summaryrefslogtreecommitdiff
path: root/migration/postcopy-ram.c
blob: 9b0477835ff07e1f00412c870acad83a7d30b224 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
/*
 * Postcopy migration for RAM
 *
 * Copyright 2013-2015 Red Hat, Inc. and/or its affiliates
 *
 * Authors:
 *  Dave Gilbert  <dgilbert@redhat.com>
 *
 * This work is licensed under the terms of the GNU GPL, version 2 or later.
 * See the COPYING file in the top-level directory.
 *
 */

/*
 * Postcopy is a migration technique where the execution flips from the
 * source to the destination before all the data has been copied.
 */

#include "qemu/osdep.h"

#include "qemu-common.h"
#include "migration/migration.h"
#include "migration/postcopy-ram.h"
#include "sysemu/sysemu.h"
#include "sysemu/balloon.h"
#include "qemu/error-report.h"
#include "trace.h"

/* Arbitrary limit on size of each discard command,
 * keeps them around ~200 bytes
 */
#define MAX_DISCARDS_PER_COMMAND 12

struct PostcopyDiscardState {
    const char *ramblock_name;
    uint64_t offset; /* Bitmap entry for the 1st bit of this RAMBlock */
    uint16_t cur_entry;
    /*
     * Start and length of a discard range (bytes)
     */
    uint64_t start_list[MAX_DISCARDS_PER_COMMAND];
    uint64_t length_list[MAX_DISCARDS_PER_COMMAND];
    unsigned int nsentwords;
    unsigned int nsentcmds;
};

/* Postcopy needs to detect accesses to pages that haven't yet been copied
 * across, and efficiently map new pages in, the techniques for doing this
 * are target OS specific.
 */
#if defined(__linux__)

#include <poll.h>
#include <sys/ioctl.h>
#include <sys/syscall.h>
#include <asm/types.h> /* for __u64 */
#endif

#if defined(__linux__) && defined(__NR_userfaultfd) && defined(CONFIG_EVENTFD)
#include <sys/eventfd.h>
#include <linux/userfaultfd.h>

static bool ufd_version_check(int ufd)
{
    struct uffdio_api api_struct;
    uint64_t ioctl_mask;

    api_struct.api = UFFD_API;
    api_struct.features = 0;
    if (ioctl(ufd, UFFDIO_API, &api_struct)) {
        error_report("postcopy_ram_supported_by_host: UFFDIO_API failed: %s",
                     strerror(errno));
        return false;
    }

    ioctl_mask = (__u64)1 << _UFFDIO_REGISTER |
                 (__u64)1 << _UFFDIO_UNREGISTER;
    if ((api_struct.ioctls & ioctl_mask) != ioctl_mask) {
        error_report("Missing userfault features: %" PRIx64,
                     (uint64_t)(~api_struct.ioctls & ioctl_mask));
        return false;
    }

    return true;
}

/*
 * Note: This has the side effect of munlock'ing all of RAM, that's
 * normally fine since if the postcopy succeeds it gets turned back on at the
 * end.
 */
bool postcopy_ram_supported_by_host(void)
{
    long pagesize = getpagesize();
    int ufd = -1;
    bool ret = false; /* Error unless we change it */
    void *testarea = NULL;
    struct uffdio_register reg_struct;
    struct uffdio_range range_struct;
    uint64_t feature_mask;

    if ((1ul << qemu_target_page_bits()) > pagesize) {
        error_report("Target page size bigger than host page size");
        goto out;
    }

    ufd = syscall(__NR_userfaultfd, O_CLOEXEC);
    if (ufd == -1) {
        error_report("%s: userfaultfd not available: %s", __func__,
                     strerror(errno));
        goto out;
    }

    /* Version and features check */
    if (!ufd_version_check(ufd)) {
        goto out;
    }

    /*
     * userfault and mlock don't go together; we'll put it back later if
     * it was enabled.
     */
    if (munlockall()) {
        error_report("%s: munlockall: %s", __func__,  strerror(errno));
        return -1;
    }

