virtio_net: document VIRTIO_NET_CTRL_GUEST_OFFLOADS

Document VIRTIO_NET_CTRL_GUEST_OFFLOADS and the
relevant feature bits.

Will allow ethtool control of the offloads down the road.

Reported-by: Yan Vugenfirer <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

include/uapi/linux/virtio_net.h

···
 /* The feature bitmap for virtio net */
 #define VIRTIO_NET_F_CSUM	0	/* Host handles pkts w/ partial csum */
 #define VIRTIO_NET_F_GUEST_CSUM	1	/* Guest handles pkts w/ partial csum */
+#define VIRTIO_NET_F_CTRL_GUEST_OFFLOADS 2 /* Dynamic offload configuration. */
 #define VIRTIO_NET_F_MAC	5	/* Host has given MAC address. */
 #define VIRTIO_NET_F_GUEST_TSO4	7	/* Guest can handle TSOv4 in. */
 #define VIRTIO_NET_F_GUEST_TSO6	8	/* Guest can handle TSOv6 in. */
···
  #define VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET        0
  #define VIRTIO_NET_CTRL_MQ_VQ_PAIRS_MIN        1
  #define VIRTIO_NET_CTRL_MQ_VQ_PAIRS_MAX        0x8000
+
+/*
+ * Control network offloads
+ *
+ * Reconfigures the network offloads that Guest can handle.
+ *
+ * Available with the VIRTIO_NET_F_CTRL_GUEST_OFFLOADS feature bit.
+ *
+ * Command data format matches the feature bit mask exactly.
+ *
+ * See VIRTIO_NET_F_GUEST_* for the list of offloads
+ * that can be enabled/disabled.
+ */
+#define VIRTIO_NET_CTRL_GUEST_OFFLOADS   5
+#define VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET        0
 
 #endif /* _LINUX_VIRTIO_NET_H */