 651
 652         bytes -= cur->iov_len;
 653         total += cur->iov_len;
 654         *iov_cnt -= 1;
 655     }
 656
 657     *iov = cur;
 658     return total;
 659 }
 660
 661 size_t iov_discard_front(struct iovec **iov, unsigned int *iov_cnt,
 662                          size_t bytes)
 663 {
 664     return iov_discard_front_undoable(iov, iov_cnt, bytes, NULL);
 665 }
 666
 667 size_t iov_discard_back_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     if (*iov_cnt == 0) {
 680         return 0;
 681     }
 682
 683     cur = iov + (*iov_cnt - 1);
 684
 685     while (*iov_cnt > 0) {
 686         if (cur->iov_len > bytes) {
 687             if (undo) {
 688                 undo->modified_iov = cur;
 689                 undo->orig = *cur;
 690             }
 691
 692             cur->iov_len -= bytes;
 693             total += bytes;
 694             break;
 695         }
 696
 697         bytes -= cur->iov_len;
 698         total += cur->iov_len;
 699         cur--;
 700         *iov_cnt -= 1;
 701     }
 702
 703     return total;
 704 }
 705
 706 size_t iov_discard_back(struct iovec *iov, unsigned int *iov_cnt,
 707                         size_t bytes)
 708 {
 709     return iov_discard_back_undoable(iov, iov_cnt, bytes, NULL);
 710 }
 711
 712 void qemu_iovec_discard_back(QEMUIOVector *qiov, size_t bytes)
 713 {
 714     size_t total;
 715     unsigned int niov = qiov->niov;
 716
 717     assert(qiov->size >= bytes);
 718     total = iov_discard_back(qiov->iov, &niov, bytes);
 719     assert(total == bytes);
 720
 721     qiov->niov = niov;
 722     qiov->size -= bytes;
 723 }