[PATCH 4/6] block, migration: add bdrv_finalize_vmstate helper

["Denis V. Lunev" <den@openvz.org>]

Right now bdrv_fclose() is just calling bdrv_flush().

The problem is that migration code is working inefficiently from block
layer terms and are frequently called for very small pieces of
unaligned data. Block layer is capable to work this way, but this is very
slow.

This patch is a preparation for the introduction of the intermediate
buffer at block driver state. It would be beneficial to separate
conventional bdrv_flush() from closing QEMU file from migration code.

The patch also forces bdrv_finalize_vmstate() operation inside
synchronous blk_save_vmstate() operation. This helper is used from
qemu-io only.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <fam@euphon.net>
CC: Juan Quintela <quintela@redhat.com>
CC: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
CC: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 block/block-backend.c |  6 +++++-
 block/io.c            | 15 +++++++++++++++
 include/block/block.h |  5 +++++
 migration/savevm.c    |  4 ++++
 4 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/block/block-backend.c b/block/block-backend.c
index 1c6e53bbde..5bb11c8e01 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -2177,16 +2177,20 @@ int blk_truncate(BlockBackend *blk, int64_t offset, bool exact,
 int blk_save_vmstate(BlockBackend *blk, const uint8_t *buf,
                      int64_t pos, int size)
 {
-    int ret;
+    int ret, ret2;
 
     if (!blk_is_available(blk)) {
         return -ENOMEDIUM;
     }
 
     ret = bdrv_save_vmstate(blk_bs(blk), buf, pos, size);
+    ret2 = bdrv_finalize_vmstate(blk_bs(blk));
     if (ret < 0) {
         return ret;
     }
+    if (ret2 < 0) {
+        return ret2;
+    }
 
     if (!blk->enable_write_cache) {
         ret = bdrv_flush(blk_bs(blk));
diff --git a/block/io.c b/block/io.c
index b6564e34c5..be1fac0be8 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2724,6 +2724,21 @@ int bdrv_readv_vmstate(BlockDriverState *bs, QEMUIOVector *qiov, int64_t pos)
     return bdrv_rw_vmstate(bs, qiov, pos, true);
 }
 
+static int coroutine_fn bdrv_co_finalize_vmstate(BlockDriverState *bs)
+{
+    return 0;
+}
+
+static int coroutine_fn bdrv_finalize_vmstate_co_entry(void *opaque)
+{
+    return bdrv_co_finalize_vmstate(opaque);
+}
+
+int bdrv_finalize_vmstate(BlockDriverState *bs)
+{
+    return bdrv_run_co(bs, bdrv_finalize_vmstate_co_entry, bs);
+}
+
 /**************************************************************/
 /* async I/Os */
 
diff --git a/include/block/block.h b/include/block/block.h
index bca3bb831c..124eb8d422 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -567,6 +567,11 @@ int bdrv_save_vmstate(BlockDriverState *bs, const uint8_t *buf,
 
 int bdrv_load_vmstate(BlockDriverState *bs, uint8_t *buf,
                       int64_t pos, int size);
+/*
+ * bdrv_finalize_vmstate() is mandatory to commit vmstate changes if
+ * bdrv_save_vmstate() was ever called.
+ */
+int bdrv_finalize_vmstate(BlockDriverState *bs);
 
 void bdrv_img_create(const char *filename, const char *fmt,
                      const char *base_filename, const char *base_fmt,
diff --git a/migration/savevm.c b/migration/savevm.c
index 45c9dd9d8a..d8a94e312c 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -150,6 +150,10 @@ static ssize_t block_get_buffer(void *opaque, uint8_t *buf, int64_t pos,
 
 static int bdrv_fclose(void *opaque, Error **errp)
 {
+    int err = bdrv_finalize_vmstate(opaque);
+    if (err < 0) {
+        return err;
+    }
     return bdrv_flush(opaque);
 }
 
-- 
2.17.1