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Message-ID: <CACGkMEs4_Zs_YpjV7CfXu7ycZxgtxqyScTi5CULdz9ZrUoHW6w@mail.gmail.com>
Date: Tue, 24 Jun 2025 08:55:57 +0800
From: Jason Wang <jasowang@...hat.com>
To: Akihiko Odaki <akihiko.odaki@...nix.com>
Cc: Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
Willem de Bruijn <willemdebruijn.kernel@...il.com>, Andrew Lunn <andrew+netdev@...n.ch>,
"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, "Michael S. Tsirkin" <mst@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
Eugenio Pérez <eperezma@...hat.com>,
Yuri Benditovich <yuri.benditovich@...nix.com>
Subject: Re: [PATCH v4 net-next 7/8] tun: enable gso over UDP tunnel support.
On Fri, Jun 20, 2025 at 12:03 AM Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
>
> On 2025/06/18 1:12, Paolo Abeni wrote:
> > Add new tun features to represent the newly introduced virtio
> > GSO over UDP tunnel offload. Allows detection and selection of
> > such features via the existing TUNSETOFFLOAD ioctl and compute
> > the expected virtio header size and tunnel header offset using
> > the current netdev features, so that we can plug almost seamless
> > the newly introduced virtio helpers to serialize the extended
> > virtio header.
> >
> > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > ---
> > v3 -> v4:
> > - virtio tnl-related fields are at fixed offset, cleanup
> > the code accordingly.
> > - use netdev features instead of flags bit to check for
> > the configured offload
> > - drop packet in case of enabled features/configured hdr
> > size mismatch
> >
> > v2 -> v3:
> > - cleaned-up uAPI comments
> > - use explicit struct layout instead of raw buf.
> > ---
> > drivers/net/tun.c | 70 ++++++++++++++++++++++++-----
> > drivers/net/tun_vnet.h | 88 +++++++++++++++++++++++++++++++++----
> > include/uapi/linux/if_tun.h | 9 ++++
> > 3 files changed, 148 insertions(+), 19 deletions(-)
> >
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index f8c5e2fd04df..bae0370a8152 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -186,7 +186,8 @@ struct tun_struct {
> > struct net_device *dev;
> > netdev_features_t set_features;
> > #define TUN_USER_FEATURES (NETIF_F_HW_CSUM|NETIF_F_TSO_ECN|NETIF_F_TSO| \
> > - NETIF_F_TSO6 | NETIF_F_GSO_UDP_L4)
> > + NETIF_F_TSO6 | NETIF_F_GSO_UDP_L4 | \
> > + NETIF_F_GSO_UDP_TUNNEL | NETIF_F_GSO_UDP_TUNNEL_CSUM)
> >
> > int align;
> > int vnet_hdr_sz;
> > @@ -925,6 +926,7 @@ static int tun_net_init(struct net_device *dev)
> > dev->hw_features = NETIF_F_SG | NETIF_F_FRAGLIST |
> > TUN_USER_FEATURES | NETIF_F_HW_VLAN_CTAG_TX |
> > NETIF_F_HW_VLAN_STAG_TX;
> > + dev->hw_enc_features = dev->hw_features;
> > dev->features = dev->hw_features;
> > dev->vlan_features = dev->features &
> > ~(NETIF_F_HW_VLAN_CTAG_TX |
> > @@ -1698,7 +1700,8 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> > struct sk_buff *skb;
> > size_t total_len = iov_iter_count(from);
> > size_t len = total_len, align = tun->align, linear;
> > - struct virtio_net_hdr gso = { 0 };
> > + struct virtio_net_hdr_v1_hash_tunnel hdr;
> > + struct virtio_net_hdr *gso;
> > int good_linear;
> > int copylen;
> > int hdr_len = 0;
> > @@ -1708,6 +1711,15 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> > int skb_xdp = 1;
> > bool frags = tun_napi_frags_enabled(tfile);
> > enum skb_drop_reason drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
> > + netdev_features_t features = 0;
> > +
> > + /*
> > + * Keep it easy and always zero the whole buffer, even if the
> > + * tunnel-related field will be touched only when the feature
> > + * is enabled and the hdr size id compatible.
> > + */
> > + memset(&hdr, 0, sizeof(hdr));
> > + gso = (struct virtio_net_hdr *)&hdr;
> >
> > if (!(tun->flags & IFF_NO_PI)) {
> > if (len < sizeof(pi))
> > @@ -1721,7 +1733,12 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> > if (tun->flags & IFF_VNET_HDR) {
> > int vnet_hdr_sz = READ_ONCE(tun->vnet_hdr_sz);
> >
> > - hdr_len = tun_vnet_hdr_get(vnet_hdr_sz, tun->flags, from, &gso);
> > + if (vnet_hdr_sz >= TUN_VNET_TNL_SIZE)
> > + features = NETIF_F_GSO_UDP_TUNNEL |
> > + NETIF_F_GSO_UDP_TUNNEL_CSUM;
> > +
> > + hdr_len = __tun_vnet_hdr_get(vnet_hdr_sz, tun->flags,
> > + features, from, gso);
> > if (hdr_len < 0)
> > return hdr_len;
> >
> > @@ -1755,7 +1772,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> > * (e.g gso or jumbo packet), we will do it at after
> > * skb was created with generic XDP routine.
