Message-ID: <64c0369eadbd5_3fe1bc2940@willemb.c.googlers.com.notmuch>
Date: Tue, 25 Jul 2023 16:54:54 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Stanislav Fomichev <sdf@...gle.com>, 
 bpf@...r.kernel.org
Cc: ast@...nel.org, 
 daniel@...earbox.net, 
 andrii@...nel.org, 
 martin.lau@...ux.dev, 
 song@...nel.org, 
 yhs@...com, 
 john.fastabend@...il.com, 
 kpsingh@...nel.org, 
 sdf@...gle.com, 
 haoluo@...gle.com, 
 jolsa@...nel.org, 
 kuba@...nel.org, 
 toke@...nel.org, 
 willemb@...gle.com, 
 dsahern@...nel.org, 
 magnus.karlsson@...el.com, 
 bjorn@...nel.org, 
 maciej.fijalkowski@...el.com, 
 hawk@...nel.org, 
 netdev@...r.kernel.org, 
 xdp-hints@...-project.net
Subject: RE: [RFC net-next v4 2/8] xsk: add TX timestamp and TX checksum
 offload support

Stanislav Fomichev wrote:
> This change actually defines the (initial) metadata layout
> that should be used by AF_XDP userspace (xsk_tx_metadata).
> The first field is flags which requests appropriate offloads,
> followed by the offload-specific fields. The supported per-device
> offloads are exported via netlink (new xsk-flags).
> 
> The offloads themselves are still implemented in a bit of a
> framework-y fashion that's left from my initial kfunc attempt.
> I'm introducing new xsk_tx_metadata_ops which drivers are
> supposed to implement. The drivers are also supposed
> to call xsk_tx_metadata_request/xsk_tx_metadata_complete in
> the right places. Since xsk_tx_metadata_{request,_complete}
> are static inline, we don't incur any extra overhead doing
> indirect calls.
> 
> The benefit of this scheme is as follows:
> - keeps all metadata layout parsing away from driver code
> - makes it easy to grep and see which drivers implement what
> - don't need any extra flags to maintain to keep track of that
>   offloads are implemented; if the callback is implemented - the offload
>   is supported (used by netlink reporting code)
> 
> Two offloads are defined right now:
> 1. XDP_TX_METADATA_CHECKSUM: skb-style csum_start+csum_offset
> 2. XDP_TX_METADATA_TIMESTAMP: writes TX timestamp back into metadata
>    area upon completion (tx_timestamp field)
> 
> The offloads are also implemented for copy mode:
> 1. Extra XDP_TX_METADATA_CHECKSUM_SW to trigger skb_checksum_help; this
>    might be useful as a reference implementation and for testing
> 2. XDP_TX_METADATA_TIMESTAMP writes SW timestamp from the skb
>    destructor (note I'm reusing hwtstamps to pass metadata pointer)
> 
> The struct is forward-compatible and can be extended in the future
> by appending more fields.
> 
> Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> ---
>  Documentation/netlink/specs/netdev.yaml | 19 ++++++++
>  include/linux/netdevice.h               | 27 +++++++++++
>  include/linux/skbuff.h                  |  5 ++-
>  include/net/xdp_sock.h                  | 60 +++++++++++++++++++++++++
>  include/net/xdp_sock_drv.h              | 13 ++++++
>  include/uapi/linux/if_xdp.h             | 35 +++++++++++++++
>  include/uapi/linux/netdev.h             | 15 +++++++
>  net/core/netdev-genl.c                  | 12 ++++-
>  net/xdp/xsk.c                           | 38 ++++++++++++++++
>  net/xdp/xsk_queue.h                     |  2 +-
>  tools/include/uapi/linux/if_xdp.h       | 50 ++++++++++++++++++---
>  11 files changed, 268 insertions(+), 8 deletions(-)
> 
> diff --git a/Documentation/netlink/specs/netdev.yaml b/Documentation/netlink/specs/netdev.yaml
> index e41015310a6e..bf9c1cc32614 100644
> --- a/Documentation/netlink/specs/netdev.yaml
> +++ b/Documentation/netlink/specs/netdev.yaml
> @@ -42,6 +42,19 @@ name: netdev
>          doc:
>            This feature informs if netdev implements non-linear XDP buffer
>            support in ndo_xdp_xmit callback.
