Re: [PATCH RFC v3 08/13] qmp: update virtio features map to support extended features

[Markus Armbruster <armbru@redhat.com>]

Paolo Abeni <pabeni@redhat.com> writes:

> Extend the VirtioDeviceFeatures struct with an additional u64
> to track unknown features in the 64-127 bit range and decode
> the full virtio features spaces for vhost and virtio devices.
>
> Also add entries for the soon-to-be-supported virtio net GSO over
> UDP features.
>
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>

[...]

> diff --git a/hw/virtio/virtio-qmp.c b/hw/virtio/virtio-qmp.c
> index 3b6377cf0d..03c6163cf4 100644
> --- a/hw/virtio/virtio-qmp.c
> +++ b/hw/virtio/virtio-qmp.c

[...]

> @@ -680,9 +715,10 @@ VirtioDeviceFeatures *qmp_decode_features(uint16_t device_id, uint64_t bitmap)
>          g_assert_not_reached();
>      }
>  
> -    features->has_unknown_dev_features = bitmap != 0;
> +    features->has_unknown_dev_features = !virtio_features_empty(bitmap);
>      if (features->has_unknown_dev_features) {
> -        features->unknown_dev_features = bitmap;
> +        features->unknown_dev_features = bitmap[0];
> +        features->unknown_dev_features2 = bitmap[1];
>      }

Why not assign unconditionally?

>  
>      return features;

[...]

> diff --git a/qapi/virtio.json b/qapi/virtio.json
> index 9d652fe4a8..f2e2dd6e97 100644
> --- a/qapi/virtio.json
> +++ b/qapi/virtio.json
> @@ -490,14 +490,18 @@
>  #     unique features)
>  #
>  # @unknown-dev-features: Virtio device features bitmap that have not
> -#     been decoded
> +#     been decoded (bits 0-63)
> +#
> +# @unknown-dev-features2: Virtio device features bitmap that have not
> +#     been decoded (bits 64-127)
>  #
>  # Since: 7.2
>  ##
>  { 'struct': 'VirtioDeviceFeatures',
>    'data': { 'transports': [ 'str' ],
>              '*dev-features': [ 'str' ],
> -            '*unknown-dev-features': 'uint64' } }
> +            '*unknown-dev-features': 'uint64',
> +            '*unknown-dev-features2': 'uint64' } }
>  
>  ##
>  # @VirtQueueStatus:

I wish we could simply widen @unknown-dev-features, but we don't have
uint128, and adding it would risk breaking QMP clients.  64 bit integers
are already troublesome in JSON.

Does the example in x-query-virtio-status's doc comment need an update?