[PATCH v4 net 0/3] ECN offload handling for AccECN series

[PATCH v4 net 0/3] ECN offload handling for AccECN series

[chia-yu.chang]

From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>

Hello,

Please find the v4 ECN offload handling for AccECN patch series for net.
It aims to avoid potential CWR flag corruption due to RFC3168 ECN offload,
because this flag is part of ACE signal used for Accurate ECN (RFC9768).

This corresponds to discussions in virtio mailing list:
https://lore.kernel.org/all/20250814120118.81787-1-chia-yu.chang@nokia-bell-labs.com/
And it was suggested to clarify SKB_GSO_TCP_ECN and SKB_GSO_TCP_ACCECN.

A prior submission is made to net-next:
https://lore.kernel.org/all/SJ0PR12MB68066115C5329872316E6B37DC98A@SJ0PR12MB6806.namprd12.prod.outlook.com/
And it was suggetsed to submit the first two patches to net.

Best regards,
Chia-Yu

---
v4:
- Fix spacing in commit message of #2 and #3 (Paolo Abeni <pabeni@redhat.com>)
- In v3, there were questions towards hisilicon and mellanox ppl but no response:
  So, we re-submit this series to ensure the discussion can stay fresh:
  - https://lore.kernel.org/netdev/b796bd57-650a-41d1-8032-f124084634c3@redhat.com/
  - https://lore.kernel.org/netdev/62393422-bc8f-4676-bf3c-4d1be15ab800@redhat.com/

v3:
- Fix commit message title typo
- Seprate prior #2 into #2 and #3 to have one patch per NIC (Paolo Abeni <pabeni@redhat.com>)

v2:
- Fix commit header title typo

---
Chia-Yu Chang (3):
  net: update comments for SKB_GSO_TCP_ECN and SKB_GSO_TCP_ACCECN
  net: mlx5e: fix CWR handling in drivers to preserve ACE signal
  net: hns3: fix CWR handling in drivers to preserve ACE signal

 drivers/net/ethernet/hisilicon/hns3/hns3_enet.c |  2 +-
 drivers/net/ethernet/mellanox/mlx5/core/en_rx.c |  4 ++--
 include/linux/skbuff.h                          | 15 ++++++++++++++-
 3 files changed, 17 insertions(+), 4 deletions(-)

-- 
2.34.1

[PATCH v4 net 1/3] net: update comments for SKB_GSO_TCP_ECN and SKB_GSO_TCP_ACCECN

[chia-yu.chang]

From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>

This patch updates the documentation of ECN‑related GSO flags, it
clarifies the limitations of SKB_GSO_TCP_ECN and explains how to preserve
the CWR flag (part of the ACE signal) in the Rx path.

For Tx, SKB_GSO_TCP_ECN and SKB_GSO_TCP_ACCECN are used respectively for
RFC3168 ECN and AccECN (RFC9768). SKB_GSO_TCP_ECN indicates that the
first segment has CWR set, while subsequent segments have CWR cleared.
In contrast, SKB_GSO_TCP_ACCECN means that the segment uses AccECN and
therefore its CWR flag must not be modified during segmentation.

For RX, SKB_GSO_TCP_ECN shall NOT be used, because the stack cannot know
whether the connection uses RFC3168 ECN or AccECN, whereas RFC3168 ECN
offload may clear CWR flag and thus corrupts the ACE signal. Instead, any
segment that arrives with CWR set must use the SKB_GSO_TCP_ACCECN flag
to prevent RFC3168 ECN offload logic from clearing the CWR flag.

Co-developed-by: Ilpo Järvinen <ij@kernel.org>
Signed-off-by: Ilpo Järvinen <ij@kernel.org>
Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
---
 include/linux/skbuff.h | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 2bcf78a4de7b..9080a6d508a3 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -671,7 +671,13 @@ enum {
 	/* This indicates the skb is from an untrusted source. */
 	SKB_GSO_DODGY = 1 << 1,
 
-	/* This indicates the tcp segment has CWR set. */
+	/* For TX, this indicates that the first TCP segment has CWR set, and
+	 * any subsequent segment in the same skb has CWR cleared. This flag
+	 * must not be used in RX, because the connection to which the segment
+	 * belongs is not tracked to use RFC3168 or AccECN. Using RFC3168 ECN
+	 * offload may clear CWR and corrupt ACE signal (CWR is part of it).
+	 * Instead, SKB_GSO_TCP_ACCECN shall be used to avoid CWR corruption.
+	 */
 	SKB_GSO_TCP_ECN = 1 << 2,
 
 	__SKB_GSO_TCP_FIXEDID = 1 << 3,
@@ -706,6 +712,13 @@ enum {
 
 	SKB_GSO_FRAGLIST = 1 << 18,
 
+	/* For TX, this indicates that the TCP segment uses the CWR flag as part
+	 * of the ACE signal, and the CWR flag must not be modified in the skb.
+	 * For RX, any incoming segment with CWR set must use this flag so that
+	 * no RFC3168 ECN offload can clear the CWR flag. This is required to
+	 * preserve ACE signal correctness (CWR is part of it) in a forwarding
+	 * scenario, e.g., from one netdevice RX to other netdevice TX
+	 */
 	SKB_GSO_TCP_ACCECN = 1 << 19,
 
 	/* These indirectly map onto the same netdev feature.
-- 
2.34.1

[PATCH v4 net 2/3] net: mlx5e: fix CWR handling in drivers to preserve ACE signal

[chia-yu.chang]

From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>

Currently, mlx5 Rx paths use the SKB_GSO_TCP_ECN flag when a TCP segment
with the CWR flag set. This is wrong because SKB_GSO_TCP_ECN is only
valid for RFC3168 ECN on Tx, and using it on Rx allows RFC3168 ECN
offload to clear the CWR flag. As a result, incoming TCP segments
may lose their ACE signal integrity required for AccECN (RFC9768),
especially when the packet is forwarded and later re-segmented by GSO.

