Re: [PATCH v1 2/3] virtio-balloon: fix free page hinting check on unrealize

[Alexander Duyck]

On Mon, May 18, 2020 at 1:37 AM David Hildenbrand <address@hidden> wrote:
>
> Checking against guest features is wrong. We allocated data structures
> based on host features. We can rely on "free_page_bh" as an indicator
> whether to un-do stuff instead.
>
> Fixes: c13c4153f76d ("virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT")
> Cc: Wei Wang <address@hidden>
> Cc: Michael S. Tsirkin <address@hidden>
> Cc: Philippe Mathieu-Daudé <address@hidden>
> Cc: Alexander Duyck <address@hidden>
> Signed-off-by: David Hildenbrand <address@hidden>
> ---
>  hw/virtio/virtio-balloon.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index dc3b1067ab..a4fcf2d777 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -818,7 +818,7 @@ static void virtio_balloon_device_unrealize(DeviceState 
> *dev)
>      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>      VirtIOBalloon *s = VIRTIO_BALLOON(dev);
>
> -    if (virtio_balloon_free_page_support(s)) {
> +    if (s->free_page_bh) {
>          qemu_bh_delete(s->free_page_bh);
>          virtio_balloon_free_page_stop(s);
>          precopy_remove_notifier(&s->free_page_report_notify);

Would it make sense to apply the same change to
virtio_balloon_device_reset and virtio_balloon_set_status? At least in
the case of virtio_balloon_set_status it seems like you could possibly
exploit it somehow as clearing the feature in the guest will prevent
the toggling of the block_iothread value.

Reviewed-by: Alexander Duyck <address@hidden>