diff --git a/homa_qdisc.c b/homa_qdisc.c
index b84ef03c..12d174c2 100755
--- a/homa_qdisc.c
+++ b/homa_qdisc.c
@@ -1400,7 +1400,9 @@ void homa_qdev_update_sysctl(struct homa_qdisc_dev *qdev)
 	qdev->link_mbps = homa->link_mbps;
 	ops = qdev->dev->ethtool_ops;
 	if (ops && ops->get_link_ksettings) {
-		if (ops->get_link_ksettings(qdev->dev, &ksettings) == 0)
+		if (ops->get_link_ksettings(qdev->dev, &ksettings) == 0 &&
+		    ksettings.base.speed != 0 &&
+		    ksettings.base.speed != SPEED_UNKNOWN)
 			qdev->link_mbps = ksettings.base.speed;
 	}
 
diff --git a/test/unit_homa_qdisc.c b/test/unit_homa_qdisc.c
index 9ea5bb8f..1efa3250 100644
--- a/test/unit_homa_qdisc.c
+++ b/test/unit_homa_qdisc.c
@@ -2502,6 +2502,22 @@ TEST_F(homa_qdisc, homa_qdev_update_sysctl__cant_get_link_speed_from_dev)
 
 	homa_qdisc_qdev_put(qdev);
 }
+TEST_F(homa_qdisc, homa_qdev_update_sysctl__link_speed_unknown)
+{
+	struct homa_qdisc_dev *qdev;
+
+	qdev = homa_qdisc_qdev_get(self->dev);
+	EXPECT_FALSE(IS_ERR(qdev));
+
+	/* Some drivers, such as virtio_net, don't know the link speed. */
+	self->homa.link_mbps = 16000;
+	mock_link_mbps = SPEED_UNKNOWN;
+	homa_qdev_update_sysctl(qdev);
+	EXPECT_EQ(16000, qdev->link_mbps);
+	EXPECT_EQ(529583, qdev->cycles_per_mibyte);
+
+	homa_qdisc_qdev_put(qdev);
+}
 
 TEST_F(homa_qdisc, homa_qdisc_update_sysctl_deps__clamp_values)
 {