On Tue, Oct 13, 2026 at 07:45:17PM +0200, Björn Töpel wrote: > "Michael S. Tsirkin" <[email protected]> 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 <[email protected]> > > 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
