Linux Virtualization - Jan 2018 - [PATCH net-next 0/2] Enable virtio to act as a master for a passthru device

This patch series enables virtio to switch over to a VF datapath when a VF
netdev is present with the same MAC address. It allows live migration of a VM
with a direct attached VF without the need to setup a bond/team between a
VF and virtio net device in the guest.

The hypervisor needs to unplug the VF device from the guest on the source
host and reset the MAC filter of the VF to initiate failover of datapath to
virtio before starting the migration. After the migration is completed, the
destination hypervisor sets the MAC filter on the VF and plugs it back to
the guest to switch over to VF datapath.

It is based on netvsc implementation and it may be possible to make this code 
generic and move it to a common location that can be shared by netvsc and
virtio.

This patch series is based on the discussion initiated by Jesse on this thread.
https://marc.info/?l=linux-virtualization&m=151189725224231&w=2

Sridhar Samudrala (2):
  virtio_net: Introduce VIRTIO_NET_F_MASTER feature bit
  virtio_net: Extend virtio to use VF datapath when available

 drivers/net/virtio_net.c        | 308 +++++++++++++++++++++++++++++++++++++++-
 include/uapi/linux/virtio_net.h |   1 +
 2 files changed, 306 insertions(+), 3 deletions(-)

-- 
2.14.3

This feature bit can be used by hypervisor to indicate virtio_net device to
act as a master for a directly attached passthru device with the same MAC
address.

Signed-off-by: Sridhar Samudrala <sridhar.samudrala at intel.com>
---
 drivers/net/virtio_net.c        | 3 ++-
 include/uapi/linux/virtio_net.h | 1 +
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 6fb7b658a6cc..46844a1d9a62 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2796,7 +2796,8 @@ static struct virtio_device_id id_table[] = {
 	VIRTIO_NET_F_CTRL_RX, VIRTIO_NET_F_CTRL_VLAN, \
 	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_MTU, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS, \
+	VIRTIO_NET_F_MASTER
 
 static unsigned int features[] = {
 	VIRTNET_FEATURES,
diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
index fc353b518288..a9b4e0836786 100644
--- a/include/uapi/linux/virtio_net.h
+++ b/include/uapi/linux/virtio_net.h
@@ -56,6 +56,7 @@
 #define VIRTIO_NET_F_MQ	22	/* Device supports Receive Flow
 					 * Steering */
 #define VIRTIO_NET_F_CTRL_MAC_ADDR 23	/* Set MAC address */
+#define VIRTIO_NET_F_MASTER	62	/* act as master for a VF device */
 
 #ifndef VIRTIO_NET_NO_LEGACY
 #define VIRTIO_NET_F_GSO	6	/* Host handles pkts w/ any GSO type */
-- 
2.14.3

This patch enables virtio to switch over to a VF datapath when a VF netdev
is present with the same MAC address.  It allows live migration of a VM
with a direct attached VF without the need to setup a bond/team between a
VF and virtio net device in the guest.

The hypervisor needs to unplug the VF device from the guest on the source
host and reset the MAC filter of the VF to initiate failover of datapath to
virtio before starting the migration. After the migration is completed, the
destination hypervisor sets the MAC filter on the VF and plugs it back to
the guest to switch over to VF datapath.

This patch is based on the discussion initiated by Jesse on this thread.
https://marc.info/?l=linux-virtualization&m=151189725224231&w=2

Signed-off-by: Sridhar Samudrala <sridhar.samudrala at intel.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg at intel.com>
---
 drivers/net/virtio_net.c | 305 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 303 insertions(+), 2 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 46844a1d9a62..6516dcb55b7d 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -31,6 +31,8 @@
 #include <linux/average.h>
 #include <linux/filter.h>
 #include <net/route.h>
+#include <linux/netdevice.h>
+#include <linux/netpoll.h>
 
 static int napi_weight = NAPI_POLL_WEIGHT;
 module_param(napi_weight, int, 0444);
@@ -117,6 +119,15 @@ struct receive_queue {
 	char name[40];
 };
 
