diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 843bb2bec8..cf7f47eaba 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -28,6 +28,10 @@
 #include "hw/virtio/virtio-bus.h"
 #include "hw/virtio/virtio-access.h"
 
+/* We don't support discard yet, hide associated config fields. */
+#define VIRTIO_BLK_CFG_SIZE offsetof(struct virtio_blk_config, \
+                                     max_discard_sectors)
+
 static void virtio_blk_init_request(VirtIOBlock *s, VirtQueue *vq,
                                     VirtIOBlockReq *req)
 {
@@ -757,7 +761,8 @@ static void virtio_blk_update_config(VirtIODevice *vdev, uint8_t *config)
     blkcfg.alignment_offset = 0;
     blkcfg.wce = blk_enable_write_cache(s->blk);
     virtio_stw_p(vdev, &blkcfg.num_queues, s->conf.num_queues);
-    memcpy(config, &blkcfg, sizeof(struct virtio_blk_config));
+    memcpy(config, &blkcfg, VIRTIO_BLK_CFG_SIZE);
+    QEMU_BUILD_BUG_ON(VIRTIO_BLK_CFG_SIZE > sizeof(blkcfg));
 }
 
 static void virtio_blk_set_config(VirtIODevice *vdev, const uint8_t *config)
@@ -765,7 +770,8 @@ static void virtio_blk_set_config(VirtIODevice *vdev, const uint8_t *config)
     VirtIOBlock *s = VIRTIO_BLK(vdev);
     struct virtio_blk_config blkcfg;
 
-    memcpy(&blkcfg, config, sizeof(blkcfg));
+    memcpy(&blkcfg, config, VIRTIO_BLK_CFG_SIZE);
+    QEMU_BUILD_BUG_ON(VIRTIO_BLK_CFG_SIZE > sizeof(blkcfg));
 
     aio_context_acquire(blk_get_aio_context(s->blk));
     blk_set_enable_write_cache(s->blk, blkcfg.wce != 0);
@@ -948,8 +954,7 @@ static void virtio_blk_device_realize(DeviceState *dev, Error **errp)
         return;
     }
 
-    virtio_init(vdev, "virtio-blk", VIRTIO_ID_BLOCK,
-                sizeof(struct virtio_blk_config));
+    virtio_init(vdev, "virtio-blk", VIRTIO_ID_BLOCK, VIRTIO_BLK_CFG_SIZE);
 
     s->blk = conf->conf.blk;
     s->rq = NULL;