Patchwork [PULL,25/36] block: vhdx - log parsing, replay, and flush support

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Submitter Stefan Hajnoczi
Date Nov. 7, 2013, 1:12 p.m.
Message ID <1383829964-32364-26-git-send-email-stefanha@redhat.com>
Download mbox | patch
Permalink /patch/289321/
State New
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Comments

Stefan Hajnoczi - Nov. 7, 2013, 1:12 p.m.
From: Jeff Cody <jcody@redhat.com>

This adds support for VHDX v0 logs, as specified in Microsoft's
VHDX Specification Format v1.00:
https://www.microsoft.com/en-us/download/details.aspx?id=34750

The following support is added:

* Log parsing, and validation - validate that an existing log
  is correct.

* Log search - search through an existing log, to find any valid
  sequence of entries.

* Log replay and flush - replay an existing log, and flush/clear
  the log when complete.

The VHDX log is a circular buffer, with elements (sectors) of 4KB.

A log entry is a variably-length number of sectors, that is
comprised of a header and 'descriptors', that describe each sector.

A log may contain multiple entries, know as a log sequence.  In a log
sequence, each log entry immediately follows the previous entry, with an
incrementing sequence number.  There can only ever be one active and
valid sequence in the log.

Each log entry must match the file log GUID in order to be valid (along
with other criteria).  Once we have flushed all valid log entries, we
marked the file log GUID to be zero, which indicates a buffer with no
valid entries.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block/Makefile.objs |   2 +-
 block/vhdx-log.c    | 728 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 block/vhdx.c        |  65 +----
 block/vhdx.h        |   7 +-
 4 files changed, 749 insertions(+), 53 deletions(-)
 create mode 100644 block/vhdx-log.c

