[PATCH RFC net-next 8/9] net: generate the start time for every skb in the rx path

[Jason Xing <kerneljasonxing@gmail.com>]

Extend net_timestamp_check(), so that it also records an in-kernel start
time in the skb's shared hwtstamps. This gives received skbs the same
start_time semantics that TCP already gives to transmitted skbs, letting
a BPF program compute how long an skb has spent in the stack.

In the receiving path, net_timestamp_check() will 1) reuse the hardware
timestamp because the hardware time always takes precedence, or 2) uses
skb->tstamp if any, or 3) generate a current timestamp.

Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
---
 net/core/dev.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 4479fc87b789..8a9d2205a884 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2501,11 +2501,17 @@ static inline void net_timestamp_set(struct sk_buff *skb)
 		skb->tstamp = ktime_get_real();
 }
 
-#define net_timestamp_check(COND, SKB)				\
-	if (static_branch_unlikely(&netstamp_needed_key)) {	\
-		if ((COND) && !(SKB)->tstamp)			\
-			(SKB)->tstamp = ktime_get_real();	\
-	}							\
+#define net_timestamp_check(COND, SKB)					\
+	if (static_branch_unlikely(&netstamp_needed_key) ||		\
+	    static_branch_unlikely(&bpfts_v2_needed_key)) {		\
+		if (static_branch_unlikely(&netstamp_needed_key) &&	\
+		    (COND) && !(SKB)->tstamp)				\
+			(SKB)->tstamp = ktime_get_real();		\
+		if (static_branch_unlikely(&bpfts_v2_needed_key) &&	\
+		    !skb_hwtstamps(SKB)->hwtstamp)			\
+			skb_hwtstamps(SKB)->start_time =		\
+				(SKB)->tstamp ?: ktime_get_real();	\
+	}								\
 
 bool is_skb_forwardable(const struct net_device *dev, const struct sk_buff *skb)
 {
-- 
2.43.7