Some/most devices implementing gso_partial need to disable the GSO partial
features when the IP ID can't be mangled; to that extend each of them
implements something alike the following[1]:

        if (skb->encapsulation && !(features & NETIF_F_TSO_MANGLEID))
                features &= ~NETIF_F_TSO;

in the ndo_features_check() op, which leads to a bit of duplicate code.

Later patch in the series will implement GSO partial support for virtual
devices, and the current status quo will require more duplicate code and
a new indirect call in the TX path for them.

Introduce the mangleid_features mask, allowing the core to disable NIC
features based on/requiring MANGLEID, without any further intervention
from the driver.

The same functionality could be alternatively implemented adding a single
boolean flag to the struct net_device, but would require an additional
checks in ndo_features_check().

Also note that [1] is incorrect if the NIC additionally implements
NETIF_F_GSO_UDP_L4, mangleid_features transparently handle even such a
case.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
v3 -> v4:
 - ensure mangleid_features includes TSO_MANGLEID for better code
   in gso_features_check() - Eric
 - some changelog clarifications.
---
 include/linux/netdevice.h | 5 ++++-
 net/core/dev.c            | 8 +++++++-
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index d99b0fbc1942..23a698b70de1 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h

@@ -1830,7 +1830,9 @@ enum netdev_reg_state {
  *                             and drivers will need to set them appropriately.
  *
  *     @mpls_features: Mask of features inheritable by MPLS
- *     @gso_partial_features: value(s) from NETIF_F_GSO\*
+ *     @gso_partial_features: value(s) from NETIF_F_GSO
+ *     @mangleid_features:     Mask of features requiring MANGLEID, will be
+ *                             disabled together with the latter.
  *
  *     @ifindex:       interface index
  *     @group:         The group the device belongs to

@@ -2219,6 +2221,7 @@ struct net_device {
        netdev_features_t       vlan_features;
        netdev_features_t       hw_enc_features;
        netdev_features_t       mpls_features;
+       netdev_features_t       mangleid_features;

        unsigned int            min_mtu;
        unsigned int            max_mtu;

diff --git a/net/core/dev.c b/net/core/dev.c
index 2661b68f5be3..3f12061ae474 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -3802,7 +3802,7 @@ static netdev_features_t gso_features_check(const struct sk_buff *skb,
                                    inner_ip_hdr(skb) : ip_hdr(skb);

                if (!(iph->frag_off & htons(IP_DF)))
-                       features &= ~NETIF_F_TSO_MANGLEID;
+                       features &= ~dev->mangleid_features;
        }

        /* NETIF_F_IPV6_CSUM does not support IPv6 extension headers,

@@ -11385,6 +11385,12 @@ int register_netdevice(struct net_device *dev)
        if (dev->hw_enc_features & NETIF_F_TSO)
                dev->hw_enc_features |= NETIF_F_TSO_MANGLEID;

+       /* Any mangleid feature disables TSO_MANGLEID; including the latter
+        * in mangleid_features allows for better code in the fastpath.
+        */
+       if (dev->mangleid_features)
+               dev->mangleid_features |= NETIF_F_TSO_MANGLEID;
+