Patch

diff --git a/block/Makefile.objs b/block/Makefile.objs
index 2fc496a..f43ecbc 100644
--- a/block/Makefile.objs
+++ b/block/Makefile.objs
@@ -2,7 +2,7 @@  block-obj-y += raw_bsd.o cow.o qcow.o vdi.o vmdk.o cloop.o dmg.o bochs.o vpc.o v
 block-obj-y += qcow2.o qcow2-refcount.o qcow2-cluster.o qcow2-snapshot.o qcow2-cache.o
 block-obj-y += qed.o qed-gencb.o qed-l2-cache.o qed-table.o qed-cluster.o
 block-obj-y += qed-check.o
-block-obj-$(CONFIG_VHDX) += vhdx.o vhdx-endian.o
+block-obj-$(CONFIG_VHDX) += vhdx.o vhdx-endian.o vhdx-log.o
 block-obj-y += parallels.o blkdebug.o blkverify.o
 block-obj-y += snapshot.o qapi.o
 block-obj-$(CONFIG_WIN32) += raw-win32.o win32-aio.o
diff --git a/block/vhdx-log.c b/block/vhdx-log.c
new file mode 100644
index 0000000..0284729
--- /dev/null
+++ b/block/vhdx-log.c
@@ -0,0 +1,728 @@ 
+/*
+ * Block driver for Hyper-V VHDX Images
+ *
+ * Copyright (c) 2013 Red Hat, Inc.,
+ *
+ * Authors:
+ *  Jeff Cody <jcody@redhat.com>
+ *
+ *  This is based on the "VHDX Format Specification v1.00", published 8/25/2012
+ *  by Microsoft:
+ *      https://www.microsoft.com/en-us/download/details.aspx?id=34750
+ *
+ * This file covers the functionality of the metadata log writing, parsing, and
+ * replay.
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+#include "qemu-common.h"
+#include "block/block_int.h"
+#include "qemu/module.h"
+#include "block/vhdx.h"
+
+
+typedef struct VHDXLogSequence {
+    bool valid;
+    uint32_t count;
+    VHDXLogEntries log;
+    VHDXLogEntryHeader hdr;
+} VHDXLogSequence;
+
+typedef struct VHDXLogDescEntries {
+    VHDXLogEntryHeader hdr;
+    VHDXLogDescriptor desc[];
+} VHDXLogDescEntries;
+
+static const MSGUID zero_guid = { 0 };
+
+/* The log located on the disk is circular buffer containing
+ * sectors of 4096 bytes each.
+ *
+ * It is assumed for the read/write functions below that the
+ * circular buffer scheme uses a 'one sector open' to indicate
+ * the buffer is full.  Given the validation methods used for each
+ * sector, this method should be compatible with other methods that
+ * do not waste a sector.
+ */
+
+
+/* Allow peeking at the hdr entry at the beginning of the current
+ * read index, without advancing the read index */
+static int vhdx_log_peek_hdr(BlockDriverState *bs, VHDXLogEntries *log,
+                             VHDXLogEntryHeader *hdr)
+{
+    int ret = 0;
+    uint64_t offset;
+    uint32_t read;
+
+    assert(hdr != NULL);
+
+    /* peek is only supported on sector boundaries */
+    if (log->read % VHDX_LOG_SECTOR_SIZE) {
+        ret = -EFAULT;
+        goto exit;
+    }
+
+    read = log->read;
+    /* we are guaranteed that a) log sectors are 4096 bytes,
+     * and b) the log length is a multiple of 1MB. So, there
+     * is always a round number of sectors in the buffer */
+    if ((read + sizeof(VHDXLogEntryHeader)) > log->length) {
+        read = 0;
+    }
+
+    if (read == log->write) {
+        ret = -EINVAL;
+        goto exit;
+    }
+
+    offset = log->offset + read;
+
+    ret = bdrv_pread(bs->file, offset, hdr, sizeof(VHDXLogEntryHeader));
+    if (ret < 0) {
+        goto exit;
+    }
+
+exit:
+    return ret;
+}
+
+/* Index increment for log, based on sector boundaries */
+static int vhdx_log_inc_idx(uint32_t idx, uint64_t length)
+{
+    idx += VHDX_LOG_SECTOR_SIZE;
+    /* we are guaranteed that a) log sectors are 4096 bytes,
+     * and b) the log length is a multiple of 1MB. So, there
+     * is always a round number of sectors in the buffer */
+    return idx >= length ? 0 : idx;
+}
+
+
+/* Reset the log to empty */
+static void vhdx_log_reset(BlockDriverState *bs, BDRVVHDXState *s)
+{
+    MSGUID guid = { 0 };
+    s->log.read = s->log.write = 0;
+    /* a log guid of 0 indicates an empty log to any parser of v0
+     * VHDX logs */
+    vhdx_update_headers(bs, s, false, &guid);
+}
+
+/* Reads num_sectors from the log (all log sectors are 4096 bytes),
+ * into buffer 'buffer'.  Upon return, *sectors_read will contain
+ * the number of sectors successfully read.
+ *
+ * It is assumed that 'buffer' is already allocated, and of sufficient
+ * size (i.e. >= 4096*num_sectors).
+ *
+ * If 'peek' is true, then the tail (read) pointer for the circular buffer is
+ * not modified.
+ *
+ * 0 is returned on success, -errno otherwise.  */
+static int vhdx_log_read_sectors(BlockDriverState *bs, VHDXLogEntries *log,
