Re: [PATCH net v3 1/8] net: account for ingress-resolved virtio net hdr state

On Tue, Oct 20, 2026 at 03:27:46PM -0400, Willem de Bruijn wrote:

Michael S. Tsirkin wrote:

quoted

The following patch will add ingress resolution of uncommon virtio net
hdr checksum metadata (software checksum completion at ingress) which
adds performance overhead to the packet processing.

We should make it visible, so a hypothetical workload relying on
uncommon metadata can be distinguished from none at all. This way issues
in the parsing will be reported and not quietly pay the host-CPU
checksum overhead instead of offload.

Flip every call to virtio_net_hdr_to_skb to test for negative errno
only, and have tun count these events per device as rx_csum_sw, exposed
via ethtool -S.

Assisted-by: LLM
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/net/tap.c        |  4 ++--
 drivers/net/tun.c        | 41 ++++++++++++++++++++++++++++++++++++++--
 drivers/net/virtio_net.c |  2 +-
 net/packet/af_packet.c   |  4 ++--
 4 files changed, 44 insertions(+), 7 deletions(-)

diff --git a/drivers/net/tap.c b/drivers/net/tap.c
index ff67d99deb39..73e883394446 100644
--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c

@@ -649,7 +649,7 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
 
 	if (vnet_hdr_len) {
 		err = tun_vnet_hdr_to_skb(q->flags, skb, &vnet_hdr);
-		if (err) {
+		if (err < 0) {
 			rcu_read_unlock();
 			drop_reason = SKB_DROP_REASON_DEV_HDR;
 			goto err_kfree;

@@ -1084,7 +1084,7 @@ static int tap_get_user_xdp(struct tap_queue *q, struct xdp_buff *xdp)
 
 	if (vnet_hdr_len) {
 		err = tun_vnet_hdr_to_skb(q->flags, skb, gso);
-		if (err) {
+		if (err < 0) {
 			rcu_read_unlock();
 			goto err_kfree;
 		}

similar to tun, should tap count this event?

quoted

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 242899f7fd07..19cb443c89b0 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c

@@ -210,6 +210,7 @@ struct tun_struct {
 	u32 flow_count;
 	u32 rx_batched;
 	atomic_long_t rx_frame_errors;
+	atomic64_t rx_csum_sw;

is a normal (and cheaper on 32b platforms) atomic_t or atomic_long_t sufficient?

well it can overflow easily. i don't know that we care that a slow
path will be even a bit slower on these ancient platforms. correct
reporting seems more important.

quoted

 	struct bpf_prog __rcu *xdp_prog;
 	struct tun_prog __rcu *steering_prog;
 	struct tun_prog __rcu *filter_prog;

@@ -1935,11 +1936,16 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		break;
 	}
 
-	if (tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, &hdr)) {
+	err = tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, &hdr);
+	if (err < 0) {
 		atomic_long_inc(&tun->rx_frame_errors);
 		err = -EINVAL;
 		goto free_skb;
 	}
+	if (err > 0) {

} else if (err > 0) {

quoted

+		atomic64_inc(&tun->rx_csum_sw);
+		err = 0;
+	}
 
 	if ((tun->flags & TUN_TYPE_MASK) == IFF_TAP)
 		skb->protocol = eth_type_trans(skb, tun->dev);

@@ -2605,12 +2611,15 @@ static int tun_xdp_one(struct tun_struct *tun,
 	tnl_hdr = (struct virtio_net_hdr_v1_hash_tunnel *)gso;
 	skb->dev = tun->dev;
 	skb_set_network_header(skb, ETH_HLEN);
-	if (tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, tnl_hdr)) {
+	ret = tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, tnl_hdr);
+	if (ret < 0) {
 		atomic_long_inc(&tun->rx_frame_errors);
 		kfree_skb(skb);
 		ret = -EINVAL;
 		goto out;
 	}
+	if (ret > 0)

same

quoted

+		atomic64_inc(&tun->rx_csum_sw);

maybe reset ret = 0 in all these branches

quoted

 
 	skb->protocol = eth_type_trans(skb, tun->dev);
 	skb_reset_network_header(skb);

@@ -3739,7 +3748,35 @@ static void tun_get_channels(struct net_device *dev,
 	channels->max_combined = tun->flags & IFF_MULTI_QUEUE ? MAX_TAP_QUEUES : 1;
 }
 
+#define TUN_STATS_LEN	1
+static const char tun_ethtool_strings[TUN_STATS_LEN][ETH_GSTRING_LEN] = {
+	"rx_csum_sw",
+};
+
+static int tun_get_sset_count(struct net_device *dev, int sset)
+{
+	return sset == ETH_SS_STATS ? TUN_STATS_LEN : -EOPNOTSUPP;
+}
+
+static void tun_get_strings(struct net_device *dev, u32 stringset, u8 *data)
+{
+	if (stringset == ETH_SS_STATS)
+		memcpy(data, tun_ethtool_strings,
+		       sizeof(tun_ethtool_strings));
+}
+
+static void tun_get_ethtool_stats(struct net_device *dev,
+				  struct ethtool_stats *stats, u64 *data)
+{
+	struct tun_struct *tun = netdev_priv(dev);
+
+	data[0] = atomic64_read(&tun->rx_csum_sw);
+}
+
 static const struct ethtool_ops tun_ethtool_ops = {
+	.get_sset_count		= tun_get_sset_count,
+	.get_strings		= tun_get_strings,
+	.get_ethtool_stats	= tun_get_ethtool_stats,
 	.supported_coalesce_params = ETHTOOL_COALESCE_RX_MAX_FRAMES,
 	.get_drvinfo	= tun_get_drvinfo,
 	.get_msglevel	= tun_get_msglevel,

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index daab43ac92ce..2e6fd81df9c4 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -2519,7 +2519,7 @@ static void virtnet_receive_done(struct virtnet_info *vi, struct receive_queue *
 	skb_set_network_header(skb, ETH_HLEN);
 	if (virtio_net_hdr_tnl_to_skb(skb, &hdr->tnl_hdr, vi->rx_tnl,
 				      vi->rx_tnl_csum,
-				      virtio_is_little_endian(vi->vdev))) {
+				      virtio_is_little_endian(vi->vdev)) < 0) {
 		net_warn_ratelimited("%s: bad gso: type: %x, size: %u, flags %x tunnel %d tnl csum %d\n",
 				     dev->name, hdr->hdr.gso_type,
 				     hdr->hdr.gso_size, hdr->hdr.flags,

should virtio_net count these too? Or do we assume that devices are
more trustworthy, so unlikely to cause this issue?

With memory encryption, from an untrusted host?
i can add that.

quoted

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 52ae3d875e97..e6aec9ecfa99 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c

@@ -2913,7 +2913,7 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 		}
 
 		if (has_vnet_hdr) {
-			if (virtio_net_hdr_to_skb(skb, &vnet_hdr, vio_le())) {
+			if (virtio_net_hdr_to_skb(skb, &vnet_hdr, vio_le()) < 0) {
 				tp_len = -EINVAL;
 				goto tpacket_error;
 			}

@@ -3136,7 +3136,7 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
 
 	if (vnet_hdr_sz) {
 		err = virtio_net_hdr_to_skb(skb, &vnet_hdr, vio_le());
-		if (err)
+		if (err < 0)
 			goto out_free;
 		len += vnet_hdr_sz;
 	}

-- 
MST