Re: [PATCH v1 1/3] virtio-balloon: fix free page hinting without an iothread

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 5/18/20 10:37 AM, David Hildenbrand wrote:
> 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_initialize().
> 
> Fixes: c13c4153f76d ("virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT")
> Reported-by: Alexander Bulekov <alxndr@bu.edu>
> 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>
> ---
>   hw/virtio/virtio-balloon.c | 35 ++++++++++++++++++-----------------
>   1 file changed, 18 insertions(+), 17 deletions(-)
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 065cd450f1..dc3b1067ab 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -789,6 +789,13 @@ static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
>           return;
>       }
>   
> +    if (virtio_has_feature(s->host_features,
> +        VIRTIO_BALLOON_F_FREE_PAGE_HINT) && !s->iothread) {
> +        error_setg(errp, "'free-page-hint' requires 'iothread' to be set");
> +        virtio_cleanup(vdev);
> +        return;

This is exactly what I had in mind!

> +    }
> +
>       s->ivq = virtio_add_queue(vdev, 128, virtio_balloon_handle_output);
>       s->dvq = virtio_add_queue(vdev, 128, virtio_balloon_handle_output);
>       s->svq = virtio_add_queue(vdev, 128, virtio_balloon_receive_stats);
> @@ -797,24 +804,11 @@ static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
>                              VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
>           s->free_page_vq = virtio_add_queue(vdev, VIRTQUEUE_MAX_SIZE,
>                                              virtio_balloon_handle_free_page_vq);
> -        s->free_page_report_status = FREE_PAGE_REPORT_S_STOP;
> -        s->free_page_report_cmd_id =
> -                           VIRTIO_BALLOON_FREE_PAGE_REPORT_CMD_ID_MIN;
> -        s->free_page_report_notify.notify =
> -                                       virtio_balloon_free_page_report_notify;
>           precopy_add_notifier(&s->free_page_report_notify);
> -        if (s->iothread) {
> -            object_ref(OBJECT(s->iothread));
> -            s->free_page_bh = aio_bh_new(iothread_get_aio_context(s->iothread),
> -                                       virtio_ballloon_get_free_page_hints, s);
> -            qemu_mutex_init(&s->free_page_lock);
> -            qemu_cond_init(&s->free_page_cond);
> -            s->block_iothread = false;
> -        } else {
> -            /* Simply disable this feature if the iothread wasn't created. */
> -            s->host_features &= ~(1 << VIRTIO_BALLOON_F_FREE_PAGE_HINT);
> -            virtio_error(vdev, "iothread is missing");
> -        }
> +
> +        object_ref(OBJECT(s->iothread));
> +        s->free_page_bh = aio_bh_new(iothread_get_aio_context(s->iothread),
> +                                     virtio_ballloon_get_free_page_hints, s);
>       }
>       reset_stats(s);
>   }
> @@ -892,6 +886,13 @@ static void virtio_balloon_instance_init(Object *obj)
>   {
>       VirtIOBalloon *s = VIRTIO_BALLOON(obj);
>   
> +    qemu_mutex_init(&s->free_page_lock);
> +    qemu_cond_init(&s->free_page_cond);
> +    s->free_page_report_status = FREE_PAGE_REPORT_S_STOP;
> +    s->free_page_report_cmd_id = VIRTIO_BALLOON_FREE_PAGE_REPORT_CMD_ID_MIN;
> +    s->free_page_report_notify.notify = virtio_balloon_free_page_report_notify;
> +    s->block_iothread = false;

This part is even cleaner.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

> +
>       object_property_add(obj, "guest-stats", "guest statistics",
>                           balloon_stats_get_all, NULL, NULL, s);
>   
>