+                                 uint32_t *sectors_read, void *buffer,
+                                 uint32_t num_sectors, bool peek)
+{
+    int ret = 0;
+    uint64_t offset;
+    uint32_t read;
+
+    read = log->read;
+
+    *sectors_read = 0;
+    while (num_sectors) {
+        if (read == log->write) {
+            /* empty */
+            break;
+        }
+        offset = log->offset + read;
+
+        ret = bdrv_pread(bs->file, offset, buffer, VHDX_LOG_SECTOR_SIZE);
+        if (ret < 0) {
+            goto exit;
+        }
+        read = vhdx_log_inc_idx(read, log->length);
+
+        *sectors_read = *sectors_read + 1;
+        num_sectors--;
+    }
+
+exit:
+    if (!peek) {
+        log->read = read;
+    }
+    return ret;
+}
+
+/* Validates a log entry header */
+static bool vhdx_log_hdr_is_valid(VHDXLogEntries *log, VHDXLogEntryHeader *hdr,
+                                  BDRVVHDXState *s)
+{
+    int valid = false;
+
+    if (memcmp(&hdr->signature, "loge", 4)) {
+        goto exit;
+    }
+
+    /* if the individual entry length is larger than the whole log
+     * buffer, that is obviously invalid */
+    if (log->length < hdr->entry_length) {
+        goto exit;
+    }
+
+    /* length of entire entry must be in units of 4KB (log sector size) */
+    if (hdr->entry_length % (VHDX_LOG_SECTOR_SIZE)) {
+        goto exit;
+    }
+
+    /* per spec, sequence # must be > 0 */
+    if (hdr->sequence_number == 0) {
+        goto exit;
+    }
+
+    /* log entries are only valid if they match the file-wide log guid
+     * found in the active header */
+    if (!guid_eq(hdr->log_guid, s->headers[s->curr_header]->log_guid)) {
+        goto exit;
+    }
+
+    if (hdr->descriptor_count * sizeof(VHDXLogDescriptor) > hdr->entry_length) {
+        goto exit;
+    }
+
+    valid = true;
+
+exit:
+    return valid;
+}
+
+/*
+ * Given a log header, this will validate that the descriptors and the
+ * corresponding data sectors (if applicable)
+ *
+ * Validation consists of:
+ *      1. Making sure the sequence numbers matches the entry header
+ *      2. Verifying a valid signature ('zero' or 'desc' for descriptors)
+ *      3. File offset field is a multiple of 4KB
+ *      4. If a data descriptor, the corresponding data sector
+ *         has its signature ('data') and matching sequence number
+ *
+ * @desc: the data buffer containing the descriptor
+ * @hdr:  the log entry header
+ *
+ * Returns true if valid
+ */
+static bool vhdx_log_desc_is_valid(VHDXLogDescriptor *desc,
+                                   VHDXLogEntryHeader *hdr)
+{
+    bool ret = false;
+
+    if (desc->sequence_number != hdr->sequence_number) {
+        goto exit;
+    }
+    if (desc->file_offset % VHDX_LOG_SECTOR_SIZE) {
+        goto exit;
+    }
+
+    if (!memcmp(&desc->signature, "zero", 4)) {
+        if (desc->zero_length % VHDX_LOG_SECTOR_SIZE == 0) {
+            /* valid */
+            ret = true;
+        }
+    } else if (!memcmp(&desc->signature, "desc", 4)) {
+            /* valid */
+            ret = true;
+    }
+
+exit:
+    return ret;
+}
+
+
+/* Prior to sector data for a log entry, there is the header
+ * and the descriptors referenced in the header:
+ *
+ * [] = 4KB sector
+ *
+ * [ hdr, desc ][   desc   ][ ... ][ data ][ ... ]
+ *
+ * The first sector in a log entry has a 64 byte header, and
+ * up to 126 32-byte descriptors.  If more descriptors than
+ * 126 are required, then subsequent sectors can have up to 128
+ * descriptors.  Each sector is 4KB.  Data follows the descriptor
+ * sectors.
+ *
+ * This will return the number of sectors needed to encompass
+ * the passed number of descriptors in desc_cnt.
+ *
+ * This will never return 0, even if desc_cnt is 0.
+ */
+static int vhdx_compute_desc_sectors(uint32_t desc_cnt)
+{
+    uint32_t desc_sectors;
+
+    desc_cnt += 2; /* account for header in first sector */
+    desc_sectors = desc_cnt / 128;
+    if (desc_cnt % 128) {
+        desc_sectors++;
+    }
+
+    return desc_sectors;
+}
+
+
+/* Reads the log header, and subsequent descriptors (if any).  This
+ * will allocate all the space for buffer, which must be NULL when
+ * passed into this function. Each descriptor will also be validated,
+ * and error returned if any are invalid. */
+static int vhdx_log_read_desc(BlockDriverState *bs, BDRVVHDXState *s,