+struct virtnet_vf_pcpu_stats {
+	u64	rx_packets;
+	u64	rx_bytes;
+	u64	tx_packets;
+	u64	tx_bytes;
+	struct u64_stats_sync   syncp;
+	u32	tx_dropped;
+};
+
 struct virtnet_info {
 	struct virtio_device *vdev;
 	struct virtqueue *cvq;
@@ -179,6 +190,10 @@ struct virtnet_info {
 	u32 speed;
 
 	unsigned long guest_offloads;
+
+	/* State to manage the associated VF interface. */
+	struct net_device __rcu *vf_netdev;
+	struct virtnet_vf_pcpu_stats __percpu *vf_stats;
 };
 
 struct padded_vnet_hdr {
@@ -1303,16 +1318,51 @@ static int xmit_skb(struct send_queue *sq, struct
sk_buff *skb)
 	return virtqueue_add_outbuf(sq->vq, sq->sg, num_sg, skb, GFP_ATOMIC);
 }
 
+/* Send skb on the slave VF device. */
+static int virtnet_vf_xmit(struct net_device *dev, struct net_device
*vf_netdev,
+			   struct sk_buff *skb)
+{
+	struct virtnet_info *vi = netdev_priv(dev);
+	unsigned int len = skb->len;
+	int rc;
+
+	skb->dev = vf_netdev;
+	skb->queue_mapping = qdisc_skb_cb(skb)->slave_dev_queue_mapping;
+
+	rc = dev_queue_xmit(skb);
+	if (likely(rc == NET_XMIT_SUCCESS || rc == NET_XMIT_CN)) {
+		struct virtnet_vf_pcpu_stats *pcpu_stats
+			= this_cpu_ptr(vi->vf_stats);
+
+		u64_stats_update_begin(&pcpu_stats->syncp);
+		pcpu_stats->tx_packets++;
+		pcpu_stats->tx_bytes += len;
+		u64_stats_update_end(&pcpu_stats->syncp);
+	} else {
+		this_cpu_inc(vi->vf_stats->tx_dropped);
+	}
+
+	return rc;
+}
+
 static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct virtnet_info *vi = netdev_priv(dev);
 	int qnum = skb_get_queue_mapping(skb);
 	struct send_queue *sq = &vi->sq[qnum];
+	struct net_device *vf_netdev;
 	int err;
 	struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
 	bool kick = !skb->xmit_more;
 	bool use_napi = sq->napi.weight;
 
+	/* If VF is present and up then redirect packets
+	 * called with rcu_read_lock_bh
+	 */
+	vf_netdev = rcu_dereference_bh(vi->vf_netdev);
+	if (vf_netdev && netif_running(vf_netdev) &&
!netpoll_tx_running(dev))
+		return virtnet_vf_xmit(dev, vf_netdev, skb);
+
 	/* Free up any pending old buffers before queueing new ones. */
 	free_old_xmit_skbs(sq);
 
@@ -1459,10 +1509,41 @@ static int virtnet_set_mac_address(struct net_device
*dev, void *p)
 	return ret;
 }
 
+static void virtnet_get_vf_stats(struct net_device *dev,
+				 struct virtnet_vf_pcpu_stats *tot)
+{
+	struct virtnet_info *vi = netdev_priv(dev);
+	int i;
+
+	memset(tot, 0, sizeof(*tot));
+
+	for_each_possible_cpu(i) {
+		const struct virtnet_vf_pcpu_stats *stats
+				= per_cpu_ptr(vi->vf_stats, i);
+		u64 rx_packets, rx_bytes, tx_packets, tx_bytes;
+		unsigned int start;
+
+		do {
+			start = u64_stats_fetch_begin_irq(&stats->syncp);
+			rx_packets = stats->rx_packets;
+			tx_packets = stats->tx_packets;
+			rx_bytes = stats->rx_bytes;
+			tx_bytes = stats->tx_bytes;
+		} while (u64_stats_fetch_retry_irq(&stats->syncp, start));
+
+		tot->rx_packets += rx_packets;
+		tot->tx_packets += tx_packets;
+		tot->rx_bytes   += rx_bytes;
+		tot->tx_bytes   += tx_bytes;
+		tot->tx_dropped += stats->tx_dropped;
+	}
+}
+
 static void virtnet_stats(struct net_device *dev,
 			  struct rtnl_link_stats64 *tot)
 {
 	struct virtnet_info *vi = netdev_priv(dev);
+	struct virtnet_vf_pcpu_stats vf_stats;
 	int cpu;
 	unsigned int start;
 