Fix this by setting SKB_GSO_TCP_ACCECN for any Rx segment with the CWR
flag set. SKB_GSO_TCP_ACCECN ensures that RFC3168 ECN offload will
not clear the CWR flag, therefore preserving the ACE signal.

Fixes: 92552d3abd329 ("net/mlx5e: HW_GRO cqe handler implementation")
Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
---
 drivers/net/ethernet/mellanox/mlx5/core/en_rx.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
index 5b60aa47c75b..9b1c80079532 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
@@ -1180,7 +1180,7 @@ static void mlx5e_shampo_update_ipv4_tcp_hdr(struct mlx5e_rq *rq, struct iphdr *
 	skb->csum_offset = offsetof(struct tcphdr, check);
 
 	if (tcp->cwr)
-		skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
+		skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ACCECN;
 }
 
 static void mlx5e_shampo_update_ipv6_tcp_hdr(struct mlx5e_rq *rq, struct ipv6hdr *ipv6,
@@ -1201,7 +1201,7 @@ static void mlx5e_shampo_update_ipv6_tcp_hdr(struct mlx5e_rq *rq, struct ipv6hdr
 	skb->csum_offset = offsetof(struct tcphdr, check);
 
 	if (tcp->cwr)
-		skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
+		skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ACCECN;
 }
 
 static void mlx5e_shampo_update_hdr(struct mlx5e_rq *rq, struct mlx5_cqe64 *cqe, bool match)
-- 
2.34.1

[PATCH v4 net 3/3] net: hns3: fix CWR handling in drivers to preserve ACE signal

[chia-yu.chang]

From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>

Currently, hns3 Rx paths use SKB_GSO_TCP_ECN flag when a TCP segment
with the CWR flag set. This is wrong because SKB_GSO_TCP_ECN is only
valid for RFC3168 ECN on Tx, and using it on Rx allows RFC3168 ECN
offload to clear the CWR flag. As a result, incoming TCP segments
lose their ACE signal integrity required for AccECN (RFC9768),
especially when the packet is forwarded and later re-segmented by GSO.

Fix this by setting SKB_GSO_TCP_ACCECN for any Rx segment with the CWR
flag set. SKB_GSO_TCP_ACCECN ensure that RFC3168 ECN offload will
not clear the CWR flag, therefore preserving the ACE signal.

Fixes: d474d88f88261 ("net: hns3: add hns3_gro_complete for HW GRO process")
Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
---
 drivers/net/ethernet/hisilicon/hns3/hns3_enet.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c b/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c
index a3206c97923e..e1b0dba56182 100644
--- a/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c
+++ b/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c
@@ -3904,7 +3904,7 @@ static int hns3_gro_complete(struct sk_buff *skb, u32 l234info)
 
 	skb_shinfo(skb)->gso_segs = NAPI_GRO_CB(skb)->count;
 	if (th->cwr)
-		skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
+		skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ACCECN;
 
 	if (l234info & BIT(HNS3_RXD_GRO_FIXID_B))
 		skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_FIXEDID;
-- 
2.34.1

Re: [PATCH v4 net 2/3] net: mlx5e: fix CWR handling in drivers to preserve ACE signal

[Paolo Abeni]

On 4/17/26 5:26 PM, chia-yu.chang@nokia-bell-labs.com wrote:
> From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
> 
> Currently, mlx5 Rx paths use the SKB_GSO_TCP_ECN flag when a TCP segment
> with the CWR flag set. This is wrong because SKB_GSO_TCP_ECN is only
> valid for RFC3168 ECN on Tx, and using it on Rx allows RFC3168 ECN
> offload to clear the CWR flag. As a result, incoming TCP segments
> may lose their ACE signal integrity required for AccECN (RFC9768),
> especially when the packet is forwarded and later re-segmented by GSO.
> 
> Fix this by setting SKB_GSO_TCP_ACCECN for any Rx segment with the CWR
> flag set. SKB_GSO_TCP_ACCECN ensures that RFC3168 ECN offload will
> not clear the CWR flag, therefore preserving the ACE signal.
> 
> Fixes: 92552d3abd329 ("net/mlx5e: HW_GRO cqe handler implementation")
> Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
> ---
>  drivers/net/ethernet/mellanox/mlx5/core/en_rx.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> index 5b60aa47c75b..9b1c80079532 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> @@ -1180,7 +1180,7 @@ static void mlx5e_shampo_update_ipv4_tcp_hdr(struct mlx5e_rq *rq, struct iphdr *
>  	skb->csum_offset = offsetof(struct tcphdr, check);
>  
>  	if (tcp->cwr)
> -		skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
> +		skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ACCECN;

Here there is an open question for nVidia:

Is the above enough or will later segmentation lead to the wrong
results? I think/guess the firmware is (still) aggregating the wire
frames using the ECN schema, i.e. the first wire packet has CWR == 1,
the later CWR==0.

If so, later segmentation of this GSO packet will emit CWR == 1 on all
the packets, making the egress stream different from ingress.

@Saeed, Leon, Tariq: could you please have a look here?

I guess that with a more conservative approach drivers update should be
omitted, and the updated documentation should be less forceful (i.e.
"TCP_ECN should not be used in RX")

/P

Thanks,

Paolo