> +  -
> +    type: flags
> +    name: xsk-flags
> +    render-max: true
> +    entries:
> +      -
> +        name: tx-timestamp
> +        doc:
> +          HW timestamping egress packets is supported by the driver.
> +      -
> +        name: tx-checksum
> +        doc:
> +          L3 checksum HW offload is supported by the driver.
>  
>  attribute-sets:
>    -
> @@ -68,6 +81,12 @@ name: netdev
>          type: u32
>          checks:
>            min: 1
> +      -
> +        name: xsk-features
> +        doc: Bitmask of enabled AF_XDP features.
> +        type: u64
> +        enum: xsk-flags
> +        enum-as-flags: true
>  
>  operations:
>    list:
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 11652e464f5d..8b40c80557aa 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -1660,6 +1660,31 @@ struct xdp_metadata_ops {
>  			       enum xdp_rss_hash_type *rss_type);
>  };
>  
> +/*
> + * This structure defines the AF_XDP TX metadata hooks for network devices.
> + * The following hooks can be defined; unless noted otherwise, they are
> + * optional and can be filled with a null pointer.
> + *
> + * int (*tmo_request_timestamp)(void *priv)
> + *     This function is called when AF_XDP frame requested egress timestamp.
> + *
> + * int (*tmo_fill_timestamp)(void *priv)
> + *     This function is called when AF_XDP frame, that had requested
> + *     egress timestamp, received a completion. The hook needs to return
> + *     the actual HW timestamp.
> + *
> + * int (*tmo_request_timestamp)(u16 csum_start, u16 csum_offset, void *priv)
> + *     This function is called when AF_XDP frame requested HW checksum
> + *     offload. csum_start indicates position where checksumming should start.
> + *     csum_offset indicates position where checksum should be stored.
> + *
> + */
> +struct xsk_tx_metadata_ops {
> +	void	(*tmo_request_timestamp)(void *priv);
> +	u64	(*tmo_fill_timestamp)(void *priv);
> +	void	(*tmo_request_checksum)(u16 csum_start, u16 csum_offset, void *priv);
> +};
> +
>  /**
>   * enum netdev_priv_flags - &struct net_device priv_flags
>   *
> @@ -1844,6 +1869,7 @@ enum netdev_ml_priv_type {
>   *	@netdev_ops:	Includes several pointers to callbacks,
>   *			if one wants to override the ndo_*() functions
>   *	@xdp_metadata_ops:	Includes pointers to XDP metadata callbacks.
> + *	@xsk_tx_metadata_ops:	Includes pointers to AF_XDP TX metadata callbacks.
>   *	@ethtool_ops:	Management operations
>   *	@l3mdev_ops:	Layer 3 master device operations
>   *	@ndisc_ops:	Includes callbacks for different IPv6 neighbour
> @@ -2100,6 +2126,7 @@ struct net_device {
>  	unsigned long long	priv_flags;
>  	const struct net_device_ops *netdev_ops;
>  	const struct xdp_metadata_ops *xdp_metadata_ops;
> +	const struct xsk_tx_metadata_ops *xsk_tx_metadata_ops;
>  	int			ifindex;
>  	unsigned short		gflags;
>  	unsigned short		hard_header_len;
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index faaba050f843..5febc1a5131e 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -581,7 +581,10 @@ struct skb_shared_info {
>  	/* Warning: this field is not always filled in (UFO)! */
>  	unsigned short	gso_segs;
>  	struct sk_buff	*frag_list;
> -	struct skb_shared_hwtstamps hwtstamps;
> +	union {
> +		struct skb_shared_hwtstamps hwtstamps;
> +		struct xsk_tx_metadata *xsk_meta;
> +	};
>  	unsigned int	gso_type;
>  	u32		tskey;
>  
> diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
> index 467b9fb56827..288fa58c4665 100644
> --- a/include/net/xdp_sock.h
> +++ b/include/net/xdp_sock.h
> @@ -90,6 +90,54 @@ int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp);
>  int __xsk_map_redirect(struct xdp_sock *xs, struct xdp_buff *xdp);
>  void __xsk_map_flush(void);
>  
> +/**
> + *  xsk_tx_metadata_request - Evaluate AF_XDP TX metadata at submission
> + *  and call appropriate xsk_tx_metadata_ops operation.