@@ -1493,6 +1574,13 @@ static void virtnet_stats(struct net_device *dev,
 	tot->rx_dropped = dev->stats.rx_dropped;
 	tot->rx_length_errors = dev->stats.rx_length_errors;
 	tot->rx_frame_errors = dev->stats.rx_frame_errors;
+
+	virtnet_get_vf_stats(dev, &vf_stats);
+	tot->rx_packets += vf_stats.rx_packets;
+	tot->tx_packets += vf_stats.tx_packets;
+	tot->rx_bytes += vf_stats.rx_bytes;
+	tot->tx_bytes += vf_stats.tx_bytes;
+	tot->tx_dropped += vf_stats.tx_dropped;
 }
 
 #ifdef CONFIG_NET_POLL_CONTROLLER
@@ -2604,6 +2692,13 @@ static int virtnet_probe(struct virtio_device *vdev)
 
 	INIT_WORK(&vi->config_work, virtnet_config_changed_work);
 
+	if (virtio_has_feature(vdev, VIRTIO_NET_F_MASTER)) {
+		vi->vf_stats +			netdev_alloc_pcpu_stats(struct virtnet_vf_pcpu_stats);
+		if (!vi->vf_stats)
+			goto free_stats;
+	}
+
 	/* If we can receive ANY GSO packets, we must allocate large ones. */
 	if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO4) ||
 	    virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO6) ||
@@ -2637,7 +2732,7 @@ static int virtnet_probe(struct virtio_device *vdev)
 			 */
 			dev_err(&vdev->dev, "device MTU appears to have changed "
 				"it is now %d < %d", mtu, dev->min_mtu);
-			goto free_stats;
+			goto free_vf_stats;
 		}
 
 		dev->mtu = mtu;
@@ -2661,7 +2756,7 @@ static int virtnet_probe(struct virtio_device *vdev)
 	/* Allocate/initialize the rx/tx queues, and invoke find_vqs */
 	err = init_vqs(vi);
 	if (err)
-		goto free_stats;
+		goto free_vf_stats;
 
 #ifdef CONFIG_SYSFS
 	if (vi->mergeable_rx_bufs)
@@ -2715,6 +2810,8 @@ static int virtnet_probe(struct virtio_device *vdev)
 	cancel_delayed_work_sync(&vi->refill);
 	free_receive_page_frags(vi);
 	virtnet_del_vqs(vi);
+free_vf_stats:
+	free_percpu(vi->vf_stats);
 free_stats:
 	free_percpu(vi->stats);
 free:
@@ -2736,19 +2833,184 @@ static void remove_vq_common(struct virtnet_info *vi)
 	virtnet_del_vqs(vi);
 }
 
