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Date:   Sat, 26 Sep 2020 22:32:08 +0300
From:   Vladimir Oltean <>
Subject: [PATCH v3 net-next 08/15] net: flow_dissector: avoid indirect call to DSA .flow_dissect for generic case

With the recent mitigations against speculative execution exploits,
indirect function calls are more expensive and it would be good to avoid
them where possible.

In the case of DSA, most switch taggers will shift the EtherType and
next headers by a fixed amount equal to that tag's length in bytes.
So we can use a generic procedure to determine that, without calling
into custom tagger code. However we still leave the flow_dissect method
inside struct dsa_device_ops as an override for the generic function.

Suggested-by: Andrew Lunn <>
Signed-off-by: Vladimir Oltean <>
Changes in v3:
Patch is new.

 net/core/flow_dissector.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 13cc4c0a8863..e21950a2c897 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -932,8 +932,14 @@ bool __skb_flow_dissect(const struct net *net,
 			int offset = 0;
 			ops = skb->dev->dsa_ptr->tag_ops;
-			if (ops->flow_dissect) {
-				ops->flow_dissect(skb, &proto, &offset);
+			/* Tail taggers don't break flow dissection */
+			if (!ops->tail_tag) {
+				if (ops->flow_dissect)
+					ops->flow_dissect(skb, &proto, &offset);
+				else
+					dsa_tag_generic_flow_dissect(skb,
+								     &proto,
+								     &offset);
 				hlen -= offset;
 				nhoff += offset;

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