> + *  @meta: pointer to AF_XDP metadata area
> + *  @ops: pointer to struct xsk_tx_metadata_ops
> + *  @priv: pointer to driver-private aread
> + *
> + *  This function should be called by the networking device when
> + *  it prepares AF_XDP egress packet.
> + */
> +static inline void xsk_tx_metadata_request(const struct xsk_tx_metadata *meta,
> +					   const struct xsk_tx_metadata_ops *ops,
> +					   void *priv)
> +{
> +	if (!meta)
> +		return;
> +
> +	if (ops->tmo_request_timestamp)
> +		if (meta->flags & XDP_TX_METADATA_TIMESTAMP)
> +			ops->tmo_request_timestamp(priv);
> +
> +	if (ops->tmo_request_checksum)
> +		if (meta->flags & XDP_TX_METADATA_CHECKSUM)
> +			ops->tmo_request_checksum(meta->csum_start, meta->csum_offset, priv);
> +}
> +
> +/**
> + *  xsk_tx_metadata_complete - Evaluate AF_XDP TX metadata at completion
> + *  and call appropriate xsk_tx_metadata_ops operation.
> + *  @meta: pointer to AF_XDP metadata area
> + *  @ops: pointer to struct xsk_tx_metadata_ops
> + *  @priv: pointer to driver-private aread
> + *
> + *  This function should be called by the networking device upon
> + *  AF_XDP egress completion.
> + */
> +static inline void xsk_tx_metadata_complete(struct xsk_tx_metadata *meta,
> +					    const struct xsk_tx_metadata_ops *ops,
> +					    void *priv)
> +{
> +	if (!meta)
> +		return;
> +
> +	if (ops->tmo_fill_timestamp)
> +		if (meta->flags & XDP_TX_METADATA_TIMESTAMP)
> +			meta->tx_timestamp = ops->tmo_fill_timestamp(priv);
> +}
> +
>  #else
>  
>  static inline int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
> @@ -106,6 +154,18 @@ static inline void __xsk_map_flush(void)
>  {
>  }
>  
> +static inline void xsk_tx_metadata_request(struct xsk_tx_metadata *meta,
> +					   const struct xsk_tx_metadata_ops *ops,
> +					   void *priv)
> +{
> +}
> +
> +static inline void xsk_tx_metadata_complete(struct xsk_tx_metadata *meta,
> +					    const struct xsk_tx_metadata_ops *ops,
> +					    void *priv)
> +{
> +}
> +
>  #endif /* CONFIG_XDP_SOCKETS */
>  
>  #endif /* _LINUX_XDP_SOCK_H */
> diff --git a/include/net/xdp_sock_drv.h b/include/net/xdp_sock_drv.h
> index 1f6fc8c7a84c..e2558ac3e195 100644
> --- a/include/net/xdp_sock_drv.h
> +++ b/include/net/xdp_sock_drv.h
> @@ -165,6 +165,14 @@ static inline void *xsk_buff_raw_get_data(struct xsk_buff_pool *pool, u64 addr)
>  	return xp_raw_get_data(pool, addr);
>  }
>  
> +static inline struct xsk_tx_metadata *xsk_buff_get_metadata(struct xsk_buff_pool *pool, u64 addr)
> +{
> +	if (!pool->tx_metadata_len)
> +		return NULL;
> +
> +	return xp_raw_get_data(pool, addr) - pool->tx_metadata_len;
> +}
> +
>  static inline void xsk_buff_dma_sync_for_cpu(struct xdp_buff *xdp, struct xsk_buff_pool *pool)
>  {
>  	struct xdp_buff_xsk *xskb = container_of(xdp, struct xdp_buff_xsk, xdp);
> @@ -324,6 +332,11 @@ static inline void *xsk_buff_raw_get_data(struct xsk_buff_pool *pool, u64 addr)
>  	return NULL;
>  }
>  
> +static inline struct xsk_tx_metadata *xsk_buff_get_metadata(struct xsk_buff_pool *pool, u64 addr)
> +{
> +	return NULL;
> +}
> +
>  static inline void xsk_buff_dma_sync_for_cpu(struct xdp_buff *xdp, struct xsk_buff_pool *pool)
>  {
>  }
> diff --git a/include/uapi/linux/if_xdp.h b/include/uapi/linux/if_xdp.h
> index b37b50102e1c..b9b1b2c4108a 100644
> --- a/include/uapi/linux/if_xdp.h
> +++ b/include/uapi/linux/if_xdp.h
> @@ -106,6 +106,38 @@ struct xdp_options {
>  #define XSK_UNALIGNED_BUF_ADDR_MASK \
>  	((1ULL << XSK_UNALIGNED_BUF_OFFSET_SHIFT) - 1)
>  
> +/* Request transmit timestamp. Upon completion, put it into tx_timestamp
> + * field of struct xsk_tx_metadata.