+static struct net_device *get_virtio_bymac(const u8 *mac)
+{
+	struct net_device *dev;
+
+	ASSERT_RTNL();
+
+	for_each_netdev(&init_net, dev) {
+		if (dev->netdev_ops != &virtnet_netdev)
+			continue;       /* not a virtio_net device */
+
+		if (ether_addr_equal(mac, dev->perm_addr))
+			return dev;
+	}
+
+	return NULL;
+}
+
+static struct net_device *get_virtio_byref(struct net_device *vf_netdev)
+{
+	struct net_device *dev;
+
+	ASSERT_RTNL();
+
+	for_each_netdev(&init_net, dev) {
+		struct virtnet_info *vi;
+
+		if (dev->netdev_ops != &virtnet_netdev)
+			continue;	/* not a virtio_net device */
+
+		vi = netdev_priv(dev);
+		if (rtnl_dereference(vi->vf_netdev) == vf_netdev)
+			return dev;	/* a match */
+	}
+
+	return NULL;
+}
+
+/* Called when VF is injecting data into network stack.
+ * Change the associated network device from VF to virtio.
+ * note: already called with rcu_read_lock
+ */
+static rx_handler_result_t virtnet_vf_handle_frame(struct sk_buff **pskb)
+{
+	struct sk_buff *skb = *pskb;
+	struct net_device *ndev = rcu_dereference(skb->dev->rx_handler_data);
+	struct virtnet_info *vi = netdev_priv(ndev);
+	struct virtnet_vf_pcpu_stats *pcpu_stats +				this_cpu_ptr(vi->vf_stats);
+
+	skb->dev = ndev;
+
+	u64_stats_update_begin(&pcpu_stats->syncp);
+	pcpu_stats->rx_packets++;
+	pcpu_stats->rx_bytes += skb->len;
+	u64_stats_update_end(&pcpu_stats->syncp);
+
+	return RX_HANDLER_ANOTHER;
+}
+
+static int virtnet_vf_join(struct net_device *vf_netdev,
+			   struct net_device *ndev)
+{
+	int ret;
+
+	ret = netdev_rx_handler_register(vf_netdev,
+					 virtnet_vf_handle_frame, ndev);
+	if (ret != 0) {
+		netdev_err(vf_netdev,
+			   "can not register virtio VF receive handler (err = %d)\n",
+			   ret);
+		goto rx_handler_failed;
+	}
+
+	ret = netdev_upper_dev_link(vf_netdev, ndev, NULL);
+	if (ret != 0) {
+		netdev_err(vf_netdev,
+			   "can not set master device %s (err = %d)\n",
+			   ndev->name, ret);
+		goto upper_link_failed;
+	}
+
+	vf_netdev->flags |= IFF_SLAVE;
+
+	/* Align MTU of VF with master */
+	ret = dev_set_mtu(vf_netdev, ndev->mtu);
+	if (ret)
+		netdev_warn(vf_netdev,
+			    "unable to change mtu to %u\n", ndev->mtu);
+
+	call_netdevice_notifiers(NETDEV_JOIN, vf_netdev);
+
+	netdev_info(vf_netdev, "joined to %s\n", ndev->name);
+	return 0;
+
+upper_link_failed:
+	netdev_rx_handler_unregister(vf_netdev);
+rx_handler_failed:
+	return ret;
+}
+
+static int virtnet_register_vf(struct net_device *vf_netdev)
+{
+	struct net_device *ndev;
+	struct virtnet_info *vi;
+
+	if (vf_netdev->addr_len != ETH_ALEN)
+		return NOTIFY_DONE;
+
+	/* We will use the MAC address to locate the virtio_net interface to
+	 * associate with the VF interface. If we don't find a matching
+	 * virtio interface, move on.
+	 */
+	ndev = get_virtio_bymac(vf_netdev->perm_addr);
+	if (!ndev)
+		return NOTIFY_DONE;
+
+	vi = netdev_priv(ndev);
+	if (!virtio_has_feature(vi->vdev, VIRTIO_NET_F_MASTER))
+		return NOTIFY_DONE;
+
+	if (rtnl_dereference(vi->vf_netdev))
+		return NOTIFY_DONE;
+
+	if (virtnet_vf_join(vf_netdev, ndev) != 0)
+		return NOTIFY_DONE;
+
+	netdev_info(ndev, "VF registering %s\n", vf_netdev->name);
+
+	dev_hold(vf_netdev);
+	rcu_assign_pointer(vi->vf_netdev, vf_netdev);
+
+	return NOTIFY_OK;
+}
+
+static int virtnet_unregister_vf(struct net_device *vf_netdev)
+{
+	struct net_device *ndev;
+	struct virtnet_info *vi;
+
+	ndev = get_virtio_byref(vf_netdev);
+	if (!ndev)
+		return NOTIFY_DONE;
+
+	vi = netdev_priv(ndev);
+	if (!virtio_has_feature(vi->vdev, VIRTIO_NET_F_MASTER))
+		return NOTIFY_DONE;
+
+	netdev_info(ndev, "VF unregistering %s\n", vf_netdev->name);
+
+	netdev_rx_handler_unregister(vf_netdev);
+	netdev_upper_dev_unlink(vf_netdev, ndev);
+	RCU_INIT_POINTER(vi->vf_netdev, NULL);
+	dev_put(vf_netdev);
+
+	return NOTIFY_OK;
+}
+
 static void virtnet_remove(struct virtio_device *vdev)
 {
 	struct virtnet_info *vi = vdev->priv;
+	struct net_device *vf_netdev;
 
 	virtnet_cpu_notif_remove(vi);
 
 	/* Make sure no work handler is accessing the device. */
 	flush_work(&vi->config_work);
 
+	rtnl_lock();
+	vf_netdev = rtnl_dereference(vi->vf_netdev);
+	if (vf_netdev)
+		virtnet_unregister_vf(vf_netdev);
+	rtnl_unlock();
+
 	unregister_netdev(vi->dev);
 
 	remove_vq_common(vi);
 