> > */
> > - skb = tun_build_skb(tun, tfile, from, &gso, len, &skb_xdp);
> > + skb = tun_build_skb(tun, tfile, from, gso, len, &skb_xdp);
> > err = PTR_ERR_OR_ZERO(skb);
> > if (err)
> > goto drop;
> > @@ -1799,7 +1816,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> > }
> > }
> >
> > - if (tun_vnet_hdr_to_skb(tun->flags, skb, &gso)) {
> > + if (tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, &hdr)) {
> > atomic_long_inc(&tun->rx_frame_errors);
> > err = -EINVAL;
> > goto free_skb;
> > @@ -2050,13 +2067,21 @@ static ssize_t tun_put_user(struct tun_struct *tun,
> > }
> >
> > if (vnet_hdr_sz) {
> > - struct virtio_net_hdr gso;
> > + struct virtio_net_hdr_v1_hash_tunnel hdr;
> > + struct virtio_net_hdr *gso;
> >
> > - ret = tun_vnet_hdr_from_skb(tun->flags, tun->dev, skb, &gso);
> > + ret = tun_vnet_hdr_tnl_from_skb(tun->flags, tun->dev, skb,
> > + &hdr);
> > if (ret)
> > return ret;
> >
> > - ret = tun_vnet_hdr_put(vnet_hdr_sz, iter, &gso);
> > + /*
> > + * Drop the packet if the configured header size is too small
> > + * WRT the enabled offloads.
> > + */
> > + gso = (struct virtio_net_hdr *)&hdr;
> > + ret = __tun_vnet_hdr_put(vnet_hdr_sz, tun->dev->features,
> > + iter, gso);
> > if (ret)
> > return ret;
> > }
> > @@ -2357,7 +2382,9 @@ static int tun_xdp_one(struct tun_struct *tun,
> > {
> > unsigned int datasize = xdp->data_end - xdp->data;
> > struct tun_xdp_hdr *hdr = xdp->data_hard_start;
> > + struct virtio_net_hdr_v1_hash_tunnel *tnl_hdr;
> > struct virtio_net_hdr *gso = &hdr->gso;
> > + netdev_features_t features = 0;
> > struct bpf_prog *xdp_prog;
> > struct sk_buff *skb = NULL;
> > struct sk_buff_head *queue;
> > @@ -2426,7 +2453,17 @@ static int tun_xdp_one(struct tun_struct *tun,
> > if (metasize > 0)
> > skb_metadata_set(skb, metasize);
> >
> > - if (tun_vnet_hdr_to_skb(tun->flags, skb, gso)) {
> > + /*
> > + * Assume tunnel offloads are enabled if the received hdr is large
> > + * enough.
> > + */
> > + if (READ_ONCE(tun->vnet_hdr_sz) >= TUN_VNET_TNL_SIZE &&
> > + xdp->data - xdp->data_hard_start >= TUN_VNET_TNL_SIZE)
> > + features = NETIF_F_GSO_UDP_TUNNEL |
> > + NETIF_F_GSO_UDP_TUNNEL_CSUM;
>
> xdp->data - xdp->data_hard_start may not represent the header size.
>
> struct tun_xdp_hdr is filled in vhost_net_build_xdp() in
> drivers/vhost/net.c. This function sets the two fields with
> xdp_prepare_buff(), but the arguments passed to xdp_prepare_buff() does
> not seem to represent the exact size of the header.
It is? In vhost_net_build_xdp() it has:
int pad = SKB_DATA_ALIGN(VHOST_NET_RX_PAD + headroom + nvq->sock_hlen);
Btw, I plan to remove tun_xdp_hdr and just use vnet header:
https://www.spinics.net/lists/netdev/msg1098039.html
Thanks
>
> > +
> > + tnl_hdr = (struct virtio_net_hdr_v1_hash_tunnel *)gso;
> > + if (tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, tnl_hdr)) {
> > atomic_long_inc(&tun->rx_frame_errors);
> > kfree_skb(skb);
> > ret = -EINVAL;
> > @@ -2812,6 +2849,8 @@ static void tun_get_iff(struct tun_struct *tun, struct ifreq *ifr)
> >
> > }
> >
> > +#define PLAIN_GSO (NETIF_F_GSO_UDP_L4 | NETIF_F_TSO | NETIF_F_TSO6)
> > +
> > /* This is like a cut-down ethtool ops, except done via tun fd so no
> > * privs required. */
> > static int set_offload(struct tun_struct *tun, unsigned long arg)
> > @@ -2841,6 +2880,18 @@ static int set_offload(struct tun_struct *tun, unsigned long arg)
> > features |= NETIF_F_GSO_UDP_L4;
> > arg &= ~(TUN_F_USO4 | TUN_F_USO6);
> > }
> > +
> > + /*
> > + * Tunnel offload is allowed only if some plain offload is
> > + * available, too.