> + */
> +#define XDP_TX_METADATA_TIMESTAMP		(1 << 0)
> +
> +/* Request transmit checksum offload. Checksum start position and offset
> + * are communicated via csum_start and csum_offset fields of struct
> + * xsk_tx_metadata.
> + */
> +#define XDP_TX_METADATA_CHECKSUM		(1 << 1)
> +
> +/* Force checksum calculation in software. Can be used for testing or
> + * working around potential HW issues. This option causes performance
> + * degradation and only works in XDP_COPY mode.
> + */
> +#define XDP_TX_METADATA_CHECKSUM_SW		(1 << 2)
> +
> +struct xsk_tx_metadata {
> +	__u32 flags;
> +
> +	/* XDP_TX_METADATA_CHECKSUM */
> +
> +	/* Offset from desc->addr where checksumming should start. */
> +	__u16 csum_start;
> +	/* Offset from csum_start where checksum should be stored. */
> +	__u16 csum_offset;
> +
> +	/* XDP_TX_METADATA_TIMESTAMP */
> +
> +	__u64 tx_timestamp;
> +};
> +
>  /* Rx/Tx descriptor */
>  struct xdp_desc {
>  	__u64 addr;
> @@ -122,4 +154,7 @@ struct xdp_desc {
>   */
>  #define XDP_PKT_CONTD (1 << 0)
>  
> +/* TX packet carries valid metadata. */
> +#define XDP_TX_METADATA (1 << 1)
> +
>  #endif /* _LINUX_IF_XDP_H */
> diff --git a/include/uapi/linux/netdev.h b/include/uapi/linux/netdev.h
> index bf71698a1e82..cf1e11c76339 100644
> --- a/include/uapi/linux/netdev.h
> +++ b/include/uapi/linux/netdev.h
> @@ -37,11 +37,26 @@ enum netdev_xdp_act {
>  	NETDEV_XDP_ACT_MASK = 127,
>  };
>  
> +/**
> + * enum netdev_xsk_flags
> + * @NETDEV_XSK_FLAGS_TX_TIMESTAMP: HW timestamping egress packets is supported
> + *   by the driver.
> + * @NETDEV_XSK_FLAGS_TX_CHECKSUM: L3 checksum HW offload is supported by the
> + *   driver.