    /*
     *  We need to check that the ops we need are supported on anon memory
     *  To do that we need to register a chunk and see the flags that
     *  are returned.
     */
    testarea = mmap(NULL, pagesize, PROT_READ | PROT_WRITE, MAP_PRIVATE |
                                    MAP_ANONYMOUS, -1, 0);
    if (testarea == MAP_FAILED) {
        error_report("%s: Failed to map test area: %s", __func__,
                     strerror(errno));
        goto out;
    }
    g_assert(((size_t)testarea & (pagesize-1)) == 0);

    reg_struct.range.start = (uintptr_t)testarea;
    reg_struct.range.len = pagesize;
    reg_struct.mode = UFFDIO_REGISTER_MODE_MISSING;

    if (ioctl(ufd, UFFDIO_REGISTER, &reg_struct)) {
        error_report("%s userfault register: %s", __func__, strerror(errno));
        goto out;
    }

    range_struct.start = (uintptr_t)testarea;
    range_struct.len = pagesize;
    if (ioctl(ufd, UFFDIO_UNREGISTER, &range_struct)) {
        error_report("%s userfault unregister: %s", __func__, strerror(errno));
        goto out;
    }

    feature_mask = (__u64)1 << _UFFDIO_WAKE |
                   (__u64)1 << _UFFDIO_COPY |
                   (__u64)1 << _UFFDIO_ZEROPAGE;
    if ((reg_struct.ioctls & feature_mask) != feature_mask) {
        error_report("Missing userfault map features: %" PRIx64,
                     (uint64_t)(~reg_struct.ioctls & feature_mask));
        goto out;
    }

    /* Success! */
    ret = true;
out:
    if (testarea) {
        munmap(testarea, pagesize);
    }
    if (ufd != -1) {
        close(ufd);
    }
    return ret;
}

/**
 * postcopy_ram_discard_range: Discard a range of memory.
 * We can assume that if we've been called postcopy_ram_hosttest returned true.
 *
 * @mis: Current incoming migration state.
 * @start, @length: range of memory to discard.
 *
 * returns: 0 on success.
 */
int postcopy_ram_discard_range(MigrationIncomingState *mis, uint8_t *start,
                               size_t length)
{
    trace_postcopy_ram_discard_range(start, length);
    if (madvise(start, length, MADV_DONTNEED)) {
        error_report("%s MADV_DONTNEED: %s", __func__, strerror(errno));
        return -1;
    }

    return 0;
}

/*
 * Setup an area of RAM so that it *can* be used for postcopy later; this
 * must be done right at the start prior to pre-copy.
 * opaque should be the MIS.
 */
static int init_range(const char *block_name, void *host_addr,
                      ram_addr_t offset, ram_addr_t length, void *opaque)
{
    MigrationIncomingState *mis = opaque;

    trace_postcopy_init_range(block_name, host_addr, offset, length);

    /*
     * We need the whole of RAM to be truly empty for postcopy, so things
     * like ROMs and any data tables built during init must be zero'd
     * - we're going to get the copy from the source anyway.
     * (Precopy will just overwrite this data, so doesn't need the discard)
     */
    if (postcopy_ram_discard_range(mis, host_addr, length)) {
        return -1;
    }

    return 0;
}

/*
 * At the end of migration, undo the effects of init_range
 * opaque should be the MIS.
 */
static int cleanup_range(const char *block_name, void *host_addr,
                        ram_addr_t offset, ram_addr_t length, void *opaque)
{
    MigrationIncomingState *mis = opaque;
    struct uffdio_range range_struct;
    trace_postcopy_cleanup_range(block_name, host_addr, offset, length);

    /*
     * We turned off hugepage for the precopy stage with postcopy enabled
     * we can turn it back on now.
     */
    qemu_madvise(host_addr, length, QEMU_MADV_HUGEPAGE);

