[PATCH RFC net-next 4/9] net: reuse skb_shared_hwtstamps for BPF Timestamping v2

[Jason Xing <kerneljasonxing@gmail.com>]

The ultimate goal is to find a place to store the start time per-packet
basis for different protocols.

It's almost unacceptable to add a new field for latency tracker (BPF
Timestamping V2) for every nornal skb because the space is so precious
nowadays. So what left we can do is to try to find a field to reuse.

The principles are 1) it should be a safe place to store the start
time, 2) it can traverse across netns (candidates like skb->tstamp
are not appropriate due to the clearance by skb_scrub_packet() in
the container scenario), 3) it works in dual directions. Detailed
comparisons and reasons can be found on slides 53-60[1].

As we discussed at Netconf 2026 in Rome, we eventually chose to reuse
hwtstamp with considering the above reasons. Now this reused field has
three meanings.

The two uses between 'hwtstamp' and 'start time' are deliberately ordered:
a real hardware time stamp always wins. The start time is only generated
when the hwtstamp slot is still empty. When hardware timestamping is present,
that means we have a better value from real hardware than the software one,
so choose the hardware timestamp as the start time in the rx path.

[1]: https://netdevconf.info/0x1A/sessions/bof/network-observability-bof.html

Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
---
 include/linux/skbuff.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 55ae1653a1cf..c5347be72ff9 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -447,6 +447,12 @@ static inline bool skb_frag_must_loop(struct page *p)
  * struct skb_shared_hwtstamps - hardware time stamps
  * @hwtstamp:		hardware time stamp transformed into duration
  *			since arbitrary point in time
+ * @start_time:		Start time stamped when SK_BPF_CB_TIMESTAMPING_V2
+ *			is enabled; shares storage with @hwtstamp. A real
+ *			hardware time stamp always takes precedence: the start
+ *			time is only recorded when @hwtstamp is still empty
+ *			(no hardware time stamp available), so the two are
+ *			never needed at the same time on a given skb.
  * @netdev_data:	address/cookie of network device driver used as
  *			reference to actual hardware time stamp
  *
@@ -462,6 +468,7 @@ static inline bool skb_frag_must_loop(struct page *p)
 struct skb_shared_hwtstamps {
 	union {
 		ktime_t	hwtstamp;
+		ktime_t	start_time;
 		void *netdev_data;
 	};
 };
-- 
2.43.7