+                              VHDXLogEntries *log, VHDXLogDescEntries **buffer)
+{
+    int ret = 0;
+    uint32_t desc_sectors;
+    uint32_t sectors_read;
+    VHDXLogEntryHeader hdr;
+    VHDXLogDescEntries *desc_entries = NULL;
+    int i;
+
+    assert(*buffer == NULL);
+
+    ret = vhdx_log_peek_hdr(bs, log, &hdr);
+    if (ret < 0) {
+        goto exit;
+    }
+    vhdx_log_entry_hdr_le_import(&hdr);
+    if (vhdx_log_hdr_is_valid(log, &hdr, s) == false) {
+        ret = -EINVAL;
+        goto exit;
+    }
+
+    desc_sectors = vhdx_compute_desc_sectors(hdr.descriptor_count);
+    desc_entries = qemu_blockalign(bs, desc_sectors * VHDX_LOG_SECTOR_SIZE);
+
+    ret = vhdx_log_read_sectors(bs, log, &sectors_read, desc_entries,
+                                desc_sectors, false);
+    if (ret < 0) {
+        goto free_and_exit;
+    }
+    if (sectors_read != desc_sectors) {
+        ret = -EINVAL;
+        goto free_and_exit;
+    }
+
+    /* put in proper endianness, and validate each desc */
+    for (i = 0; i < hdr.descriptor_count; i++) {
+        vhdx_log_desc_le_import(&desc_entries->desc[i]);
+        if (vhdx_log_desc_is_valid(&desc_entries->desc[i], &hdr) == false) {
+            ret = -EINVAL;
+            goto free_and_exit;
+        }
+    }
+
+    *buffer = desc_entries;
+    goto exit;
+
+free_and_exit:
+    qemu_vfree(desc_entries);
+exit:
+    return ret;
+}
+
+
+/* Flushes the descriptor described by desc to the VHDX image file.
+ * If the descriptor is a data descriptor, than 'data' must be non-NULL,
+ * and >= 4096 bytes (VHDX_LOG_SECTOR_SIZE), containing the data to be
+ * written.
+ *
+ * Verification is performed to make sure the sequence numbers of a data
+ * descriptor match the sequence number in the desc.
+ *
+ * For a zero descriptor, it may describe multiple sectors to fill with zeroes.
+ * In this case, it should be noted that zeroes are written to disk, and the
+ * image file is not extended as a sparse file.  */
+static int vhdx_log_flush_desc(BlockDriverState *bs, VHDXLogDescriptor *desc,
+                               VHDXLogDataSector *data)
+{
+    int ret = 0;
+    uint64_t seq, file_offset;
+    uint32_t offset = 0;
+    void *buffer = NULL;
+    uint64_t count = 1;
+    int i;
+
+    buffer = qemu_blockalign(bs, VHDX_LOG_SECTOR_SIZE);
+
+    if (!memcmp(&desc->signature, "desc", 4)) {
+        /* data sector */
+        if (data == NULL) {
+            ret = -EFAULT;
+            goto exit;
+        }
+
+        /* The sequence number of the data sector must match that
+         * in the descriptor */
+        seq = data->sequence_high;
+        seq <<= 32;
+        seq |= data->sequence_low & 0xffffffff;
+
+        if (seq != desc->sequence_number) {
+            ret = -EINVAL;
+            goto exit;
+        }
+
+        /* Each data sector is in total 4096 bytes, however the first
+         * 8 bytes, and last 4 bytes, are located in the descriptor */
+        memcpy(buffer, &desc->leading_bytes, 8);
+        offset += 8;
+
+        memcpy(buffer+offset, data->data, 4084);
+        offset += 4084;
+
+        memcpy(buffer+offset, &desc->trailing_bytes, 4);
+
+    } else if (!memcmp(&desc->signature, "zero", 4)) {
+        /* write 'count' sectors of sector */
+        memset(buffer, 0, VHDX_LOG_SECTOR_SIZE);
+        count = desc->zero_length / VHDX_LOG_SECTOR_SIZE;
+    }
+
+    file_offset = desc->file_offset;
+
+    /* count is only > 1 if we are writing zeroes */
+    for (i = 0; i < count; i++) {
+        ret = bdrv_pwrite_sync(bs->file, file_offset, buffer,
+                               VHDX_LOG_SECTOR_SIZE);
+        if (ret < 0) {
+            goto exit;
+        }
+        file_offset += VHDX_LOG_SECTOR_SIZE;
+    }
+
+exit:
+    qemu_vfree(buffer);
+    return ret;
+}
+
+/* Flush the entire log (as described by 'logs') to the VHDX image
+ * file, and then set the log to 'empty' status once complete.
+ *
+ * The log entries should be validate prior to flushing */
+static int vhdx_log_flush(BlockDriverState *bs, BDRVVHDXState *s,
+                          VHDXLogSequence *logs)
+{
+    int ret = 0;
+    int i;
+    uint32_t cnt, sectors_read;
+    uint64_t new_file_size;
+    void *data = NULL;
+    VHDXLogDescEntries *desc_entries = NULL;
+    VHDXLogEntryHeader hdr_tmp = { 0 };
+
+    cnt = logs->count;
+
+    data = qemu_blockalign(bs, VHDX_LOG_SECTOR_SIZE);
