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Date:   Fri, 12 Aug 2022 18:44:59 +0800
From:   Zhu Lingshan <>
Cc:,,,,,, Zhu Lingshan <>
Subject: [PATCH V5 5/6] vDPA: Conditionally read fields in virtio-net dev config space

Some fields of virtio-net device config space are
conditional on the feature bits, the spec says:

"The mac address field always exists
(though is only valid if VIRTIO_NET_F_MAC is set)"

"max_virtqueue_pairs only exists if VIRTIO_NET_F_MQ
or VIRTIO_NET_F_RSS is set"

"mtu only exists if VIRTIO_NET_F_MTU is set"

so we should read MTU, MAC and MQ in the device config
space only when these feature bits are offered.

For MQ, if both VIRTIO_NET_F_MQ and VIRTIO_NET_F_RSS are
not set, the virtio device should have
one queue pair as default value, so when userspace querying queue pair numbers,
it should return mq=1 than zero.

For MTU, if VIRTIO_NET_F_MTU is not set, we should not read
MTU from the device config sapce.
RFC894 <A Standard for the Transmission of IP Datagrams over Ethernet Networks>
says:"The minimum length of the data field of a packet sent over an
Ethernet is 1500 octets, thus the maximum length of an IP datagram
sent over an Ethernet is 1500 octets.  Implementations are encouraged
to support full-length packets"

virtio spec says:"The virtio network device is a virtual ethernet card",
so the default MTU value should be 1500 for virtio-net.

For MAC, the spec says:"If the VIRTIO_NET_F_MAC feature bit is set,
the configuration space mac entry indicates the “physical” address
of the network card, otherwise the driver would typically
generate a random local MAC address." So there is no
default MAC address if VIRTIO_NET_F_MAC not set.

This commits introduces functions vdpa_dev_net_mtu_config_fill()
and vdpa_dev_net_mac_config_fill() to fill MTU and MAC.
It also fixes vdpa_dev_net_mq_config_fill() to report correct
MQ when _F_MQ is not present.

These functions should check devices features than driver
features, and struct vdpa_device is not needed as a parameter

The test & userspace tool output:

and VIRTIO_NET_F_MAC can be mask out by hardcode.

However, it is challenging to "disable" the related fields
in the HW device config space, so let's just assume the values
are meaningless if the feature bits are not set.

Before this change, when feature bits for RSS, MQ, MTU and MAC
are not set, iproute2 output:
$vdpa vdpa0: mac 00:e8:ca:11:be:05 link up link_announce false mtu 1500

without this commit, function vdpa_dev_net_config_fill()
reads all config space fields unconditionally, so let's
assume the MAC and MTU are meaningless, and it checks
MQ with driver_features, so we don't see max_vq_pairs.

After applying this commit, when feature bits for
MQ, RSS, MAC and MTU are not set 0,iproute2 output:
$vdpa dev config show vdpa0
vdpa0: link up link_announce false max_vq_pairs 1 mtu 1500

As explained above:
Here is no MAC, because VIRTIO_NET_F_MAC is not set,
and there is no default value for MAC. It shows
max_vq_paris = 1 because even without MQ feature,
a functional virtio-net must have one queue pair.
mtu = 1500 is the default value as ethernet

Signed-off-by: Zhu Lingshan <>
 drivers/vdpa/vdpa.c | 51 ++++++++++++++++++++++++++++++++-------------
 1 file changed, 37 insertions(+), 14 deletions(-)

diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
index bf312d9c59ab..f6172c8dd262 100644
--- a/drivers/vdpa/vdpa.c
+++ b/drivers/vdpa/vdpa.c
@@ -801,19 +801,44 @@ static int vdpa_nl_cmd_dev_get_dumpit(struct sk_buff *msg, struct netlink_callba
 	return msg->len;
-static int vdpa_dev_net_mq_config_fill(struct vdpa_device *vdev,
-				       struct sk_buff *msg, u64 features,
+static int vdpa_dev_net_mq_config_fill(struct sk_buff *msg, u64 features,
 				       const struct virtio_net_config *config)
 	u16 val_u16;
-	if ((features & BIT_ULL(VIRTIO_NET_F_MQ)) == 0)
-		return 0;
+	if ((features & BIT_ULL(VIRTIO_NET_F_MQ)) == 0 &&
+	    (features & BIT_ULL(VIRTIO_NET_F_RSS)) == 0)
+		val_u16 = 1;
+	else
+		val_u16 = __virtio16_to_cpu(true, config->max_virtqueue_pairs);
-	val_u16 = le16_to_cpu(config->max_virtqueue_pairs);
 	return nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MAX_VQP, val_u16);
+static int vdpa_dev_net_mtu_config_fill(struct sk_buff *msg, u64 features,
+					const struct virtio_net_config *config)
+	u16 val_u16;
+	if ((features & BIT_ULL(VIRTIO_NET_F_MTU)) == 0)
+		val_u16 = 1500;
+	else
+		val_u16 = __virtio16_to_cpu(true, config->mtu);
+	return nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MTU, val_u16);
+static int vdpa_dev_net_mac_config_fill(struct sk_buff *msg, u64 features,
+					const struct virtio_net_config *config)
+	if ((features & BIT_ULL(VIRTIO_NET_F_MAC)) == 0)
+		return 0;
+	else
+		return  nla_put(msg, VDPA_ATTR_DEV_NET_CFG_MACADDR,
+				sizeof(config->mac), config->mac);
 static int vdpa_dev_net_config_fill(struct vdpa_device *vdev, struct sk_buff *msg)
 	struct virtio_net_config config = {};
@@ -822,18 +847,10 @@ static int vdpa_dev_net_config_fill(struct vdpa_device *vdev, struct sk_buff *ms
 	vdpa_get_config_unlocked(vdev, 0, &config, sizeof(config));
-	if (nla_put(msg, VDPA_ATTR_DEV_NET_CFG_MACADDR, sizeof(config.mac),
-		    config.mac))
-		return -EMSGSIZE;
 	val_u16 = le16_to_cpu(config.status);
 	if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_STATUS, val_u16))
 		return -EMSGSIZE;
-	val_u16 = le16_to_cpu(config.mtu);
-	if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MTU, val_u16))
-		return -EMSGSIZE;
 	features_driver = vdev->config->get_driver_features(vdev);
 	if (nla_put_u64_64bit(msg, VDPA_ATTR_DEV_NEGOTIATED_FEATURES, features_driver,
@@ -846,7 +863,13 @@ static int vdpa_dev_net_config_fill(struct vdpa_device *vdev, struct sk_buff *ms
 		return -EMSGSIZE;
-	return vdpa_dev_net_mq_config_fill(vdev, msg, features_driver, &config);
+	if (vdpa_dev_net_mac_config_fill(msg, features_device, &config))
+		return -EMSGSIZE;
+	if (vdpa_dev_net_mtu_config_fill(msg, features_device, &config))
+		return -EMSGSIZE;
+	return vdpa_dev_net_mq_config_fill(msg, features_device, &config);
 static int

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