Re: [PATCH net-next 4/4] net: account for ingress-resolved virtio net hdr state

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Oct 13, 2026 at 07:45:17PM +0200, Björn Töpel wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > Ingress resolution of uncommon virtio net hdr checksum metadata
> > (software checksum completion at ingress) adds performance overhead but
> > is currently invisible: no counter tracks it, so a hypothetical workload
> > relying on uncommon metadata cannot be distinguished from none at all,
> > and packets quietly pay the host-CPU checksum overhead instead of offload.
> >
> > Have virtio_net_hdr_to_skb() return 1 when it completes an uncommon
> > checksum in software, and flip every converter call site to test for
> > negative errno.  Have tun count these events per device as
> > rx_csum_sw and expose the counter via ethtool -S.
> >
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > Assisted-by: LLM
> > Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr")
> >
> > ---
> >  drivers/net/tap.c          |  4 ++--
> >  drivers/net/tun.c          | 44 ++++++++++++++++++++++++++++++++++++--
> >  drivers/net/virtio_net.c   |  2 +-
> >  include/linux/virtio_net.h | 12 +++++++++--
> >  net/packet/af_packet.c     |  4 ++--
> >  5 files changed, 57 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/net/tap.c b/drivers/net/tap.c
> > index ff67d99deb39e..73e8833944463 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;
> >  		}
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index 242899f7fd071..138ca8987dfc6 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;
> > +	atomic_long_t rx_csum_sw;
> >  	struct bpf_prog __rcu *xdp_prog;
> >  	struct tun_prog __rcu *steering_prog;
> >  	struct tun_prog __rcu *filter_prog;
> > @@ -1935,11 +1936,19 @@ 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)
> > +		atomic_long_inc(&tun->rx_csum_sw);
> > +	/* The status is consumed: keep err clear so later exit paths
> > +	 * (XDP drop via unlock_frags) do not return it to write() as
> > +	 * a short count.
> > +	 */
> > +	err = 0;
> >  
> >  	if ((tun->flags & TUN_TYPE_MASK) == IFF_TAP)
> >  		skb->protocol = eth_type_trans(skb, tun->dev);
> > @@ -2605,12 +2614,15 @@ 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)
> > +		atomic_long_inc(&tun->rx_csum_sw);
> >  
> >  	skb->protocol = eth_type_trans(skb, tun->dev);
> >  	skb_reset_network_header(skb);
> > @@ -3739,7 +3751,35 @@ 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] = atomic_long_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 daab43ac92ce4..2e6fd81df9c41 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,
> > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> > index b181183118b3a..d8447db58ac83 100644
> > --- a/include/linux/virtio_net.h
> > +++ b/include/linux/virtio_net.h
> > @@ -14,6 +14,14 @@
> >  #include <uapi/linux/tcp.h>
> >  #include <uapi/linux/virtio_net.h>
> >  
> > +/*
> > + * virtio_net_hdr_to_skb() and virtio_net_hdr_tnl_to_skb() return 0 on
> > + * success and a negative errno on failure.  On success they may also
> > + * return 1, indicating that an uncommon checksum was completed in
> > + * software at ingress rather than deferred to the egress path;
> > + * drivers can use this to account for such packets.
> > + */
> > +
> >  static inline bool virtio_net_hdr_match_proto(__be16 protocol, __u8 gso_type)
> >  {
> >  	switch (gso_type & ~VIRTIO_NET_HDR_GSO_ECN) {
> > @@ -292,7 +300,7 @@ static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb,
> >  
> >  		err = skb_checksum_help(skb);
> >  		skb->transport_header = old_transport;
> > -		return err;
> > +		return err ?: 1;
> >  	}
> >  
> >  	return 0;
> > @@ -441,7 +449,7 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
> >  	/* Let the basic parsing deal with plain GSO features. */
> >  	ret = __virtio_net_hdr_to_skb(skb, hdr, true,
> >  				      hdr->gso_type & ~gso_tunnel_type, false);
> > -	if (ret)
> > +	if (ret < 0)
> >  		return ret;
> >  
> >  	/* In case of USO, the inner protocol is still unknown and
> > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> > index 52ae3d875e971..e6aec9ecfa994 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) {
> 
> nit: I don't know if we care, but this sort of breaks bisectability.
> Previous patch does return ret ?: 1, but it's this patch that does the
> if-statement fixup. (Same for tun/tap... ^^^)
> 

Oh ugh. You are right, I kept rebasing to tweak things and moved it
by chance. I'll fix it up in v2.


> Björn