block: drop bs_snapshots global variable

The bs_snapshots global variable points to the BlockDriverState which
will be used to save vmstate.  This is really a savevm.c concept but was
moved into block.c:bdrv_snapshots() when it became clear that hotplug
could result in a dangling pointer.

While auditing the block layer's global state I came upon bs_snapshots
and realized that a variable is not necessary here.  Simply find the
first BlockDriverState capable of internal snapshots each time this is
needed.

The behavior of bdrv_snapshots() is preserved across hotplug because new
drives are always appended to the bdrv_states list.  This means that
calling the new find_vmstate_bs() function is idempotent - it returns
the same BlockDriverState unless it was hot-unplugged.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NWenchao Xia <xiawenc@linux.vnet.ibm.com>
Signed-off-by: NWenchao Xia <xiawenc@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -99,9 +99,6 @@ static QTAILQ_HEAD(, BlockDriverState) bdrv_states =
 static QLIST_HEAD(, BlockDriver) bdrv_drivers =
     QLIST_HEAD_INITIALIZER(bdrv_drivers);
 
-/* The device to use for VM snapshots */
-static BlockDriverState *bs_snapshots;
-
 /* If non-zero, use only whitelisted block drivers */
 static int use_bdrv_whitelist;
 
@@ -1368,9 +1365,6 @@ void bdrv_close(BlockDriverState *bs)
     notifier_list_notify(&bs->close_notifiers, bs);
 
     if (bs->drv) {
-        if (bs == bs_snapshots) {
-            bs_snapshots = NULL;
-        }
         if (bs->backing_hd) {
             bdrv_delete(bs->backing_hd);
             bs->backing_hd = NULL;
@@ -1602,7 +1596,6 @@ void bdrv_delete(BlockDriverState *bs)
 
     bdrv_close(bs);
 
-    assert(bs != bs_snapshots);
     g_free(bs);
 }
 
@@ -1646,9 +1639,6 @@ void bdrv_set_dev_ops(BlockDriverState *bs, const BlockDevOps *ops,
 {
     bs->dev_ops = ops;
     bs->dev_opaque = opaque;
-    if (bdrv_dev_has_removable_media(bs) && bs == bs_snapshots) {
-        bs_snapshots = NULL;
-    }
 }
 
 void bdrv_emit_qmp_error_event(const BlockDriverState *bdrv,
@@ -3392,24 +3382,6 @@ int bdrv_is_snapshot(BlockDriverState *bs)
     return !!(bs->open_flags & BDRV_O_SNAPSHOT);
 }
 
-BlockDriverState *bdrv_snapshots(void)
-{
-    BlockDriverState *bs;
-
-    if (bs_snapshots) {
-        return bs_snapshots;
-    }
-
-    bs = NULL;
-    while ((bs = bdrv_next(bs))) {
-        if (bdrv_can_snapshot(bs)) {
-            bs_snapshots = bs;
-            return bs;
-        }
-    }
-    return NULL;
-}
-
 int bdrv_snapshot_create(BlockDriverState *bs,
                          QEMUSnapshotInfo *sn_info)
 {

include/block/block.h

@@ -333,7 +333,6 @@ BlockInfo *bdrv_query_info(BlockDriverState *s);
 BlockStats *bdrv_query_stats(const BlockDriverState *bs);
 int bdrv_can_snapshot(BlockDriverState *bs);
 int bdrv_is_snapshot(BlockDriverState *bs);
-BlockDriverState *bdrv_snapshots(void);
 int bdrv_snapshot_create(BlockDriverState *bs,
                          QEMUSnapshotInfo *sn_info);
 int bdrv_snapshot_goto(BlockDriverState *bs,

savevm.c

@@ -2262,6 +2262,17 @@ out:
     return ret;
 }
 
+static BlockDriverState *find_vmstate_bs(void)
+{
+    BlockDriverState *bs = NULL;
+    while ((bs = bdrv_next(bs))) {
+        if (bdrv_can_snapshot(bs)) {
+            return bs;
+        }
+    }
+    return NULL;
+}
+
 static int bdrv_snapshot_find(BlockDriverState *bs, QEMUSnapshotInfo *sn_info,
                               const char *name)
 {
@@ -2338,7 +2349,7 @@ void do_savevm(Monitor *mon, const QDict *qdict)
         }
     }
 
-    bs = bdrv_snapshots();
+    bs = find_vmstate_bs();
     if (!bs) {
         monitor_printf(mon, "No block device can accept snapshots\n");
         return;
@@ -2440,7 +2451,7 @@ int load_vmstate(const char *name)
     QEMUFile *f;
     int ret;
 
-    bs_vm_state = bdrv_snapshots();
+    bs_vm_state = find_vmstate_bs();
     if (!bs_vm_state) {
         error_report("No block device supports snapshots");
         return -ENOTSUP;
@@ -2519,7 +2530,7 @@ void do_delvm(Monitor *mon, const QDict *qdict)
     int ret;
     const char *name = qdict_get_str(qdict, "name");
 
-    bs = bdrv_snapshots();
+    bs = find_vmstate_bs();
     if (!bs) {
         monitor_printf(mon, "No block device supports snapshots\n");
         return;
@@ -2551,7 +2562,7 @@ void do_info_snapshots(Monitor *mon, const QDict *qdict)
     int *available_snapshots;
     char buf[256];
 
-    bs = bdrv_snapshots();
+    bs = find_vmstate_bs();
     if (!bs) {
         monitor_printf(mon, "No available block device supports snapshots\n");
         return;