virtio-net: Introduce extended RSC feature

VIRTIO_NET_F_RSC_EXT feature bit indicates that the device
is able to provide extended RSC information. When the feature
is negotiatede and 'gso_type' field in received packet is not
GSO_NONE, the device reports number of coalesced packets in
'csum_start' field and number of duplicated acks in 'csum_offset'
field and sets VIRTIO_NET_HDR_F_RSC_INFO in 'flags' field.
Signed-off-by: Yuri Benditovich <yuri.benditovich@daynix.com>
Link: https://lore.kernel.org/r/20200302115003.14877-2-yuri.benditovich@daynix.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

include/uapi/linux/virtio_net.h

@@ -57,6 +57,7 @@
 					 * Steering */
 #define VIRTIO_NET_F_CTRL_MAC_ADDR 23	/* Set MAC address */
 
+#define VIRTIO_NET_F_RSC_EXT	  61	/* extended coalescing info */
 #define VIRTIO_NET_F_STANDBY	  62	/* Act as standby for another device
 					 * with the same MAC.
 					 */
@@ -104,6 +105,7 @@ struct virtio_net_config {
 struct virtio_net_hdr_v1 {
 #define VIRTIO_NET_HDR_F_NEEDS_CSUM	1	/* Use csum_start, csum_offset */
 #define VIRTIO_NET_HDR_F_DATA_VALID	2	/* Csum is valid */
+#define VIRTIO_NET_HDR_F_RSC_INFO	4	/* rsc info in csum_ fields */
 	__u8 flags;
 #define VIRTIO_NET_HDR_GSO_NONE		0	/* Not a GSO frame */
 #define VIRTIO_NET_HDR_GSO_TCPV4	1	/* GSO frame, IPv4 TCP (TSO) */
@@ -113,8 +115,26 @@ struct virtio_net_hdr_v1 {
 	__u8 gso_type;
 	__virtio16 hdr_len;	/* Ethernet + IP + tcp/udp hdrs */
 	__virtio16 gso_size;	/* Bytes to append to hdr_len per frame */
-	__virtio16 csum_start;	/* Position to start checksumming from */
-	__virtio16 csum_offset;	/* Offset after that to place checksum */
+	union {
+		struct {
+			__virtio16 csum_start;
+			__virtio16 csum_offset;
+		};
+		/* Checksum calculation */
+		struct {
+			/* Position to start checksumming from */
+			__virtio16 start;
+			/* Offset after that to place checksum */
+			__virtio16 offset;
+		} csum;
+		/* Receive Segment Coalescing */
+		struct {
+			/* Number of coalesced segments */
+			__le16 segments;
+			/* Number of duplicated acks */
+			__le16 dup_acks;
+		} rsc;
+	};
 	__virtio16 num_buffers;	/* Number of merged rx buffers */
 };