[PATCH rfc 1/3] virtio-net: support transmit hash report

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

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

Virtio-net supports sharing the flow hash from host to guest on rx.
Do the same on transmit, to allow the host to infer connection state
for more robust routing and telemetry.

Linux derives ipv6 flowlabel and ECMP multipath from sk->sk_txhash,
and updates these fields on error with sk_rethink_txhash. This feature
allows the host to make similar decisions.

Besides the raw hash, optionally also convey connection state for
this hash. Specifically, the hash rotates on transmit timeout. To
avoid having to keep a stateful table in the host to detect flow
changes, explicitly notify when a hash changed due to timeout.

Signed-off-by: Willem de Bruijn <willemb@google.com>
---
 drivers/net/virtio_net.c        | 24 +++++++++++++++++++++---
 include/uapi/linux/virtio_net.h | 10 +++++++++-
 2 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 21b71148c532..b917b7333928 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -201,6 +201,9 @@ struct virtnet_info {
 	/* Host will merge rx buffers for big packets (shake it! shake it!) */
 	bool mergeable_rx_bufs;
 
+	/* Guest will pass tx path info to the host */
+	bool has_tx_hash;
+
 	/* Has control virtqueue */
 	bool has_cvq;
 
@@ -394,9 +397,9 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
 
 	hdr_len = vi->hdr_len;
 	if (vi->mergeable_rx_bufs)
-		hdr_padded_len = sizeof(*hdr);
+		hdr_padded_len = max_t(unsigned int, hdr_len, sizeof(*hdr));
 	else
-		hdr_padded_len = sizeof(struct padded_vnet_hdr);
+		hdr_padded_len = ALIGN(hdr_len, 16);
 
 	/* hdr_valid means no XDP, so we can copy the vnet header */
 	if (hdr_valid)
@@ -1534,6 +1537,7 @@ 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;
 	int num_sg;
 	unsigned hdr_len = vi->hdr_len;
 	bool can_push;
@@ -1558,6 +1562,14 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
 	if (vi->mergeable_rx_bufs)
 		hdr->num_buffers = 0;
 
+	ht = (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;
+	}
+
 	sg_init_table(sq->sg, skb_shinfo(skb)->nr_frags + (can_push ? 1 : 2));
 	if (can_push) {
 		__skb_push(skb, hdr_len);
@@ -3054,6 +3066,11 @@ static int virtnet_probe(struct virtio_device *vdev)
 	else
 		vi->hdr_len = sizeof(struct virtio_net_hdr);
 
+	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_TX_HASH)) {
+		vi->has_tx_hash = true;
+		vi->hdr_len = sizeof(struct virtio_net_hdr_v1_hash);
+	}
+
 	if (virtio_has_feature(vdev, VIRTIO_F_ANY_LAYOUT) ||
 	    virtio_has_feature(vdev, VIRTIO_F_VERSION_1))
 		vi->any_header_sg = true;
@@ -3243,7 +3260,8 @@ static struct virtio_device_id id_table[] = {
 	VIRTIO_NET_F_GUEST_ANNOUNCE, VIRTIO_NET_F_MQ, \
 	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_SPEED_DUPLEX, VIRTIO_NET_F_STANDBY, \
+	VIRTIO_NET_F_TX_HASH
 
 static unsigned int features[] = {
 	VIRTNET_FEATURES,
diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
index 3f55a4215f11..f6881b5b77ee 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_HASH	  56	/* Guest sends hash report */
 #define VIRTIO_NET_F_HASH_REPORT  57	/* Supports hash report */
 #define VIRTIO_NET_F_RSS	  60	/* Supports RSS RX steering */
 #define VIRTIO_NET_F_RSC_EXT	  61	/* extended coalescing info */
@@ -170,8 +171,15 @@ struct virtio_net_hdr_v1_hash {
 #define VIRTIO_NET_HASH_REPORT_IPv6_EX         7
 #define VIRTIO_NET_HASH_REPORT_TCPv6_EX        8
 #define VIRTIO_NET_HASH_REPORT_UDPv6_EX        9
+#define VIRTIO_NET_HASH_REPORT_L4              10
+#define VIRTIO_NET_HASH_REPORT_OTHER           11
 	__le16 hash_report;
-	__le16 padding;
+	union {
+		__le16 padding;
+#define VIRTIO_NET_HASH_STATE_DEFAULT          0
+#define VIRTIO_NET_HASH_STATE_TIMEOUT_BIT      0x1
+		__le16 hash_state;
+	};
 };
 
 #ifndef VIRTIO_NET_NO_LEGACY
-- 
2.29.2.729.g45daf8777d-goog