+
+    ret = vhdx_user_visible_write(bs, s);
+    if (ret < 0) {
+        goto exit;
+    }
+
+    /* each iteration represents one log sequence, which may span multiple
+     * sectors */
+    while (cnt--) {
+        ret = vhdx_log_peek_hdr(bs, &logs->log, &hdr_tmp);
+        if (ret < 0) {
+            goto exit;
+        }
+        /* if the log shows a FlushedFileOffset larger than our current file
+         * size, then that means the file has been truncated / corrupted, and
+         * we must refused to open it / use it */
+        if (hdr_tmp.flushed_file_offset > bdrv_getlength(bs->file)) {
+            ret = -EINVAL;
+            goto exit;
+        }
+
+        ret = vhdx_log_read_desc(bs, s, &logs->log, &desc_entries);
+        if (ret < 0) {
+            goto exit;
+        }
+
+        for (i = 0; i < desc_entries->hdr.descriptor_count; i++) {
+            if (!memcmp(&desc_entries->desc[i].signature, "desc", 4)) {
+                /* data sector, so read a sector to flush */
+                ret = vhdx_log_read_sectors(bs, &logs->log, &sectors_read,
+                                            data, 1, false);
+                if (ret < 0) {
+                    goto exit;
+                }
+                if (sectors_read != 1) {
+                    ret = -EINVAL;
+                    goto exit;
+                }
+            }
+
+            ret = vhdx_log_flush_desc(bs, &desc_entries->desc[i], data);
+            if (ret < 0) {
+                goto exit;
+            }
+        }
+        if (bdrv_getlength(bs->file) < desc_entries->hdr.last_file_offset) {
+            new_file_size = desc_entries->hdr.last_file_offset;
+            if (new_file_size % (1024*1024)) {
+                /* round up to nearest 1MB boundary */
+                new_file_size = ((new_file_size >> 20) + 1) << 20;
+                bdrv_truncate(bs->file, new_file_size);
+            }
+        }
+        qemu_vfree(desc_entries);
+        desc_entries = NULL;
+    }
+
+    bdrv_flush(bs);
+    /* once the log is fully flushed, indicate that we have an empty log
+     * now.  This also sets the log guid to 0, to indicate an empty log */
+    vhdx_log_reset(bs, s);
+
+exit:
+    qemu_vfree(data);
+    qemu_vfree(desc_entries);
+    return ret;
+}
+
+static int vhdx_validate_log_entry(BlockDriverState *bs, BDRVVHDXState *s,
+                                   VHDXLogEntries *log, uint64_t seq,
+                                   bool *valid, VHDXLogEntryHeader *entry)
+{
+    int ret = 0;
+    VHDXLogEntryHeader hdr;
+    void *buffer = NULL;
+    uint32_t i, desc_sectors, total_sectors, crc;
+    uint32_t sectors_read = 0;
+    VHDXLogDescEntries *desc_buffer = NULL;
+
+    *valid = false;
+
+    ret = vhdx_log_peek_hdr(bs, log, &hdr);
+    if (ret < 0) {
+        goto inc_and_exit;
+    }
+
+    vhdx_log_entry_hdr_le_import(&hdr);
+
+
+    if (vhdx_log_hdr_is_valid(log, &hdr, s) == false) {
+        goto inc_and_exit;
+    }
+
+    if (seq > 0) {
+        if (hdr.sequence_number != seq + 1) {
+            goto inc_and_exit;
+        }
+    }
+
+    desc_sectors = vhdx_compute_desc_sectors(hdr.descriptor_count);
+
+    /* Read desc sectors, and calculate log checksum */
+
+    total_sectors = hdr.entry_length / VHDX_LOG_SECTOR_SIZE;
+
+
+    /* read_desc() will incrememnt the read idx */
+    ret = vhdx_log_read_desc(bs, s, log, &desc_buffer);
+    if (ret < 0) {
+        goto free_and_exit;
+    }
+
+    crc = vhdx_checksum_calc(0xffffffff, (void *)desc_buffer,
+                            desc_sectors * VHDX_LOG_SECTOR_SIZE, 4);
+    crc ^= 0xffffffff;
+
+    buffer = qemu_blockalign(bs, VHDX_LOG_SECTOR_SIZE);
+    if (total_sectors > desc_sectors) {
+        for (i = 0; i < total_sectors - desc_sectors; i++) {
+            sectors_read = 0;
+            ret = vhdx_log_read_sectors(bs, log, &sectors_read, buffer,
+                                        1, false);
+            if (ret < 0 || sectors_read != 1) {
+                goto free_and_exit;
+            }
+            crc = vhdx_checksum_calc(crc, buffer, VHDX_LOG_SECTOR_SIZE, -1);
+            crc ^= 0xffffffff;
+        }
+    }
+    crc ^= 0xffffffff;
+    if (crc != desc_buffer->hdr.checksum) {
+        goto free_and_exit;
+    }
+
+    *valid = true;
+    *entry = hdr;
+    goto free_and_exit;
+
+inc_and_exit:
+    log->read = vhdx_log_inc_idx(log->read, log->length);
