Add qemu 2.4.0

Change-Id: Ic99cbad4b61f8b127b7dc74d04576c0bcbaaf4f5
Signed-off-by: Yang Zhang <yang.z.zhang@intel.com>

1 files changed, 1039 insertions, 0 deletions

diff --git a/qemu/block/vhdx-log.c b/qemu/block/vhdx-log.c
new file mode 100644
index 000000000..47fec63c6
--- /dev/null
+++ b/qemu/block/vhdx-log.c
@@ -0,0 +1,1039 @@
+/*
+ * 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/error-report.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;
+    }
+    vhdx_log_entry_hdr_le_import(hdr);
+
+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;
+}
+
+/* Writes num_sectors to the log (all log sectors are 4096 bytes),
+ * from buffer 'buffer'.  Upon return, *sectors_written will contain
+ * the number of sectors successfully written.
+ *
+ * It is assumed that 'buffer' is at least 4096*num_sectors large.
+ *
+ * 0 is returned on success, -errno otherwise */
+static int vhdx_log_write_sectors(BlockDriverState *bs, VHDXLogEntries *log,
+                                  uint32_t *sectors_written, void *buffer,
+                                  uint32_t num_sectors)
+{
+    int ret = 0;
+    uint64_t offset;
+    uint32_t write;
+    void *buffer_tmp;
+    BDRVVHDXState *s = bs->opaque;
+
+    ret = vhdx_user_visible_write(bs, s);
+    if (ret < 0) {
+        goto exit;
+    }
+
+    write = log->write;
+
+    buffer_tmp = buffer;
+    while (num_sectors) {
+
+        offset = log->offset + write;
+        write = vhdx_log_inc_idx(write, log->length);
+        if (write == log->read) {
+            /* full */
+            break;
+        }
+        ret = bdrv_pwrite(bs->file, offset, buffer_tmp, VHDX_LOG_SECTOR_SIZE);
+        if (ret < 0) {
+            goto exit;
+        }
+        buffer_tmp += VHDX_LOG_SECTOR_SIZE;
+
+        log->write = write;
+        *sectors_written = *sectors_written + 1;
+        num_sectors--;
+    }
+
+exit:
+    return ret;
+}
+
+
+/* Validates a log entry header */
+static bool vhdx_log_hdr_is_valid(VHDXLogEntries *log, VHDXLogEntryHeader *hdr,
+                                  BDRVVHDXState *s)
+{
+    int valid = false;
+
+    if (hdr->signature != VHDX_LOG_SIGNATURE) {
+        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 (desc->signature == VHDX_LOG_ZERO_SIGNATURE) {
+        if (desc->zero_length % VHDX_LOG_SECTOR_SIZE == 0) {
+            /* valid */
+            ret = true;
+        }
+    } else if (desc->signature == VHDX_LOG_DESC_SIGNATURE) {
+            /* 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,
+                              bool convert_endian)
+{
+    int ret = 0;
+    uint32_t desc_sectors;
+    uint32_t sectors_read;
+    VHDXLogEntryHeader hdr;
+    VHDXLogDescEntries *desc_entries = NULL;
+    VHDXLogDescriptor desc;
+    int i;
+
+    assert(*buffer == NULL);
+
+    ret = vhdx_log_peek_hdr(bs, log, &hdr);
+    if (ret < 0) {
+        goto exit;
+    }
+
+    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_try_blockalign(bs->file,
+                                       desc_sectors * VHDX_LOG_SECTOR_SIZE);
+    if (desc_entries == NULL) {
+        ret = -ENOMEM;
+        goto exit;
+    }
+
+    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++) {
+        desc = desc_entries->desc[i];
+        vhdx_log_desc_le_import(&desc);
+        if (convert_endian) {
+            desc_entries->desc[i] = desc;
+        }
+        if (vhdx_log_desc_is_valid(&desc, &hdr) == false) {
+            ret = -EINVAL;
+            goto free_and_exit;
+        }
+    }
+    if (convert_endian) {
+        desc_entries->hdr = hdr;
+    }
+
+    *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 (desc->signature == VHDX_LOG_DESC_SIGNATURE) {
+        /* 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 (desc->signature == VHDX_LOG_ZERO_SIGNATURE) {
+        /* write 'count' sectors of sector */
+        memset(buffer, 0, VHDX_LOG_SECTOR_SIZE);
+        count = desc->zero_length / VHDX_LOG_SECTOR_SIZE;
+    } else {
+        error_report("Invalid VHDX log descriptor entry signature 0x%" PRIx32,
+                      desc->signature);
+        ret = -EINVAL;
+        goto exit;
+    }
+
+    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, true);
+        if (ret < 0) {
+            goto exit;
+        }
+
+        for (i = 0; i < desc_entries->hdr.descriptor_count; i++) {
+            if (desc_entries->desc[i].signature == VHDX_LOG_DESC_SIGNATURE) {
+                /* 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;
