block: Add bdrv_(p)write_sync

Add new functions that write and flush the written data to disk immediately.
This is what needs to be used for image format metadata to maintain integrity
for cache=... modes that don't use O_DSYNC. (Actually, we only need barriers,
and therefore the functions are defined as such, but flushes is what is
implemented in this patch - we can try to change that later)

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit f08145fe16470aca09304099888f68cfbc5d1de7)

1 files changed, 39 insertions, 0 deletions

diff --git a/block.c b/block.c
index 298414c92..e3b3a5504 100644
--- a/block.c
+++ b/block.c
@@ -452,6 +452,8 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
             (flags & (BDRV_O_CACHE_MASK|BDRV_O_NATIVE_AIO));
     else
         open_flags = flags & ~(BDRV_O_FILE | BDRV_O_SNAPSHOT);
+
+    bs->open_flags = open_flags;
     if (use_bdrv_whitelist && !bdrv_is_whitelisted(drv))
         ret = -ENOTSUP;
     else
@@ -779,6 +781,43 @@ int bdrv_pwrite(BlockDriverState *bs, int64_t offset,
     return count1;
 }
 
+/*
+ * Writes to the file and ensures that no writes are reordered across this
+ * request (acts as a barrier)
+ *
+ * Returns 0 on success, -errno in error cases.
+ */
+int bdrv_pwrite_sync(BlockDriverState *bs, int64_t offset,
+    const void *buf, int count)
+{
+    int ret;
+
+    ret = bdrv_pwrite(bs, offset, buf, count);
+    if (ret < 0) {
+        return ret;
+    }
+
+    /* No flush needed for cache=writethrough, it uses O_DSYNC */
+    if ((bs->open_flags & BDRV_O_CACHE_MASK) != 0) {
+        bdrv_flush(bs);
+    }
+
+    return 0;
+}
+
+/*
+ * Writes to the file and ensures that no writes are reordered across this
+ * request (acts as a barrier)
+ *
+ * Returns 0 on success, -errno in error cases.
+ */
+int bdrv_write_sync(BlockDriverState *bs, int64_t sector_num,
+    const uint8_t *buf, int nb_sectors)
+{
+    return bdrv_pwrite_sync(bs, BDRV_SECTOR_SIZE * sector_num,
+        buf, BDRV_SECTOR_SIZE * nb_sectors);
+}
+
 /**
  * Truncate file to 'offset' bytes (needed only for file protocols)
  */