We need an ability to insert filters above top block node, attached to
block device. It can't be achieved with blockdev-reopen command. So, we
want do it with help of qom-set.

Intended usage:

Assume there is a node A that is attached to some guest device.

1. blockdev-add to create a filter node B that has A as its child.

2. qom-set to change the node attached to the guest device’s
   BlockBackend from A to B.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 hw/core/qdev-properties-system.c | 43 +++++++++++++++++++++++---------
 1 file changed, 31 insertions(+), 12 deletions(-)

diff --git a/hw/core/qdev-properties-system.c b/hw/core/qdev-properties-system.c
index 2760c21f11..e71f5d64d1 100644
--- a/hw/core/qdev-properties-system.c
+++ b/hw/core/qdev-properties-system.c
@@ -36,11 +36,11 @@
 
 static bool check_prop_still_unset(Object *obj, const char *name,
                                    const void *old_val, const char *new_val,
-                                   Error **errp)
+                                   bool allow_override, Error **errp)
 {
     const GlobalProperty *prop = qdev_find_global_prop(obj, name);
 
-    if (!old_val) {
+    if (!old_val || (!prop && allow_override)) {
         return true;
     }
 
@@ -93,16 +93,34 @@ static void set_drive_helper(Object *obj, Visitor *v, const char *name,
     BlockBackend *blk;
     bool blk_created = false;
     int ret;
+    BlockDriverState *bs;
+    AioContext *ctx;
 
     if (!visit_type_str(v, name, &str, errp)) {
         return;
     }
 
-    /*
-     * TODO Should this really be an error?  If no, the old value
-     * needs to be released before we store the new one.
-     */
-    if (!check_prop_still_unset(obj, name, *ptr, str, errp)) {
+    if (!check_prop_still_unset(obj, name, *ptr, str, true, errp)) {
+        return;
+    }
+
+    if (*ptr) {
+        /* BlockBackend alread exists. So, we want to change attached node */
+        blk = *ptr;
+        ctx = blk_get_aio_context(blk);
+        bs = bdrv_lookup_bs(NULL, str, errp);
+        if (!bs) {
+            return;
+        }
+
+        if (ctx != bdrv_get_aio_context(bs)) {
+            error_setg(errp, "Different aio context is not supported for new "
+                       "node");
+        }
+
+        aio_context_acquire(ctx);
+        blk_replace_bs(blk, bs, errp);
+        aio_context_release(ctx);
         return;
     }
 
@@ -114,7 +132,7 @@ static void set_drive_helper(Object *obj, Visitor *v, const char *name,
 
     blk = blk_by_name(str);
     if (!blk) {
-        BlockDriverState *bs = bdrv_lookup_bs(NULL, str, NULL);
+        bs = bdrv_lookup_bs(NULL, str, NULL);
         if (bs) {
             /*
              * If the device supports iothreads, it will make sure to move the
@@ -123,8 +141,7 @@ static void set_drive_helper(Object *obj, Visitor *v, const char *name,
              * aware of iothreads require their BlockBackends to be in the main
              * AioContext.
              */
-            AioContext *ctx = iothread ? bdrv_get_aio_context(bs) :
-                                         qemu_get_aio_context();
+            ctx = iothread ? bdrv_get_aio_context(bs) : qemu_get_aio_context();
             blk = blk_new(ctx, 0, BLK_PERM_ALL);
             blk_created = true;
 
@@ -196,6 +213,7 @@ static void release_drive(Object *obj, const char *name, void *opaque)
 const PropertyInfo qdev_prop_drive = {
     .name  = "str",
     .description = "Node name or ID of a block device to use as a backend",
+    .realized_set_allowed = true,
     .get   = get_drive,
     .set   = set_drive,
     .release = release_drive,
@@ -204,6 +222,7 @@ const PropertyInfo qdev_prop_drive = {
 const PropertyInfo qdev_prop_drive_iothread = {
     .name  = "str",
     .description = "Node name or ID of a block device to use as a backend",
+    .realized_set_allowed = true,
     .get   = get_drive,
     .set   = set_drive_iothread,
     .release = release_drive,
@@ -238,7 +257,7 @@ static void set_chr(Object *obj, Visitor *v, const char *name, void *opaque,
      * TODO Should this really be an error?  If no, the old value
      * needs to be released before we store the new one.
      */
-    if (!check_prop_still_unset(obj, name, be->chr, str, errp)) {
+    if (!check_prop_still_unset(obj, name, be->chr, str, false, errp)) {
         return;
     }
 
@@ -408,7 +427,7 @@ static void set_netdev(Object *obj, Visitor *v, const char *name,
          * TODO Should this really be an error?  If no, the old value
          * needs to be released before we store the new one.
          */
-        if (!check_prop_still_unset(obj, name, ncs[i], str, errp)) {
+        if (!check_prop_still_unset(obj, name, ncs[i], str, false, errp)) {
             goto out;
         }
 
-- 
2.29.2

On 20.05.21 16:21, Vladimir Sementsov-Ogievskiy wrote:
> We need an ability to insert filters above top block node, attached to
> block device. It can't be achieved with blockdev-reopen command. So, we
> want do it with help of qom-set.
>
> Intended usage:
>
> Assume there is a node A that is attached to some guest device.
>
> 1. blockdev-add to create a filter node B that has A as its child.
>
> 2. qom-set to change the node attached to the guest device’s
>     BlockBackend from A to B.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   hw/core/qdev-properties-system.c | 43 +++++++++++++++++++++++---------
>   1 file changed, 31 insertions(+), 12 deletions(-)

Tentative

Reviewed-by: Max Reitz <mreitz@redhat.com>