+	free_percpu(vi->vf_stats);
 	free_percpu(vi->stats);
 	free_netdev(vi->dev);
 }
@@ -2827,6 +3089,42 @@ static struct virtio_driver virtio_net_driver = {
 #endif
 };
 
+static int virtio_netdev_event(struct notifier_block *this,
+			       unsigned long event, void *ptr)
+{
+	struct net_device *event_dev = netdev_notifier_info_to_dev(ptr);
+
+	/* Skip our own events */
+	if (event_dev->netdev_ops == &virtnet_netdev)
+		return NOTIFY_DONE;
+
+	/* Avoid non-Ethernet type devices */
+	if (event_dev->type != ARPHRD_ETHER)
+		return NOTIFY_DONE;
+
+	/* Avoid Vlan dev with same MAC registering as VF */
+	if (is_vlan_dev(event_dev))
+		return NOTIFY_DONE;
+
+	/* Avoid Bonding master dev with same MAC registering as VF */
+	if ((event_dev->priv_flags & IFF_BONDING) &&
+	    (event_dev->flags & IFF_MASTER))
+		return NOTIFY_DONE;
+
+	switch (event) {
+	case NETDEV_REGISTER:
+		return virtnet_register_vf(event_dev);
+	case NETDEV_UNREGISTER:
+		return virtnet_unregister_vf(event_dev);
+	default:
+		return NOTIFY_DONE;
+	}
+}
+
+static struct notifier_block virtio_netdev_notifier = {
+	.notifier_call = virtio_netdev_event,
+};
+
 static __init int virtio_net_driver_init(void)
 {
 	int ret;
@@ -2845,6 +3143,8 @@ static __init int virtio_net_driver_init(void)
         ret = register_virtio_driver(&virtio_net_driver);
 	if (ret)
 		goto err_virtio;
+
+	register_netdevice_notifier(&virtio_netdev_notifier);
 	return 0;
 err_virtio:
 	cpuhp_remove_multi_state(CPUHP_VIRT_NET_DEAD);
@@ -2857,6 +3157,7 @@ module_init(virtio_net_driver_init);
 
 static __exit void virtio_net_driver_exit(void)
 {
+	unregister_netdevice_notifier(&virtio_netdev_notifier);
 	unregister_virtio_driver(&virtio_net_driver);
 	cpuhp_remove_multi_state(CPUHP_VIRT_NET_DEAD);
 	cpuhp_remove_multi_state(virtionet_online);
-- 
2.14.3

On Tue,  2 Jan 2018 16:35:36 -0800, Sridhar Samudrala
wrote:
> This patch series enables virtio to switch over to a VF datapath when a VF
> netdev is present with the same MAC address. It allows live migration of a
VM
> with a direct attached VF without the need to setup a bond/team between a
> VF and virtio net device in the guest.
> 
> The hypervisor needs to unplug the VF device from the guest on the source
> host and reset the MAC filter of the VF to initiate failover of datapath to
> virtio before starting the migration. After the migration is completed, the
> destination hypervisor sets the MAC filter on the VF and plugs it back to
> the guest to switch over to VF datapath.
> 
> It is based on netvsc implementation and it may be possible to make this
code
> generic and move it to a common location that can be shared by netvsc and
virtio.
> 
> This patch series is based on the discussion initiated by Jesse on this
thread.
> https://marc.info/?l=linux-virtualization&m=151189725224231&w=2
How does the notion of a device which is both a bond and a leg of a
bond fit with Alex's recent discussions about feature propagation?
Which propagation rules will apply to VirtIO master?  Meaning of the
flags on a software upper device may be different.  Why muddy the
architecture like this and not introduce a synthetic bond device?

From: Sridhar Samudrala <sridhar.samudrala at intel.com>
Date: Tue,  2 Jan 2018 16:35:37 -0800
> diff --git a/include/uapi/linux/virtio_net.h
b/include/uapi/linux/virtio_net.h
> index fc353b518288..a9b4e0836786 100644
> --- a/include/uapi/linux/virtio_net.h
> +++ b/include/uapi/linux/virtio_net.h
> @@ -56,6 +56,7 @@
>  #define VIRTIO_NET_F_MQ	22	/* Device supports Receive Flow
>  					 * Steering */
>  #define VIRTIO_NET_F_CTRL_MAC_ADDR 23	/* Set MAC address */
> +#define VIRTIO_NET_F_MASTER	62	/* act as master for a VF device */
Please respin this series with something in this commit message which
explains why we are using "62" rather than something like
"24" here.