    /*
     * We can also turn off userfault now since we should have all the
     * pages.   It can be useful to leave it on to debug postcopy
     * if you're not sure it's always getting every page.
     */
    range_struct.start = (uintptr_t)host_addr;
    range_struct.len = length;

    if (ioctl(mis->userfault_fd, UFFDIO_UNREGISTER, &range_struct)) {
        error_report("%s: userfault unregister %s", __func__, strerror(errno));

        return -1;
    }

    return 0;
}

/*
 * Initialise postcopy-ram, setting the RAM to a state where we can go into
 * postcopy later; must be called prior to any precopy.
 * called from arch_init's similarly named ram_postcopy_incoming_init
 */
int postcopy_ram_incoming_init(MigrationIncomingState *mis, size_t ram_pages)
{
    if (qemu_ram_foreach_block(init_range, mis)) {
        return -1;
    }

    return 0;
}

/*
 * At the end of a migration where postcopy_ram_incoming_init was called.
 */
int postcopy_ram_incoming_cleanup(MigrationIncomingState *mis)
{
    trace_postcopy_ram_incoming_cleanup_entry();

    if (mis->have_fault_thread) {
        uint64_t tmp64;

        if (qemu_ram_foreach_block(cleanup_range, mis)) {
            return -1;
        }
        /*
         * Tell the fault_thread to exit, it's an eventfd that should
         * currently be at 0, we're going to increment it to 1
         */
        tmp64 = 1;
        if (write(mis->userfault_quit_fd, &tmp64, 8) == 8) {
            trace_postcopy_ram_incoming_cleanup_join();
            qemu_thread_join(&mis->fault_thread);
        } else {
            /* Not much we can do here, but may as well report it */
            error_report("%s: incrementing userfault_quit_fd: %s", __func__,
                         strerror(errno));
        }
        trace_postcopy_ram_incoming_cleanup_closeuf();
        close(mis->userfault_fd);
        close(mis->userfault_quit_fd);
        mis->have_fault_thread = false;
    }

    qemu_balloon_inhibit(false);

    if (enable_mlock) {
        if (os_mlock() < 0) {
            error_report("mlock: %s", strerror(errno));
            /*
             * It doesn't feel right to fail at this point, we have a valid
             * VM state.
             */
        }
    }

    postcopy_state_set(POSTCOPY_INCOMING_END);
    migrate_send_rp_shut(mis, qemu_file_get_error(mis->from_src_file) != 0);

    if (mis->postcopy_tmp_page) {
        munmap(mis->postcopy_tmp_page, getpagesize());
        mis->postcopy_tmp_page = NULL;
    }
    trace_postcopy_ram_incoming_cleanup_exit();
    return 0;
}

/*
 * Disable huge pages on an area
 */
static int nhp_range(const char *block_name, void *host_addr,
                    ram_addr_t offset, ram_addr_t length, void *opaque)
{
    trace_postcopy_nhp_range(block_name, host_addr, offset, length);

    /*
     * Before we do discards we need to ensure those discards really
     * do delete areas of the page, even if THP thinks a hugepage would
     * be a good idea, so force hugepages off.
     */
    qemu_madvise(host_addr, length, QEMU_MADV_NOHUGEPAGE);

    return 0;
}

/*
 * Userfault requires us to mark RAM as NOHUGEPAGE prior to discard
 * however leaving it until after precopy means that most of the precopy
 * data is still THPd
 */
int postcopy_ram_prepare_discard(MigrationIncomingState *mis)
{
    if (qemu_ram_foreach_block(nhp_range, mis)) {
        return -1;
    }

    postcopy_state_set(POSTCOPY_INCOMING_DISCARD);

    return 0;
}

/*
 * Mark the given area of RAM as requiring notification to unwritten areas
 * Used as a  callback on qemu_ram_foreach_block.
 *   host_addr: Base of area to mark
 *   offset: Offset in the whole ram arena
 *   length: Length of the section
 *   opaque: MigrationIncomingState pointer
 * Returns 0 on success
 */
static int ram_block_enable_notify(const char *block_name, void *host_addr,
                                   ram_addr_t offset, ram_addr_t length,
                                   void *opaque)
{
    MigrationIncomingState *mis = opaque;
    struct uffdio_register reg_struct;

    reg_struct.range.start = (uintptr_t)host_addr;
    reg_struct.range.len = length;
    reg_struct.mode = UFFDIO_REGISTER_MODE_MISSING;

