Re: [PATCH v3 01/12] vhost-user-blk: fix blkcfg->num_queues endianness

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Feb 23, 2021 at 02:46:42PM +0000, Stefan Hajnoczi wrote:
> Treat the num_queues field as virtio-endian. On big-endian hosts the
> vhost-user-blk num_queues field was in the wrong endianness.
> 
> Move the blkcfg.num_queues store operation from realize to
> vhost_user_blk_update_config() so feature negotiation has finished and
> we know the endianness of the device. VIRTIO 1.0 devices are
> little-endian, but in case someone wants to use legacy VIRTIO we support
> all endianness cases.
> 
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
> Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

okay but as we recently discovered config space can in theory
be read before FEATURES_OK. Nasty, I know. Things kind of work
right now but we really need some other path to notify backends
when legacy guest is active. E.g. VDPA also has this problem.

> ---
>  hw/block/vhost-user-blk.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index da4fbf9084..b870a50e6b 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -54,6 +54,9 @@ static void vhost_user_blk_update_config(VirtIODevice *vdev, uint8_t *config)
>  {
>      VHostUserBlk *s = VHOST_USER_BLK(vdev);
>  
> +    /* Our num_queues overrides the device backend */
> +    virtio_stw_p(vdev, &s->blkcfg.num_queues, s->num_queues);
> +
>      memcpy(config, &s->blkcfg, sizeof(struct virtio_blk_config));
>  }
>  
> @@ -491,10 +494,6 @@ reconnect:
>          goto reconnect;
>      }
>  
> -    if (s->blkcfg.num_queues != s->num_queues) {
> -        s->blkcfg.num_queues = s->num_queues;
> -    }
> -
>      return;
>  
>  virtio_err:
> -- 
> 2.29.2
>