Thank you.

On Tue, Jan 2, 2018 at 6:16 PM, Jakub Kicinski <kubakici at wp.pl>
wrote:
> On Tue,  2 Jan 2018 16:35:36 -0800, Sridhar Samudrala wrote:
>> This patch series enables virtio to switch over to a VF datapath when a
VF
>> netdev is present with the same MAC address. It allows live migration
of a VM
>> with a direct attached VF without the need to setup a bond/team between
a
>> VF and virtio net device in the guest.
>>
>> The hypervisor needs to unplug the VF device from the guest on the
source
>> host and reset the MAC filter of the VF to initiate failover of
datapath to
>> virtio before starting the migration. After the migration is completed,
the
>> destination hypervisor sets the MAC filter on the VF and plugs it back
to
>> the guest to switch over to VF datapath.
>>
>> It is based on netvsc implementation and it may be possible to make
this code
>> generic and move it to a common location that can be shared by netvsc
and virtio.
>>
>> This patch series is based on the discussion initiated by Jesse on this
thread.
>> https://marc.info/?l=linux-virtualization&m=151189725224231&w=2
>
> How does the notion of a device which is both a bond and a leg of a
> bond fit with Alex's recent discussions about feature propagation?
> Which propagation rules will apply to VirtIO master?  Meaning of the
> flags on a software upper device may be different.  Why muddy the
> architecture like this and not introduce a synthetic bond device?
It doesn't really fit with the notion I had. I think there may have
been a bit of a disconnect as I have been out for the last week or so
for the holidays.

My thought on this was that the feature bit should be spawning a new
para-virtual bond device and that bond should have the virto and the
VF as slaves. Also I thought there was some discussion about trying to
reuse as much of the netvsc code as possible for this so that we could
avoid duplication of effort and have the two drivers use the same
approach. It seems like it should be pretty straight forward since you
would have the feature bit in the case of virto, and netvsc just does
this sort of thing by default if I am not mistaken.

- Alex

On Tue, Jan 02, 2018 at 04:35:37PM -0800, Sridhar Samudrala
wrote:
> This feature bit can be used by hypervisor to indicate virtio_net device to
> act as a master for a directly attached passthru device with the same MAC
> address.
> 
> Signed-off-by: Sridhar Samudrala <sridhar.samudrala at intel.com>
> ---
>  drivers/net/virtio_net.c        | 3 ++-
>  include/uapi/linux/virtio_net.h | 1 +
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 6fb7b658a6cc..46844a1d9a62 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2796,7 +2796,8 @@ static struct virtio_device_id id_table[] = {
>  	VIRTIO_NET_F_CTRL_RX, VIRTIO_NET_F_CTRL_VLAN, \
>  	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_MTU, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS, \
> +	VIRTIO_NET_F_MASTER
>  
>  static unsigned int features[] = {
>  	VIRTNET_FEATURES,
> diff --git a/include/uapi/linux/virtio_net.h
b/include/uapi/linux/virtio_net.h
> index fc353b518288..a9b4e0836786 100644
> --- a/include/uapi/linux/virtio_net.h
> +++ b/include/uapi/linux/virtio_net.h
> @@ -56,6 +56,7 @@
>  #define VIRTIO_NET_F_MQ	22	/* Device supports Receive Flow
>  					 * Steering */
>  #define VIRTIO_NET_F_CTRL_MAC_ADDR 23	/* Set MAC address */
> +#define VIRTIO_NET_F_MASTER	62	/* act as master for a VF device */
Well the virtio hardware doesn't really know whether it's a master or
the slave of a bond. In fact quite possibly down the road we'll add a
virtual device on top as others seem to want to do - that other one will
be the master then.

And we do nothing do check it's a VF.

So maybe a better name would be

#define VIRTIO_NET_F_BACKUP	62	/* Act as backup for another device with the same
MAC */
>  
>  #ifndef VIRTIO_NET_NO_LEGACY
>  #define VIRTIO_NET_F_GSO	6	/* Host handles pkts w/ any GSO type */
> -- 
> 2.14.3