[PATCH RFC v2 4/4] virtio-net: support future packet transmit time

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

From: Willem de Bruijn <willemb@google.com>

Add optional transmit time (SO_TXTIME) offload for virtio-net.

The Linux TCP/IP stack tries to avoid bursty transmission and network
congestion through pacing: computing an skb delivery time based on
congestion information. Userspace protocol implementations can achieve
the same with SO_TXTIME. This may also reduce scheduling jitter and
improve RTT estimation.

Pacing can be implemented in ETF or FQ qdiscs or offloaded to NIC
hardware. Allow virtio-net driver to offload for the same reasons.

The timestamp straddles (virtual) hardware domains. Like PTP, use
international atomic time (CLOCK_TAI) as global clock base. The driver
must sync with the device, e.g., through kvm-clock.

Changes RFC - RFCv2
  - rename from transmit timestamp to future packet transmit time
  - convert cpu_to_virtioXX to cpu_to_leXX

Signed-off-by: Willem de Bruijn <willemb@google.com>
---
 drivers/net/virtio_net.c        | 13 ++++++++++++-
 include/uapi/linux/virtio_net.h |  1 +
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index fc8ecd3a333a..c09d19b97f42 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -216,6 +216,9 @@ struct virtnet_info {
 	/* Device will pass tx timestamp. Requires has_tx_tstamp */
 	bool enable_tx_tstamp;
 
+	/* Driver will pass CLOCK_TAI delivery time to the device */
+	bool has_tx_time;
+
 	/* Has control virtqueue */
 	bool has_cvq;
 
@@ -1616,6 +1619,8 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
 	}
 	if (vi->enable_tx_tstamp && skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP)
 		ht->hdr.flags |= VIRTIO_NET_HDR_F_TSTAMP;
+	if (vi->has_tx_time && skb->tstamp)
+		ht->tstamp = cpu_to_le64(skb->tstamp);
 
 	sg_init_table(sq->sg, skb_shinfo(skb)->nr_frags + (can_push ? 1 : 2));
 	if (can_push) {
@@ -3221,6 +3226,11 @@ static int virtnet_probe(struct virtio_device *vdev)
 		vi->hdr_len = sizeof(struct virtio_net_hdr_hash_ts);
 	}
 
+	if (virtio_has_feature(vdev, VIRTIO_NET_F_TX_TIME)) {
+		vi->has_tx_time = true;
+		vi->hdr_len = sizeof(struct virtio_net_hdr_hash_ts);
+	}
+
 	if (virtio_has_feature(vdev, VIRTIO_F_ANY_LAYOUT) ||
 	    virtio_has_feature(vdev, VIRTIO_F_VERSION_1))
 		vi->any_header_sg = true;
@@ -3412,7 +3422,8 @@ static struct virtio_device_id id_table[] = {
 	VIRTIO_NET_F_CTRL_MAC_ADDR, \
 	VIRTIO_NET_F_MTU, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS, \
 	VIRTIO_NET_F_SPEED_DUPLEX, VIRTIO_NET_F_STANDBY, \
-	VIRTIO_NET_F_TX_HASH, VIRTIO_NET_F_RX_TSTAMP, VIRTIO_NET_F_TX_TSTAMP
+	VIRTIO_NET_F_TX_HASH, VIRTIO_NET_F_RX_TSTAMP, VIRTIO_NET_F_TX_TSTAMP, \
+	VIRTIO_NET_F_TX_TIME
 
 static unsigned int features[] = {
 	VIRTNET_FEATURES,
diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
index b5d6f0c6cead..7ca99a2ee1a3 100644
--- a/include/uapi/linux/virtio_net.h
+++ b/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_TX_TIME	  53	/* Driver sets TAI delivery time */
 #define VIRTIO_NET_F_TX_TSTAMP	  54	/* Device sends TAI transmit time */
 #define VIRTIO_NET_F_RX_TSTAMP	  55	/* Device sends TAI receive time */
 #define VIRTIO_NET_F_TX_HASH	  56	/* Driver sends hash report */
-- 
2.30.0.478.g8a0d178c01-goog