> > + */
> > + if (features & PLAIN_GSO && arg & TUN_F_UDP_TUNNEL_GSO) {
> > + features |= NETIF_F_GSO_UDP_TUNNEL;
> > + if (arg & TUN_F_UDP_TUNNEL_GSO_CSUM)
> > + features |= NETIF_F_GSO_UDP_TUNNEL_CSUM;
> > + arg &= ~(TUN_F_UDP_TUNNEL_GSO |
> > + TUN_F_UDP_TUNNEL_GSO_CSUM);
> > + }
> > }
> >
> > /* This gives the user a way to test for new features in future by
> > @@ -2852,7 +2903,6 @@ static int set_offload(struct tun_struct *tun, unsigned long arg)
> > tun->dev->wanted_features &= ~TUN_USER_FEATURES;
> > tun->dev->wanted_features |= features;
> > netdev_update_features(tun->dev);
> > -
> > return 0;
> > }
> >
> > diff --git a/drivers/net/tun_vnet.h b/drivers/net/tun_vnet.h
> > index 58b9ac7a5fc4..7450fc153bb4 100644
> > --- a/drivers/net/tun_vnet.h
> > +++ b/drivers/net/tun_vnet.h
> > @@ -6,6 +6,8 @@
> > #define TUN_VNET_LE 0x80000000
> > #define TUN_VNET_BE 0x40000000
> >
> > +#define TUN_VNET_TNL_SIZE sizeof(struct virtio_net_hdr_v1_hash_tunnel)
> > +
> > static inline bool tun_vnet_legacy_is_little_endian(unsigned int flags)
> > {
> > bool be = IS_ENABLED(CONFIG_TUN_VNET_CROSS_LE) &&
> > @@ -107,16 +109,26 @@ static inline long tun_vnet_ioctl(int *vnet_hdr_sz, unsigned int *flags,
> > }
> > }
> >
> > -static inline int tun_vnet_hdr_get(int sz, unsigned int flags,
> > - struct iov_iter *from,
> > - struct virtio_net_hdr *hdr)
> > +static inline unsigned int tun_vnet_parse_size(netdev_features_t features)
> > +{
> > + if (!(features & NETIF_F_GSO_UDP_TUNNEL))
> > + return sizeof(struct virtio_net_hdr);
> > +
> > + return TUN_VNET_TNL_SIZE;
> > +}
> > +
> > +static inline int __tun_vnet_hdr_get(int sz, unsigned int flags,
> > + netdev_features_t features,
> > + struct iov_iter *from,
> > + struct virtio_net_hdr *hdr)
> > {
> > + unsigned int parsed_size = tun_vnet_parse_size(features);
> > u16 hdr_len;
> >
> > if (iov_iter_count(from) < sz)
> > return -EINVAL;
> >
> > - if (!copy_from_iter_full(hdr, sizeof(*hdr), from))
> > + if (!copy_from_iter_full(hdr, parsed_size, from))
> > return -EFAULT;
> >
> > hdr_len = tun_vnet16_to_cpu(flags, hdr->hdr_len);
> > @@ -129,32 +141,59 @@ static inline int tun_vnet_hdr_get(int sz, unsigned int flags,
> > if (hdr_len > iov_iter_count(from))
> > return -EINVAL;
> >
> > - iov_iter_advance(from, sz - sizeof(*hdr));
> > + iov_iter_advance(from, sz - parsed_size);
> >
> > return hdr_len;
> > }
> >
> > -static inline int tun_vnet_hdr_put(int sz, struct iov_iter *iter,
> > - const struct virtio_net_hdr *hdr)
> > +static inline int tun_vnet_hdr_get(int sz, unsigned int flags,
> > + struct iov_iter *from,
> > + struct virtio_net_hdr *hdr)
> > +{
> > + return __tun_vnet_hdr_get(sz, flags, 0, from, hdr);
> > +}
> > +
> > +static inline int __tun_vnet_hdr_put(int sz, netdev_features_t features,
> > + struct iov_iter *iter,
> > + const struct virtio_net_hdr *hdr)
> > {
> > + unsigned int parsed_size = tun_vnet_parse_size(features);
> > +
> > if (unlikely(iov_iter_count(iter) < sz))
> > return -EINVAL;
> >
> > - if (unlikely(copy_to_iter(hdr, sizeof(*hdr), iter) != sizeof(*hdr)))
> > + if (unlikely(copy_to_iter(hdr, parsed_size, iter) != parsed_size))
