[PATCH v3] ibmveth: Disable GSO for packets with small MSS

[PATCH v3] ibmveth: Disable GSO for packets with small MSS

[Mingming Cao]

Some physical adapters on Power systems do not support segmentation
offload when the MSS is less than 224 bytes. Attempting to send such
packets causes the adapter to freeze, stopping all traffic until
manually reset.

Implement ndo_features_check to disable GSO for packets with small MSS
values. The network stack will perform software segmentation instead.

The 224-byte minimum matches ibmvnic
commit <f10b09ef687f> ("ibmvnic: Enforce stronger sanity checks
on GSO packets")
which uses the same physical adapters in SEA configurations.

The issue occurs specifically when the hardware attempts to perform
segmentation (gso_segs > 1) with a small MSS. Single-segment GSO packets
(gso_segs == 1) do not trigger the problematic LSO code path and are
transmitted normally without segmentation.

Add an ndo_features_check callback to disable GSO when MSS < 224 bytes.
Also call vlan_features_check() to ensure proper handling of VLAN packets,
particularly QinQ (802.1ad) configurations where the hardware parser may
not support certain offload features.

Validated using iptables to force small MSS values. Without the fix,
the adapter freezes. With the fix, packets are segmented in software
and transmission succeeds. Comprehensive regression testing completedd 
(MSS tests, performance, stability).

Fixes: 8641dd85799f ("ibmveth: Add support for TSO")
Cc: stable@vger.kernel.org
Reviewed-by: Brian King <bjking1@linux.ibm.com>
Tested-by: Shaik Abdulla <shaik.abdulla1@ibm.com>
Tested-by: Naveed Ahmed <naveedaus@in.ibm.com>
Signed-off-by: Mingming Cao <mmc@linux.ibm.com>
---
Changes in v3:
- Call vlan_features_check() to handle VLAN packets correctly
- Clarified that gso_segs == 1 check is not needed for ibmveth
  (unlike ibmvnic, single-segment packets don't enter LSO path)
- Updated commit message to explain the difference

Changes in v2:
- Add Fixes tag as requested by automated checks
---
 drivers/net/ethernet/ibm/ibmveth.c | 22 ++++++++++++++++++++++
 drivers/net/ethernet/ibm/ibmveth.h |  1 +
 2 files changed, 23 insertions(+)

diff --git a/drivers/net/ethernet/ibm/ibmveth.c b/drivers/net/ethernet/ibm/ibmveth.c
index 58cc3147afe2..73e051d26b9d 100644
--- a/drivers/net/ethernet/ibm/ibmveth.c
+++ b/drivers/net/ethernet/ibm/ibmveth.c
@@ -1756,6 +1756,27 @@ static int ibmveth_set_mac_addr(struct net_device *dev, void *p)
 	return 0;
 }
 
+static netdev_features_t ibmveth_features_check(struct sk_buff *skb,
+						struct net_device *dev,
+						netdev_features_t features)
+{
+	/* Some physical adapters do not support segmentation offload with
+	 * MSS < 224. Disable GSO for such packets to avoid adapter freeze.
+	 * Note: Single-segment packets (gso_segs == 1) don't need this check
+	 * as they bypass the LSO path and are transmitted without segmentation.
+	 */
+	if (skb_is_gso(skb)) {
+		if (skb_shinfo(skb)->gso_size < IBMVETH_MIN_LSO_MSS) {
+			netdev_warn_once(dev,
+					 "MSS %u too small for LSO, disabling GSO\n",
+					 skb_shinfo(skb)->gso_size);
+			features &= ~NETIF_F_GSO_MASK;
+		}
+	}
+
+	return vlan_features_check(skb, features);
+}
+
 static const struct net_device_ops ibmveth_netdev_ops = {
 	.ndo_open		= ibmveth_open,
 	.ndo_stop		= ibmveth_close,
@@ -1767,6 +1788,7 @@ static const struct net_device_ops ibmveth_netdev_ops = {
 	.ndo_set_features	= ibmveth_set_features,
 	.ndo_validate_addr	= eth_validate_addr,
 	.ndo_set_mac_address    = ibmveth_set_mac_addr,
+	.ndo_features_check	= ibmveth_features_check,
 #ifdef CONFIG_NET_POLL_CONTROLLER
 	.ndo_poll_controller	= ibmveth_poll_controller,
 #endif
diff --git a/drivers/net/ethernet/ibm/ibmveth.h b/drivers/net/ethernet/ibm/ibmveth.h
index 068f99df133e..d87713668ed3 100644
--- a/drivers/net/ethernet/ibm/ibmveth.h
+++ b/drivers/net/ethernet/ibm/ibmveth.h
@@ -37,6 +37,7 @@
 #define IBMVETH_ILLAN_IPV4_TCP_CSUM		0x0000000000000002UL
 #define IBMVETH_ILLAN_ACTIVE_TRUNK		0x0000000000000001UL
 
+#define IBMVETH_MIN_LSO_MSS		224	/* Minimum MSS for LSO */
 /* hcall macros */
 #define h_register_logical_lan(ua, buflst, rxq, fltlst, mac) \
   plpar_hcall_norets(H_REGISTER_LOGICAL_LAN, ua, buflst, rxq, fltlst, mac)
-- 
2.39.3 (Apple Git-146)