    /* Now tell our userfault_fd that it's responsible for this area */
    if (ioctl(mis->userfault_fd, UFFDIO_REGISTER, &reg_struct)) {
        error_report("%s userfault register: %s", __func__, strerror(errno));
        return -1;
    }

    return 0;
}

/*
 * Handle faults detected by the USERFAULT markings
 */
static void *postcopy_ram_fault_thread(void *opaque)
{
    MigrationIncomingState *mis = opaque;
    struct uffd_msg msg;
    int ret;
    size_t hostpagesize = getpagesize();
    RAMBlock *rb = NULL;
    RAMBlock *last_rb = NULL; /* last RAMBlock we sent part of */

    trace_postcopy_ram_fault_thread_entry();
    qemu_sem_post(&mis->fault_thread_sem);

    while (true) {
        ram_addr_t rb_offset;
        struct pollfd pfd[2];

        /*
         * We're mainly waiting for the kernel to give us a faulting HVA,
         * however we can be told to quit via userfault_quit_fd which is
         * an eventfd
         */
        pfd[0].fd = mis->userfault_fd;
        pfd[0].events = POLLIN;
        pfd[0].revents = 0;
        pfd[1].fd = mis->userfault_quit_fd;
        pfd[1].events = POLLIN; /* Waiting for eventfd to go positive */
        pfd[1].revents = 0;

        if (poll(pfd, 2, -1 /* Wait forever */) == -1) {
            error_report("%s: userfault poll: %s", __func__, strerror(errno));
            break;
        }

        if (pfd[1].revents) {
            trace_postcopy_ram_fault_thread_quit();
            break;
        }

        ret = read(mis->userfault_fd, &msg, sizeof(msg));
        if (ret != sizeof(msg)) {
            if (errno == EAGAIN) {
                /*
                 * if a wake up happens on the other thread just after
                 * the poll, there is nothing to read.
                 */
                continue;
            }
            if (ret < 0) {
                error_report("%s: Failed to read full userfault message: %s",
                             __func__, strerror(errno));
                break;
            } else {
                error_report("%s: Read %d bytes from userfaultfd expected %zd",
                             __func__, ret, sizeof(msg));
                break; /* Lost alignment, don't know what we'd read next */
            }
        }
        if (msg.event != UFFD_EVENT_PAGEFAULT) {
            error_report("%s: Read unexpected event %ud from userfaultfd",
                         __func__, msg.event);
            continue; /* It's not a page fault, shouldn't happen */
        }

        rb = qemu_ram_block_from_host(
                 (void *)(uintptr_t)msg.arg.pagefault.address,
                 true, &rb_offset);
        if (!rb) {
            error_report("postcopy_ram_fault_thread: Fault outside guest: %"
                         PRIx64, (uint64_t)msg.arg.pagefault.address);
            break;
        }

        rb_offset &= ~(hostpagesize - 1);
        trace_postcopy_ram_fault_thread_request(msg.arg.pagefault.address,
                                                qemu_ram_get_idstr(rb),
                                                rb_offset);

        /*
         * Send the request to the source - we want to request one
         * of our host page sizes (which is >= TPS)
         */
        if (rb != last_rb) {
            last_rb = rb;
            migrate_send_rp_req_pages(mis, qemu_ram_get_idstr(rb),
                                     rb_offset, hostpagesize);
        } else {
            /* Save some space */
            migrate_send_rp_req_pages(mis, NULL,
                                     rb_offset, hostpagesize);
        }
    }
    trace_postcopy_ram_fault_thread_exit();
    return NULL;
}

int postcopy_ram_enable_notify(MigrationIncomingState *mis)
{
    /* Open the fd for the kernel to give us userfaults */
    mis->userfault_fd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
    if (mis->userfault_fd == -1) {
        error_report("%s: Failed to open userfault fd: %s", __func__,
                     strerror(errno));
        return -1;
    }