> > return -EFAULT;
> >
> > - if (iov_iter_zero(sz - sizeof(*hdr), iter) != sz - sizeof(*hdr))
> > + if (iov_iter_zero(sz - parsed_size, iter) != sz - parsed_size)
> > return -EFAULT;
> >
> > return 0;
> > }
> >
> > +static inline int tun_vnet_hdr_put(int sz, struct iov_iter *iter,
> > + const struct virtio_net_hdr *hdr)
> > +{
> > + return __tun_vnet_hdr_put(sz, 0, iter, hdr);
> > +}
> > +
> > static inline int tun_vnet_hdr_to_skb(unsigned int flags, struct sk_buff *skb,
> > const struct virtio_net_hdr *hdr)
> > {
> > return virtio_net_hdr_to_skb(skb, hdr, tun_vnet_is_little_endian(flags));
> > }
> >
> > +static inline int
> > +tun_vnet_hdr_tnl_to_skb(unsigned int flags, netdev_features_t features,
> > + struct sk_buff *skb,
> > + const struct virtio_net_hdr_v1_hash_tunnel *hdr)
> > +{
> > + return virtio_net_hdr_tnl_to_skb(skb, hdr,
> > + !!(features & NETIF_F_GSO_UDP_TUNNEL),
> > + !!(features & NETIF_F_GSO_UDP_TUNNEL_CSUM),
> > + tun_vnet_is_little_endian(flags));
> > +}
> > +
> > static inline int tun_vnet_hdr_from_skb(unsigned int flags,
> > const struct net_device *dev,
> > const struct sk_buff *skb,
> > @@ -183,4 +222,35 @@ static inline int tun_vnet_hdr_from_skb(unsigned int flags,
> > return 0;
> > }
> >
> > +static inline int
> > +tun_vnet_hdr_tnl_from_skb(unsigned int flags,
> > + const struct net_device *dev,
> > + const struct sk_buff *skb,
> > + struct virtio_net_hdr_v1_hash_tunnel *tnl_hdr)
> > +{
> > + bool has_tnl_offload = !!(dev->features & NETIF_F_GSO_UDP_TUNNEL);
> > + int vlan_hlen = skb_vlan_tag_present(skb) ? VLAN_HLEN : 0;
> > +
> > + if (virtio_net_hdr_tnl_from_skb(skb, tnl_hdr, has_tnl_offload,
> > + tun_vnet_is_little_endian(flags),
> > + vlan_hlen)) {
> > + struct virtio_net_hdr_v1 *hdr = &tnl_hdr->hash_hdr.hdr;
> > + struct skb_shared_info *sinfo = skb_shinfo(skb);
> > +
> > + if (net_ratelimit()) {
> > + netdev_err(dev, "unexpected GSO type: 0x%x, gso_size %d, hdr_len %d\n",
> > + sinfo->gso_type, tun_vnet16_to_cpu(flags, hdr->gso_size),
> > + tun_vnet16_to_cpu(flags, hdr->hdr_len));
> > + print_hex_dump(KERN_ERR, "tun: ",
> > + DUMP_PREFIX_NONE,
> > + 16, 1, skb->head,
> > + min(tun_vnet16_to_cpu(flags, hdr->hdr_len), 64), true);
> > + }
> > + WARN_ON_ONCE(1);
> > + return -EINVAL;
> > + }
> > +
> > + return 0;
> > +}
> > +
> > #endif /* TUN_VNET_H */
> > diff --git a/include/uapi/linux/if_tun.h b/include/uapi/linux/if_tun.h
> > index 287cdc81c939..79d53c7a1ebd 100644
> > --- a/include/uapi/linux/if_tun.h
> > +++ b/include/uapi/linux/if_tun.h
> > @@ -93,6 +93,15 @@
> > #define TUN_F_USO4 0x20 /* I can handle USO for IPv4 packets */
> > #define TUN_F_USO6 0x40 /* I can handle USO for IPv6 packets */
> >
> > +/* I can handle TSO/USO for UDP tunneled packets */
> > +#define TUN_F_UDP_TUNNEL_GSO 0x080
> > +
> > +/*
> > + * I can handle TSO/USO for UDP tunneled packets requiring csum offload for
> > + * the outer header
> > + */
> > +#define TUN_F_UDP_TUNNEL_GSO_CSUM 0x100
> > +
> > /* Protocol info prepended to the packets (when IFF_NO_PI is not set) */
> > #define TUN_PKT_STRIP 0x0001
> > struct tun_pi {
>
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