+
+free_and_exit:
+    qemu_vfree(buffer);
+    qemu_vfree(desc_buffer);
+    return ret;
+}
+
+/* Search through the log circular buffer, and find the valid, active
+ * log sequence, if any exists
+ * */
+static int vhdx_log_search(BlockDriverState *bs, BDRVVHDXState *s,
+                           VHDXLogSequence *logs)
+{
+    int ret = 0;
+    uint32_t tail;
+    bool seq_valid = false;
+    VHDXLogSequence candidate = { 0 };
+    VHDXLogEntryHeader hdr = { 0 };
+    VHDXLogEntries curr_log;
+
+    memcpy(&curr_log, &s->log, sizeof(VHDXLogEntries));
+    curr_log.write = curr_log.length;   /* assume log is full */
+    curr_log.read = 0;
+
+
+    /* now we will go through the whole log sector by sector, until
+     * we find a valid, active log sequence, or reach the end of the
+     * log buffer */
+    for (;;) {
+        uint64_t curr_seq = 0;
+        VHDXLogSequence current = { 0 };
+
+        tail = curr_log.read;
+
+        ret = vhdx_validate_log_entry(bs, s, &curr_log, curr_seq,
+                                      &seq_valid, &hdr);
+        if (ret < 0) {
+            goto exit;
+        }
+
+        if (seq_valid) {
+            current.valid     = true;
+            current.log       = curr_log;
+            current.log.read  = tail;
+            current.log.write = curr_log.read;
+            current.count     = 1;
+            current.hdr       = hdr;
+
+
+            for (;;) {
+                ret = vhdx_validate_log_entry(bs, s, &curr_log, curr_seq,
+                                              &seq_valid, &hdr);
+                if (ret < 0) {
+                    goto exit;
+                }
+                if (seq_valid == false) {
+                    break;
+                }
+                current.log.write = curr_log.read;
+                current.count++;
+
+                curr_seq = hdr.sequence_number;
+            }
+        }
+
+        if (current.valid) {
+            if (candidate.valid == false ||
+                current.hdr.sequence_number > candidate.hdr.sequence_number) {
+                candidate = current;
+            }
+        }
+
+        if (curr_log.read < tail) {
+            break;
+        }
+    }
+
+    *logs = candidate;
+
+    if (candidate.valid) {
+        /* this is the next sequence number, for writes */
+        s->log.sequence = candidate.hdr.sequence_number + 1;
+    }
+
+
+exit:
+    return ret;
+}
+
+/* Parse the replay log.  Per the VHDX spec, if the log is present
+ * it must be replayed prior to opening the file, even read-only.
+ *
+ * If read-only, we must replay the log in RAM (or refuse to open
+ * a dirty VHDX file read-only) */
+int vhdx_parse_log(BlockDriverState *bs, BDRVVHDXState *s, bool *flushed)
+{
+    int ret = 0;
+    VHDXHeader *hdr;
+    VHDXLogSequence logs = { 0 };
+
+    hdr = s->headers[s->curr_header];
+
+    *flushed = false;
+
+    /* s->log.hdr is freed in vhdx_close() */
+    if (s->log.hdr == NULL) {
+        s->log.hdr = qemu_blockalign(bs, sizeof(VHDXLogEntryHeader));
+    }
+
+    s->log.offset = hdr->log_offset;
+    s->log.length = hdr->log_length;
+
+    if (s->log.offset < VHDX_LOG_MIN_SIZE ||
+        s->log.offset % VHDX_LOG_MIN_SIZE) {
+        ret = -EINVAL;
+        goto exit;
+    }
+
+    /* per spec, only log version of 0 is supported */
+    if (hdr->log_version != 0) {
+        ret = -EINVAL;
+        goto exit;
+    }
+
+    /* If either the log guid, or log length is zero,
+     * then a replay log is not present */
+    if (guid_eq(hdr->log_guid, zero_guid)) {
+        goto exit;
+    }
+
+    if (hdr->log_length == 0) {
+        goto exit;
+    }
+
+    if (hdr->log_length % VHDX_LOG_MIN_SIZE) {
+        ret = -EINVAL;
+        goto exit;
+    }
+
+
+    /* The log is present, we need to find if and where there is an active
+     * sequence of valid entries present in the log.  */
+
+    ret = vhdx_log_search(bs, s, &logs);
+    if (ret < 0) {
+        goto exit;
+    }
+
+    if (logs.valid) {
+        /* now flush the log */
+        ret = vhdx_log_flush(bs, s, &logs);
+        if (ret < 0) {
+            goto exit;
+        }
+        *flushed = true;
+    }
+
+
+exit:
+    return ret;
+}
+
+
diff --git a/block/vhdx.c b/block/vhdx.c
index 3f06ce3..8fbfbd6 100644
--- a/block/vhdx.c
+++ b/block/vhdx.c
@@ -735,58 +735,22 @@  exit:
     return ret;
 }
 