> + */
> +enum netdev_xsk_flags {
> +	NETDEV_XSK_FLAGS_TX_TIMESTAMP = 1,
> +	NETDEV_XSK_FLAGS_TX_CHECKSUM = 2,
> +
> +	NETDEV_XSK_FLAGS_MASK = 3,
> +};
> +
>  enum {
>  	NETDEV_A_DEV_IFINDEX = 1,
>  	NETDEV_A_DEV_PAD,
>  	NETDEV_A_DEV_XDP_FEATURES,
>  	NETDEV_A_DEV_XDP_ZC_MAX_SEGS,
> +	NETDEV_A_DEV_XSK_FEATURES,
>  
>  	__NETDEV_A_DEV_MAX,
>  	NETDEV_A_DEV_MAX = (__NETDEV_A_DEV_MAX - 1)
> diff --git a/net/core/netdev-genl.c b/net/core/netdev-genl.c
> index 65ef4867fc49..9e8c1f3caf36 100644
> --- a/net/core/netdev-genl.c
> +++ b/net/core/netdev-genl.c
> @@ -12,15 +12,25 @@ static int
>  netdev_nl_dev_fill(struct net_device *netdev, struct sk_buff *rsp,
>  		   u32 portid, u32 seq, int flags, u32 cmd)
>  {
> +	u64 xsk_flags = 0;
>  	void *hdr;
>  
>  	hdr = genlmsg_put(rsp, portid, seq, &netdev_nl_family, flags, cmd);
>  	if (!hdr)
>  		return -EMSGSIZE;
>  
> +	if (netdev->xsk_tx_metadata_ops) {
> +		if (netdev->xsk_tx_metadata_ops->tmo_fill_timestamp)
> +			xsk_flags |= NETDEV_XSK_FLAGS_TX_TIMESTAMP;
> +		if (netdev->xsk_tx_metadata_ops->tmo_request_checksum)
> +			xsk_flags |= NETDEV_XSK_FLAGS_TX_CHECKSUM;
> +	}
> +
>  	if (nla_put_u32(rsp, NETDEV_A_DEV_IFINDEX, netdev->ifindex) ||
>  	    nla_put_u64_64bit(rsp, NETDEV_A_DEV_XDP_FEATURES,
> -			      netdev->xdp_features, NETDEV_A_DEV_PAD)) {
> +			      netdev->xdp_features, NETDEV_A_DEV_PAD) ||
> +	    nla_put_u64_64bit(rsp, NETDEV_A_DEV_XSK_FEATURES,
> +			      xsk_flags, NETDEV_A_DEV_PAD)) {
>  		genlmsg_cancel(rsp, hdr);
>  		return -EINVAL;
>  	}
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 81106e4d6e0f..9a5c4e63898d 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -542,6 +542,15 @@ static u32 xsk_get_num_desc(struct sk_buff *skb)
>  
>  static void xsk_destruct_skb(struct sk_buff *skb)
>  {
> +	struct xsk_tx_metadata *meta = skb_shinfo(skb)->xsk_meta;
> +
> +	if (meta) {
> +		if (meta->flags & XDP_TX_METADATA_TIMESTAMP) {
> +			/* sw completion timestamp, not a real one */
> +			meta->tx_timestamp = ktime_get_tai_fast_ns();
> +		}
> +	}
> +
>  	xsk_cq_submit_locked(xdp_sk(skb->sk), xsk_get_num_desc(skb));
>  	sock_wfree(skb);
>  }
> @@ -626,6 +635,7 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
>  static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>  				     struct xdp_desc *desc)
>  {
> +	struct xsk_tx_metadata *meta = NULL;
>  	struct net_device *dev = xs->dev;
>  	struct sk_buff *skb = xs->skb;
>  	int err;
> @@ -678,12 +688,40 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>  
>  			skb_add_rx_frag(skb, nr_frags, page, 0, len, 0);
>  		}
> +
> +		if (desc->options & XDP_TX_METADATA) {
> +			if (unlikely(xs->tx_metadata_len == 0)) {
> +				err = -EINVAL;
> +				goto free_err;
> +			}
> +
> +			meta = buffer - xs->tx_metadata_len;
> +
> +			if (meta->flags & XDP_TX_METADATA_CHECKSUM) {
> +				if (unlikely(meta->csum_start + meta->csum_offset +
> +					     sizeof(__sum16) > len)) {
> +					err = -EINVAL;
> +					goto free_err;
> +				}
> +
> +				skb->csum_start = hr + meta->csum_start;
> +				skb->csum_offset = meta->csum_offset;
> +				skb->ip_summed = CHECKSUM_PARTIAL;
> +
> +				if (unlikely(meta->flags & XDP_TX_METADATA_CHECKSUM_SW)) {
> +					err = skb_checksum_help(skb);