    /*
     * Although the host check already tested the API, we need to
     * do the check again as an ABI handshake on the new fd.
     */
    if (!ufd_version_check(mis->userfault_fd)) {
        return -1;
    }

    /* Now an eventfd we use to tell the fault-thread to quit */
    mis->userfault_quit_fd = eventfd(0, EFD_CLOEXEC);
    if (mis->userfault_quit_fd == -1) {
        error_report("%s: Opening userfault_quit_fd: %s", __func__,
                     strerror(errno));
        close(mis->userfault_fd);
        return -1;
    }

    qemu_sem_init(&mis->fault_thread_sem, 0);
    qemu_thread_create(&mis->fault_thread, "postcopy/fault",
                       postcopy_ram_fault_thread, mis, QEMU_THREAD_JOINABLE);
    qemu_sem_wait(&mis->fault_thread_sem);
    qemu_sem_destroy(&mis->fault_thread_sem);
    mis->have_fault_thread = true;

    /* Mark so that we get notified of accesses to unwritten areas */
    if (qemu_ram_foreach_block(ram_block_enable_notify, mis)) {
        return -1;
    }

    /*
     * Ballooning can mark pages as absent while we're postcopying
     * that would cause false userfaults.
     */
    qemu_balloon_inhibit(true);

    trace_postcopy_ram_enable_notify();

    return 0;
}

/*
 * Place a host page (from) at (host) atomically
 * returns 0 on success
 */
int postcopy_place_page(MigrationIncomingState *mis, void *host, void *from)
{
    struct uffdio_copy copy_struct;

    copy_struct.dst = (uint64_t)(uintptr_t)host;
    copy_struct.src = (uint64_t)(uintptr_t)from;
    copy_struct.len = getpagesize();
    copy_struct.mode = 0;

    /* copy also acks to the kernel waking the stalled thread up
     * TODO: We can inhibit that ack and only do it if it was requested
     * which would be slightly cheaper, but we'd have to be careful
     * of the order of updating our page state.
     */
    if (ioctl(mis->userfault_fd, UFFDIO_COPY, &copy_struct)) {
        int e = errno;
        error_report("%s: %s copy host: %p from: %p",
                     __func__, strerror(e), host, from);

        return -e;
    }

    trace_postcopy_place_page(host);
    return 0;
}

/*
 * Place a zero page at (host) atomically
 * returns 0 on success
 */
int postcopy_place_page_zero(MigrationIncomingState *mis, void *host)
{
    struct uffdio_zeropage zero_struct;

    zero_struct.range.start = (uint64_t)(uintptr_t)host;
    zero_struct.range.len = getpagesize();
    zero_struct.mode = 0;

    if (ioctl(mis->userfault_fd, UFFDIO_ZEROPAGE, &zero_struct)) {
        int e = errno;
        error_report("%s: %s zero host: %p",
                     __func__, strerror(e), host);

        return -e;
    }

    trace_postcopy_place_page_zero(host);
    return 0;
}

/*
 * Returns a target page of memory that can be mapped at a later point in time
 * using postcopy_place_page
 * The same address is used repeatedly, postcopy_place_page just takes the
 * backing page away.
 * Returns: Pointer to allocated page
 *
 */
void *postcopy_get_tmp_page(MigrationIncomingState *mis)
{
    if (!mis->postcopy_tmp_page) {
        mis->postcopy_tmp_page = mmap(NULL, getpagesize(),
                             PROT_READ | PROT_WRITE, MAP_PRIVATE |
                             MAP_ANONYMOUS, -1, 0);
        if (mis->postcopy_tmp_page == MAP_FAILED) {
            mis->postcopy_tmp_page = NULL;
            error_report("%s: %s", __func__, strerror(errno));
            return NULL;
        }
    }

    return mis->postcopy_tmp_page;
}

#else
/* No target OS support, stubs just fail */
bool postcopy_ram_supported_by_host(void)
{
    error_report("%s: No OS support", __func__);
    return false;
}

