In case we don't have an iothread, we mark the feature as abscent but still add the queue. 'free_page_bh' remains set to NULL. qemu-system-i386 \ -M microvm \ -nographic \ -device virtio-balloon-device,free-page-hint=true \ -nographic \ -display none \ -monitor none \ -serial none \ -qtest stdio Doing a "write 0xc0000e30 0x24 0x030000000300000003000000030000000300000003000000030000000300000003000000" We will trigger a SEGFAULT. Let's move the check and bail out. While at it, move the static initializations to instance_init(). free_page_report_status and block_iothread are implicitly set to the right values (0/false) already, so drop the initialization. Reviewed-by: Alexander Duyck <alexander.h.duyck@linux.intel.com> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com> Fixes: c13c4153f76d ("virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT") Reported-by: Alexander Bulekov <alxndr@bu.edu> Cc: qemu-stable@nongnu.org Cc: Wei Wang <wei.w.wang@intel.com> Cc: Michael S. Tsirkin <mst@redhat.com> Cc: Philippe Mathieu-Daudé <philmd@redhat.com> Cc: Alexander Duyck <alexander.duyck@gmail.com> Signed-off-by: David Hildenbrand <david@redhat.com> Message-Id: <20200520100439.19872-2-david@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>