netdev - [PATCH rfc 3/3] virtio-net: support transmit timestamp

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Date:   Mon, 28 Dec 2020 11:22:33 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     virtualization@...ts.linux-foundation.org
Cc:     netdev@...r.kernel.org, mst@...hat.com, jasowang@...hat.com,
        Willem de Bruijn <willemb@...gle.com>
Subject: [PATCH rfc 3/3] virtio-net: support transmit timestamp

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

Add optional delivery 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 guests to offload for the same reasons.

The timestamp straddles (virtual) hardware domains. Like PTP, use
international atomic time (CLOCK_TAI) as global clock base. It is
guest responsibility to sync with host, e.g., through kvm-clock.

Signed-off-by: Willem de Bruijn <willemb@...gle.com>
---
 drivers/net/virtio_net.c        | 24 +++++++++++++++++-------
 include/uapi/linux/virtio_net.h |  1 +
 2 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 57744bb6a141..d40be688aed0 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -207,6 +207,9 @@ struct virtnet_info {
 	/* Host will pass CLOCK_TAI receive time to the guest */
 	bool has_rx_tstamp;
 
+	/* Guest will pass CLOCK_TAI delivery time to the host */
+	bool has_tx_tstamp;
+
 	/* Has control virtqueue */
 	bool has_cvq;
 
@@ -1550,7 +1553,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
 	struct virtio_net_hdr_mrg_rxbuf *hdr;
 	const unsigned char *dest = ((struct ethhdr *)skb->data)->h_dest;
 	struct virtnet_info *vi = sq->vq->vdev->priv;
-	struct virtio_net_hdr_v1_hash *ht;
+	struct virtio_net_hdr_v12 *h12;
 	int num_sg;
 	unsigned hdr_len = vi->hdr_len;
 	bool can_push;
@@ -1575,13 +1578,15 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
 	if (vi->mergeable_rx_bufs)
 		hdr->num_buffers = 0;
 
-	ht = (void *)hdr;
+	h12 = (void *)hdr;
 	if (vi->has_tx_hash) {
-		ht->hash_value = cpu_to_virtio32(vi->vdev, skb->hash);
-		ht->hash_report = skb->l4_hash ? VIRTIO_NET_HASH_REPORT_L4 :
-						 VIRTIO_NET_HASH_REPORT_OTHER;
-		ht->hash_state = VIRTIO_NET_HASH_STATE_DEFAULT;
+		h12->hash.value = cpu_to_virtio32(vi->vdev, skb->hash);
+		h12->hash.report = skb->l4_hash ? VIRTIO_NET_HASH_REPORT_L4 :
+						  VIRTIO_NET_HASH_REPORT_OTHER;
+		h12->hash.flow_state = VIRTIO_NET_HASH_STATE_DEFAULT;
 	}
+	if (vi->has_tx_tstamp)
+		h12->tstamp = cpu_to_virtio64(vi->vdev, skb->tstamp);
 
 	sg_init_table(sq->sg, skb_shinfo(skb)->nr_frags + (can_push ? 1 : 2));
 	if (can_push) {
@@ -3089,6 +3094,11 @@ static int virtnet_probe(struct virtio_device *vdev)
 		vi->hdr_len = sizeof(struct virtio_net_hdr_v12);
 	}
 
+	if (virtio_has_feature(vdev, VIRTIO_NET_F_TX_TSTAMP)) {
+		vi->has_tx_tstamp = true;
+		vi->hdr_len = sizeof(struct virtio_net_hdr_v12);
+	}
+
 	if (virtio_has_feature(vdev, VIRTIO_F_ANY_LAYOUT) ||
 	    virtio_has_feature(vdev, VIRTIO_F_VERSION_1))
 		vi->any_header_sg = true;
@@ -3279,7 +3289,7 @@ 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_HASH, VIRTIO_NET_F_RX_TSTAMP, VIRTIO_NET_F_TX_TSTAMP
 
 static unsigned int features[] = {
 	VIRTNET_FEATURES,
diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
index 0ffe2eeebd4a..da017a47791d 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_TSTAMP	  54	/* Guest sets TAI delivery time */
 #define VIRTIO_NET_F_RX_TSTAMP	  55	/* Host sends TAI receive time */
 #define VIRTIO_NET_F_TX_HASH	  56	/* Guest sends hash report */
 #define VIRTIO_NET_F_HASH_REPORT  57	/* Supports hash report */
-- 
2.29.2.729.g45daf8777d-goog