-/* Parse the replay log.  Per the VHDX spec, if the log is present
- * it must be replayed prior to opening the file, even read-only.
- *
- * If read-only, we must replay the log in RAM (or refuse to open
- * a dirty VHDX file read-only */
-static int vhdx_parse_log(BlockDriverState *bs, BDRVVHDXState *s)
-{
-    int ret = 0;
-    int i;
-    VHDXHeader *hdr;
-
-    hdr = s->headers[s->curr_header];
-
-    /* either the log guid, or log length is zero,
-     * then a replay log is present */
-    for (i = 0; i < sizeof(hdr->log_guid.data4); i++) {
-        ret |= hdr->log_guid.data4[i];
-    }
-    if (hdr->log_guid.data1 == 0 &&
-        hdr->log_guid.data2 == 0 &&
-        hdr->log_guid.data3 == 0 &&
-        ret == 0) {
-        goto exit;
-    }
-
-    /* per spec, only log version of 0 is supported */
-    if (hdr->log_version != 0) {
-        ret = -EINVAL;
-        goto exit;
-    }
-
-    if (hdr->log_length == 0) {
-        goto exit;
-    }
-
-    /* We currently do not support images with logs to replay */
-    ret = -ENOTSUP;
-
-exit:
-    return ret;
-}
-
 
