virtio-blk: Allow config-wce in dataplane

Dataplane now uses block layer. Protect bdrv_set_enable_write_cache with
aio_context_acquire and aio_context_release, so we can enable config-wce
to allow guest to modify the write cache online.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

hw/block/dataplane/virtio-blk.c

@@ -336,12 +336,6 @@ void virtio_blk_data_plane_create(VirtIODevice *vdev, VirtIOBlkConf *blk,
         return;
     }
 
-    if (blk->config_wce) {
-        error_setg(errp, "device is incompatible with x-data-plane, "
-                         "use config-wce=off");
-        return;
-    }
-
     /* If dataplane is (re-)enabled while the guest is running there could be
      * block jobs that can conflict.
      */

hw/block/virtio-blk.c

@@ -523,7 +523,10 @@ static void virtio_blk_set_config(VirtIODevice *vdev, const uint8_t *config)
     struct virtio_blk_config blkcfg;
 
     memcpy(&blkcfg, config, sizeof(blkcfg));
+
+    aio_context_acquire(bdrv_get_aio_context(s->bs));
     bdrv_set_enable_write_cache(s->bs, blkcfg.wce != 0);
+    aio_context_release(bdrv_get_aio_context(s->bs));
 }
 
 static uint32_t virtio_blk_get_features(VirtIODevice *vdev, uint32_t features)
@@ -582,7 +585,10 @@ static void virtio_blk_set_status(VirtIODevice *vdev, uint8_t status)
      * s->bs would erroneously be placed in writethrough mode.
      */
     if (!(features & (1 << VIRTIO_BLK_F_CONFIG_WCE))) {
-        bdrv_set_enable_write_cache(s->bs, !!(features & (1 << VIRTIO_BLK_F_WCE)));
+        aio_context_acquire(bdrv_get_aio_context(s->bs));
+        bdrv_set_enable_write_cache(s->bs,
+                                    !!(features & (1 << VIRTIO_BLK_F_WCE)));
+        aio_context_release(bdrv_get_aio_context(s->bs));
     }
 }