netdev - [PATCH net] virtio-net: Fix operstate for virtio when no VIRTIO_NET_F

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Date:   Thu, 22 Mar 2018 09:05:52 +0000
From:   Jay Vosburgh <jay.vosburgh@...onical.com>
To:     netdev@...r.kernel.org
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David Miller" <davem@...emloft.net>,
        Ben Hutchings <ben@...adent.org.uk>
Subject: [PATCH net] virtio-net: Fix operstate for virtio when no VIRTIO_NET_F_STATUS

	The operstate update logic will leave an interface in the
default UNKNOWN operstate if the interface carrier state never changes
from the default carrier up state set at creation.  This includes the
case of an explicit call to netif_carrier_on, as the carrier on to on
transition has no effect on operstate.

	This affects virtio-net for the case that the virtio peer does
not support VIRTIO_NET_F_STATUS (the feature that provides carrier state
updates).  Without this feature, the virtio specification states that
"the link should be assumed active," so, logically, the operstate should
be UP instead of UNKNOWN.  This has impact on user space applications
that use the operstate to make availability decisions for the interface.

	Resolve this by changing the virtio probe logic slightly to call
netif_carrier_off for both the "with" and "without" VIRTIO_NET_F_STATUS
cases, and then the existing call to netif_carrier_on for the "without"
case will cause an operstate transition.

Cc: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>
Cc: Ben Hutchings <ben@...adent.org.uk>
Fixes: 167c25e4c550 ("virtio-net: init link state correctly")
Signed-off-by: Jay Vosburgh <jay.vosburgh@...onical.com>

---

	I considered resolving this by changing linkwatch_init_dev to
unconditionally call rfc2863_policy, as that would always set operstate
for all interfaces.

	This would not have any impact on most cases (as most drivers
call netif_carrier_off during probe), except for the loopback device,
which currently has an operstate of UNKNOWN (because it never does any
carrier state transitions).  This change would add a round trip on the
dev_base_lock for every loopback device creation, which could have a
negative impact when creating many loopback devices, e.g., when
concurrently creating large numbers of containers.


 drivers/net/virtio_net.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 23374603e4d9..7b187ec7411e 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2857,8 +2857,8 @@ static int virtnet_probe(struct virtio_device *vdev)
 
 	/* Assume link up if device can't report link status,
 	   otherwise get link status from config. */
+	netif_carrier_off(dev);
 	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_STATUS)) {
-		netif_carrier_off(dev);
 		schedule_work(&vi->config_work);
 	} else {
 		vi->status = VIRTIO_NET_S_LINK_UP;
-- 
2.14.1