+                }
+                vhdx_log_data_le_import(data);
+            }
+
+            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;
+    }
+
+    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 all log sectors, and calculate log checksum */
+
+    total_sectors = hdr.entry_length / VHDX_LOG_SECTOR_SIZE;
+
+
+    /* read_desc() will increment the read idx */
+    ret = vhdx_log_read_desc(bs, s, log, &desc_buffer, false);
+    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 != 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,
+                   Error **errp)
+{
+    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) {
+        if (bs->read_only) {
+            ret = -EPERM;
+            error_setg_errno(errp, EPERM,
+                             "VHDX image file '%s' opened read-only, but "
+                             "contains a log that needs to be replayed.  To "
+                             "replay the log, execute:\n qemu-img check -r "
+                             "all '%s'",
+                             bs->filename, bs->filename);
+            goto exit;
+        }
+        /* now flush the log */
+        ret = vhdx_log_flush(bs, s, &logs);
+        if (ret < 0) {
+            goto exit;
+        }
+        *flushed = true;
+    }
+
+
+exit:
+    return ret;
+}
+
+
+
+static void vhdx_log_raw_to_le_sector(VHDXLogDescriptor *desc,
+                                      VHDXLogDataSector *sector, void *data,
+                                      uint64_t seq)
+{
+    /* 8 + 4084 + 4 = 4096, 1 log sector */
+    memcpy(&desc->leading_bytes, data, 8);
+    data += 8;
+    cpu_to_le64s(&desc->leading_bytes);
+    memcpy(sector->data, data, 4084);
+    data += 4084;
+    memcpy(&desc->trailing_bytes, data, 4);
+    cpu_to_le32s(&desc->trailing_bytes);
+    data += 4;
+
+    sector->sequence_high  = (uint32_t) (seq >> 32);
+    sector->sequence_low   = (uint32_t) (seq & 0xffffffff);
+    sector->data_signature = VHDX_LOG_DATA_SIGNATURE;
+
+    vhdx_log_desc_le_export(desc);
+    vhdx_log_data_le_export(sector);
+}
+
+
+static int vhdx_log_write(BlockDriverState *bs, BDRVVHDXState *s,
+                          void *data, uint32_t length, uint64_t offset)
+{
+    int ret = 0;
+    void *buffer = NULL;
+    void *merged_sector = NULL;
+    void *data_tmp, *sector_write;
+    unsigned int i;
+    int sector_offset;
+    uint32_t desc_sectors, sectors, total_length;
+    uint32_t sectors_written = 0;
+    uint32_t aligned_length;
+    uint32_t leading_length = 0;
+    uint32_t trailing_length = 0;
+    uint32_t partial_sectors = 0;
+    uint32_t bytes_written = 0;
+    uint64_t file_offset;
+    VHDXHeader *header;
+    VHDXLogEntryHeader new_hdr;
+    VHDXLogDescriptor *new_desc = NULL;
+    VHDXLogDataSector *data_sector = NULL;
+    MSGUID new_guid = { 0 };
+
+    header = s->headers[s->curr_header];
+
+    /* need to have offset read data, and be on 4096 byte boundary */
+
+    if (length > header->log_length) {
+        /* no log present.  we could create a log here instead of failing */
+        ret = -EINVAL;
+        goto exit;
+    }
+
+    if (guid_eq(header->log_guid, zero_guid)) {
+        vhdx_guid_generate(&new_guid);
+        vhdx_update_headers(bs, s, false, &new_guid);
+    } else {
+        /* currently, we require that the log be flushed after
+         * every write. */
+        ret = -ENOTSUP;
+        goto exit;
+    }
+
+    /* 0 is an invalid sequence number, but may also represent the first
+     * log write (or a wrapped seq) */
+    if (s->log.sequence == 0) {
+        s->log.sequence = 1;
+    }
+
+    sector_offset = offset % VHDX_LOG_SECTOR_SIZE;
+    file_offset = (offset / VHDX_LOG_SECTOR_SIZE) * VHDX_LOG_SECTOR_SIZE;
+
+    aligned_length = length;
+
+    /* add in the unaligned head and tail bytes */
+    if (sector_offset) {
+        leading_length = (VHDX_LOG_SECTOR_SIZE - sector_offset);
+        leading_length = leading_length > length ? length : leading_length;
+        aligned_length -= leading_length;
+        partial_sectors++;
+    }
+
+    sectors = aligned_length / VHDX_LOG_SECTOR_SIZE;
+    trailing_length = aligned_length - (sectors * VHDX_LOG_SECTOR_SIZE);
+    if (trailing_length) {
+        partial_sectors++;
+    }
+
+    sectors += partial_sectors;
+
+    /* sectors is now how many sectors the data itself takes, not
+     * including the header and descriptor metadata */
+
+    new_hdr = (VHDXLogEntryHeader) {
