Re: [PATCH 02/33] vhost: drop backend_features field

[Markus Armbruster <armbru@redhat.com>]

Cc: libvirt

Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> writes:

> This field is mostly unused and sometimes confusing (we even have
> a TODO-like comment to drop it). Let's finally do.
>
> The field is used to held VHOST_USER_F_PROTOCOL_FEATURES for vhost-user
> and/or VHOST_NET_F_VIRTIO_NET_HDR for vhost-net (which may be
> vhoust-user-net). But we can simply recalculte these two flags inplace
> from hdev->features, and from net-client for
> VHOST_NET_F_VIRTIO_NET_HDR.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

[...]

> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> index 66be6afc88..9f9dd2d46d 100644
> --- a/include/hw/virtio/vhost.h
> +++ b/include/hw/virtio/vhost.h
> @@ -100,16 +100,9 @@ struct vhost_dev {
>       *
>       * @features: available features provided by the backend
>       * @acked_features: final negotiated features with front-end driver
> -     *
> -     * @backend_features: this is used in a couple of places to either
> -     * store VHOST_USER_F_PROTOCOL_FEATURES to apply to
> -     * VHOST_USER_SET_FEATURES or VHOST_NET_F_VIRTIO_NET_HDR. Its
> -     * future use should be discouraged and the variable retired as
> -     * its easy to confuse with the VirtIO backend_features.

I guess this is the TODO-like comment mentioned in the commit message.

>       */
>      uint64_t features;
>      uint64_t acked_features;
> -    uint64_t backend_features;
>  
>      /**
>       * @protocol_features: is the vhost-user only feature set by
> diff --git a/qapi/virtio.json b/qapi/virtio.json
> index 9d652fe4a8..0aae77340d 100644
> --- a/qapi/virtio.json
> +++ b/qapi/virtio.json
> @@ -85,8 +85,6 @@
>  #
>  # @acked-features: vhost_dev acked_features
>  #
> -# @backend-features: vhost_dev backend_features
> -#
>  # @protocol-features: vhost_dev protocol_features
>  #
>  # @max-queues: vhost_dev max_queues
> @@ -106,7 +104,6 @@
>              'vq-index': 'int',
>              'features': 'VirtioDeviceFeatures',
>              'acked-features': 'VirtioDeviceFeatures',
> -            'backend-features': 'VirtioDeviceFeatures',
>              'protocol-features': 'VhostDeviceProtocols',
>              'max-queues': 'uint64',
>              'backend-cap': 'uint64',

Incompatible change.  We can do this because it's only visible in the
return value of x-query-virtio-status, which is unstable.  Recommend to
note this in the commit message.

Acked-by: Markus Armbruster <armbru@redhat.com>