> +					if (err)
> +						goto free_err;
> +				}
> +			}
> +		}
>  	}
>  
>  	skb->dev = dev;
>  	skb->priority = xs->sk.sk_priority;
>  	skb->mark = xs->sk.sk_mark;
>  	skb->destructor = xsk_destruct_skb;
> +	skb_shinfo(skb)->xsk_meta = meta;
>  	xsk_set_destructor_arg(skb);
>  
>  	return skb;
> diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h
> index c74a1372bcb9..6f2d1621c992 100644
> --- a/net/xdp/xsk_queue.h
> +++ b/net/xdp/xsk_queue.h
> @@ -137,7 +137,7 @@ static inline bool xskq_cons_read_addr_unchecked(struct xsk_queue *q, u64 *addr)
>  
>  static inline bool xp_unused_options_set(u32 options)
>  {
> -	return options & ~XDP_PKT_CONTD;
> +	return options & ~(XDP_PKT_CONTD | XDP_TX_METADATA);
>  }
>  
>  static inline bool xp_aligned_validate_desc(struct xsk_buff_pool *pool,
> diff --git a/tools/include/uapi/linux/if_xdp.h b/tools/include/uapi/linux/if_xdp.h
> index 73a47da885dc..b9b1b2c4108a 100644
> --- a/tools/include/uapi/linux/if_xdp.h
> +++ b/tools/include/uapi/linux/if_xdp.h
> @@ -26,11 +26,11 @@
>   */
>  #define XDP_USE_NEED_WAKEUP (1 << 3)
>  /* By setting this option, userspace application indicates that it can
> - * handle multiple descriptors per packet thus enabling xsk core to split
> + * handle multiple descriptors per packet thus enabling AF_XDP to split
>   * multi-buffer XDP frames into multiple Rx descriptors. Without this set
> - * such frames will be dropped by xsk.
> + * such frames will be dropped.
>   */
> -#define XDP_USE_SG     (1 << 4)
> +#define XDP_USE_SG	(1 << 4)
>  
>  /* Flags for xsk_umem_config flags */
>  #define XDP_UMEM_UNALIGNED_CHUNK_FLAG (1 << 0)
> @@ -69,6 +69,7 @@ struct xdp_mmap_offsets {
>  #define XDP_UMEM_COMPLETION_RING	6
>  #define XDP_STATISTICS			7
>  #define XDP_OPTIONS			8
> +#define XDP_TX_METADATA_LEN		9
>  
>  struct xdp_umem_reg {
>  	__u64 addr; /* Start of packet data area */
> @@ -105,6 +106,38 @@ struct xdp_options {
>  #define XSK_UNALIGNED_BUF_ADDR_MASK \
>  	((1ULL << XSK_UNALIGNED_BUF_OFFSET_SHIFT) - 1)
>  
> +/* Request transmit timestamp. Upon completion, put it into tx_timestamp
> + * field of struct xsk_tx_metadata.
> + */
> +#define XDP_TX_METADATA_TIMESTAMP		(1 << 0)
> +
> +/* Request transmit checksum offload. Checksum start position and offset
> + * are communicated via csum_start and csum_offset fields of struct
> + * xsk_tx_metadata.
> + */
> +#define XDP_TX_METADATA_CHECKSUM		(1 << 1)
> +
> +/* Force checksum calculation in software. Can be used for testing or
> + * working around potential HW issues. This option causes performance
> + * degradation and only works in XDP_COPY mode.
> + */
> +#define XDP_TX_METADATA_CHECKSUM_SW		(1 << 2)

Not sure how useful this is, especially if only for copy mode.

> +struct xsk_tx_metadata {
> +	__u32 flags;
> +
> +	/* XDP_TX_METADATA_CHECKSUM */
> +
> +	/* Offset from desc->addr where checksumming should start. */
> +	__u16 csum_start;
> +	/* Offset from csum_start where checksum should be stored. */
> +	__u16 csum_offset;
> +
> +	/* XDP_TX_METADATA_TIMESTAMP */
> +
> +	__u64 tx_timestamp;
> +};

Is this structure easily extensible for future offloads,
such as USO?

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