[PATCH 3/4] block, migration: add bdrv_flush_vmstate helper

[Denis V. Lunev]

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

The problem is that migration code is working inefficently from black
layer terms and are frequently called for very small pieces of not
properly aligned 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_flush_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>
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: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
CC: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 block/block-backend.c |  6 +++++-
 block/io.c            | 39 +++++++++++++++++++++++++++++++++++++++
 include/block/block.h |  1 +
 migration/savevm.c    |  3 +++
 4 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/block/block-backend.c b/block/block-backend.c
index 9342a475cb..2107ace699 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -2177,7 +2177,7 @@ 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;
@@ -2187,6 +2187,10 @@ int blk_save_vmstate(BlockBackend *blk, const uint8_t 
*buf,
     if (ret < 0) {
         return ret;
     }
+    ret2 = bdrv_flush_vmstate(blk_bs(blk));
+    if (ret2 < 0) {
+        return ret;
+    }
 
     if (ret == size && !blk->enable_write_cache) {
         ret = bdrv_flush(blk_bs(blk));
diff --git a/block/io.c b/block/io.c
index 121ce17a49..fbf352f39d 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2725,6 +2725,45 @@ int bdrv_readv_vmstate(BlockDriverState *bs, 
QEMUIOVector *qiov, int64_t pos)
     return bdrv_rw_vmstate(bs, qiov, pos, true);
 }
 
+
+typedef struct FlushVMStateCo {
+    BlockDriverState *bs;
+    int ret;
+} FlushVMStateCo;
+
+static int coroutine_fn bdrv_co_flush_vmstate(BlockDriverState *bs)
+{
+    return 0;
+}
+
+static void coroutine_fn bdrv_flush_vmstate_co_entry(void *opaque)
+{
+    FlushVMStateCo *rwco = opaque;
+
+    rwco->ret = bdrv_co_flush_vmstate(rwco->bs);
+    aio_wait_kick();
+}
+
+int bdrv_flush_vmstate(BlockDriverState *bs)
+{
+    Coroutine *co;
+    FlushVMStateCo flush_co = {
+        .bs = bs,
+        .ret = NOT_DONE,
+    };
+
+    if (qemu_in_coroutine()) {
+        /* Fast-path if already in coroutine context */
+        bdrv_flush_vmstate_co_entry(&flush_co);
+    } else {
+        co = qemu_coroutine_create(bdrv_flush_vmstate_co_entry, &flush_co);
+        bdrv_coroutine_enter(bs, co);
+        BDRV_POLL_WHILE(bs, flush_co.ret == NOT_DONE);
+    }
+
+    return flush_co.ret;
+}
+
 /**************************************************************/
 /* async I/Os */
 
diff --git a/include/block/block.h b/include/block/block.h
index 25e299605e..024525b87d 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -572,6 +572,7 @@ int bdrv_save_vmstate(BlockDriverState *bs, const uint8_t 
*buf,
 
 int bdrv_load_vmstate(BlockDriverState *bs, uint8_t *buf,
                       int64_t pos, int size);
+int bdrv_flush_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 0ff5bb40ed..9698c909d7 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -150,6 +150,9 @@ 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_flush_vmstate(opaque);
+    if (err < 0)
+        return err;
     return bdrv_flush(opaque);
 }
 
-- 
2.17.1