int postcopy_ram_incoming_init(MigrationIncomingState *mis, size_t ram_pages)
{
    error_report("postcopy_ram_incoming_init: No OS support");
    return -1;
}

int postcopy_ram_incoming_cleanup(MigrationIncomingState *mis)
{
    assert(0);
    return -1;
}

int postcopy_ram_discard_range(MigrationIncomingState *mis, uint8_t *start,
                               size_t length)
{
    assert(0);
    return -1;
}

int postcopy_ram_prepare_discard(MigrationIncomingState *mis)
{
    assert(0);
    return -1;
}

int postcopy_ram_enable_notify(MigrationIncomingState *mis)
{
    assert(0);
    return -1;
}

int postcopy_place_page(MigrationIncomingState *mis, void *host, void *from)
{
    assert(0);
    return -1;
}

int postcopy_place_page_zero(MigrationIncomingState *mis, void *host)
{
    assert(0);
    return -1;
}

void *postcopy_get_tmp_page(MigrationIncomingState *mis)
{
    assert(0);
    return NULL;
}

#endif

/* ------------------------------------------------------------------------- */

/**
 * postcopy_discard_send_init: Called at the start of each RAMBlock before
 *   asking to discard individual ranges.
 *
 * @ms: The current migration state.
 * @offset: the bitmap offset of the named RAMBlock in the migration
 *   bitmap.
 * @name: RAMBlock that discards will operate on.
 *
 * returns: a new PDS.
 */
PostcopyDiscardState *postcopy_discard_send_init(MigrationState *ms,
                                                 unsigned long offset,
                                                 const char *name)
{
    PostcopyDiscardState *res = g_malloc0(sizeof(PostcopyDiscardState));

    if (res) {
        res->ramblock_name = name;
        res->offset = offset;
    }

    return res;
}

/**
 * postcopy_discard_send_range: Called by the bitmap code for each chunk to
 *   discard. May send a discard message, may just leave it queued to
 *   be sent later.
 *
 * @ms: Current migration state.
 * @pds: Structure initialised by postcopy_discard_send_init().
 * @start,@length: a range of pages in the migration bitmap in the
 *   RAM block passed to postcopy_discard_send_init() (length=1 is one page)
 */
void postcopy_discard_send_range(MigrationState *ms, PostcopyDiscardState *pds,
                                unsigned long start, unsigned long length)
{
    size_t tp_bits = qemu_target_page_bits();
    /* Convert to byte offsets within the RAM block */
    pds->start_list[pds->cur_entry] = (start - pds->offset) << tp_bits;
    pds->length_list[pds->cur_entry] = length << tp_bits;
    trace_postcopy_discard_send_range(pds->ramblock_name, start, length);
    pds->cur_entry++;
    pds->nsentwords++;

    if (pds->cur_entry == MAX_DISCARDS_PER_COMMAND) {
        /* Full set, ship it! */
        qemu_savevm_send_postcopy_ram_discard(ms->to_dst_file,
                                              pds->ramblock_name,
                                              pds->cur_entry,
                                              pds->start_list,
                                              pds->length_list);
        pds->nsentcmds++;
        pds->cur_entry = 0;
    }
}

/**
 * postcopy_discard_send_finish: Called at the end of each RAMBlock by the
 * bitmap code. Sends any outstanding discard messages, frees the PDS
 *
 * @ms: Current migration state.
 * @pds: Structure initialised by postcopy_discard_send_init().
 */
void postcopy_discard_send_finish(MigrationState *ms, PostcopyDiscardState *pds)
{
    /* Anything unsent? */
    if (pds->cur_entry) {
        qemu_savevm_send_postcopy_ram_discard(ms->to_dst_file,
                                              pds->ramblock_name,
                                              pds->cur_entry,
                                              pds->start_list,
                                              pds->length_list);
        pds->nsentcmds++;
    }

    trace_postcopy_discard_send_finish(pds->ramblock_name, pds->nsentwords,
                                       pds->nsentcmds);

    g_free(pds);
}