 static void vhdx_close(BlockDriverState *bs)
 {
     BDRVVHDXState *s = bs->opaque;
     qemu_vfree(s->headers[0]);
+    s->headers[0] = NULL;
     qemu_vfree(s->headers[1]);
+    s->headers[1] = NULL;
     qemu_vfree(s->bat);
+    s->bat = NULL;
     qemu_vfree(s->parent_entries);
+    s->parent_entries = NULL;
     migrate_del_blocker(s->migration_blocker);
     error_free(s->migration_blocker);
+    qemu_vfree(s->log.hdr);
+    s->log.hdr = NULL;
 }
 
 static int vhdx_open(BlockDriverState *bs, QDict *options, int flags,
@@ -797,6 +761,7 @@  static int vhdx_open(BlockDriverState *bs, QDict *options, int flags,
     uint32_t i;
     uint64_t signature;
     uint32_t data_blocks_cnt, bitmap_blocks_cnt;
+    bool log_flushed = false;
 
 
     s->bat = NULL;
@@ -820,24 +785,25 @@  static int vhdx_open(BlockDriverState *bs, QDict *options, int flags,
     vhdx_guid_generate(&s->session_guid);
 
     ret = vhdx_parse_header(bs, s);
-    if (ret) {
+    if (ret < 0) {
         goto fail;
     }
 
-    ret = vhdx_parse_log(bs, s);
-    if (ret) {
+    ret = vhdx_parse_log(bs, s, &log_flushed);
+    if (ret < 0) {
         goto fail;
     }
 
     ret = vhdx_open_region_tables(bs, s);
-    if (ret) {
+    if (ret < 0) {
         goto fail;
     }
 
     ret = vhdx_parse_metadata(bs, s);
-    if (ret) {
+    if (ret < 0) {
         goto fail;
     }
+
     s->block_size = s->params.block_size;
 
     /* the VHDX spec dictates that virtual_disk_size is always a multiple of
@@ -897,10 +863,7 @@  static int vhdx_open(BlockDriverState *bs, QDict *options, int flags,
 
     return 0;
 fail:
-    qemu_vfree(s->headers[0]);
-    qemu_vfree(s->headers[1]);
-    qemu_vfree(s->bat);
-    qemu_vfree(s->parent_entries);
+    vhdx_close(bs);
     return ret;
 }
 
diff --git a/block/vhdx.h b/block/vhdx.h
index 6c35737..584ebec 100644
--- a/block/vhdx.h
+++ b/block/vhdx.h
@@ -326,7 +326,11 @@  typedef struct VHDXMetadataEntries {
 typedef struct VHDXLogEntries {
     uint64_t offset;
     uint64_t length;
-    uint32_t head;
+    uint32_t write;
+    uint32_t read;
+    VHDXLogEntryHeader *hdr;
+    void *desc_buffer;
+    uint64_t sequence;
     uint32_t tail;
 } VHDXLogEntries;
 
@@ -383,6 +387,7 @@  uint32_t vhdx_checksum_calc(uint32_t crc, uint8_t *buf, size_t size,
 
 bool vhdx_checksum_is_valid(uint8_t *buf, size_t size, int crc_offset);
 
+int vhdx_parse_log(BlockDriverState *bs, BDRVVHDXState *s, bool *flushed);
 
 static inline void leguid_to_cpus(MSGUID *guid)
 {