+                .signature           = VHDX_LOG_SIGNATURE,
+                .tail                = s->log.tail,
+                .sequence_number     = s->log.sequence,
+                .descriptor_count    = sectors,
+                .reserved            = 0,
+                .flushed_file_offset = bdrv_getlength(bs->file),
+                .last_file_offset    = bdrv_getlength(bs->file),
+              };
+
+    new_hdr.log_guid = header->log_guid;
+
+    desc_sectors = vhdx_compute_desc_sectors(new_hdr.descriptor_count);
+
+    total_length = (desc_sectors + sectors) * VHDX_LOG_SECTOR_SIZE;
+    new_hdr.entry_length = total_length;
+
+    vhdx_log_entry_hdr_le_export(&new_hdr);
+
+    buffer = qemu_blockalign(bs, total_length);
+    memcpy(buffer, &new_hdr, sizeof(new_hdr));
+
+    new_desc = buffer + sizeof(new_hdr);
+    data_sector = buffer + (desc_sectors * VHDX_LOG_SECTOR_SIZE);
+    data_tmp = data;
+
+    /* All log sectors are 4KB, so for any partial sectors we must
+     * merge the data with preexisting data from the final file
+     * destination */
+    merged_sector = qemu_blockalign(bs, VHDX_LOG_SECTOR_SIZE);
+
+    for (i = 0; i < sectors; i++) {
+        new_desc->signature       = VHDX_LOG_DESC_SIGNATURE;
+        new_desc->sequence_number = s->log.sequence;
+        new_desc->file_offset     = file_offset;
+
+        if (i == 0 && leading_length) {
+            /* partial sector at the front of the buffer */
+            ret = bdrv_pread(bs->file, file_offset, merged_sector,
+                             VHDX_LOG_SECTOR_SIZE);
+            if (ret < 0) {
+                goto exit;
+            }
+            memcpy(merged_sector + sector_offset, data_tmp, leading_length);
+            bytes_written = leading_length;
+            sector_write = merged_sector;
+        } else if (i == sectors - 1 && trailing_length) {
+            /* partial sector at the end of the buffer */
+            ret = bdrv_pread(bs->file,
+                            file_offset,
+                            merged_sector + trailing_length,
+                            VHDX_LOG_SECTOR_SIZE - trailing_length);
+            if (ret < 0) {
+                goto exit;
+            }
+            memcpy(merged_sector, data_tmp, trailing_length);
+            bytes_written = trailing_length;
+            sector_write = merged_sector;
+        } else {
+            bytes_written = VHDX_LOG_SECTOR_SIZE;
+            sector_write = data_tmp;
+        }
+
+        /* populate the raw sector data into the proper structures,
+         * as well as update the descriptor, and convert to proper
+         * endianness */
+        vhdx_log_raw_to_le_sector(new_desc, data_sector, sector_write,
+                                  s->log.sequence);
+
+        data_tmp += bytes_written;
+        data_sector++;
+        new_desc++;
+        file_offset += VHDX_LOG_SECTOR_SIZE;
+    }
+
+    /* checksum covers entire entry, from the log header through the
+     * last data sector */
+    vhdx_update_checksum(buffer, total_length,
+                         offsetof(VHDXLogEntryHeader, checksum));
+
+    /* now write to the log */
+    ret = vhdx_log_write_sectors(bs, &s->log, &sectors_written, buffer,
+                                 desc_sectors + sectors);
+    if (ret < 0) {
+        goto exit;
+    }
+
+    if (sectors_written != desc_sectors + sectors) {
+        /* instead of failing, we could flush the log here */
+        ret = -EINVAL;
+        goto exit;
+    }
+
+    s->log.sequence++;
+    /* write new tail */
+    s->log.tail = s->log.write;
+
+exit:
+    qemu_vfree(buffer);
+    qemu_vfree(merged_sector);
+    return ret;
+}
+
+/* Perform a log write, and then immediately flush the entire log */
+int vhdx_log_write_and_flush(BlockDriverState *bs, BDRVVHDXState *s,
+                             void *data, uint32_t length, uint64_t offset)
+{
+    int ret = 0;
+    VHDXLogSequence logs = { .valid = true,
+                             .count = 1,
+                             .hdr = { 0 } };
+
+
+    /* Make sure data written (new and/or changed blocks) is stable
+     * on disk, before creating log entry */
+    bdrv_flush(bs);
+    ret = vhdx_log_write(bs, s, data, length, offset);
+    if (ret < 0) {
+        goto exit;
+    }
+    logs.log = s->log;
+
+    /* Make sure log is stable on disk */
+    bdrv_flush(bs);
+    ret = vhdx_log_flush(bs, s, &logs);
+    if (ret < 0) {
+        goto exit;
+    }
+
+    s->log = logs.log;
+
+exit:
+    return ret;
+}
+