Re: [PATCH net-next 1/5] net: add a common function to compute features from lowers devices

[Sabrina Dubroca <sd@queasysnail.net>]

2025-08-31, 18:35:49 +0300, Ido Schimmel wrote:
> On Fri, Aug 29, 2025 at 09:54:26AM +0000, Hangbin Liu wrote:
> > Some high level virtual drivers need to compute features from lower
> > devices. But each has their own implementations and may lost some
> > feature compute. Let's use one common function to compute features
> > for kinds of these devices.
> > 
> > The new helper uses the current bond implementation as the reference
> > one, as the latter already handles all the relevant aspects: netdev
> > features, TSO limits and dst retention.
> > 
> > Suggested-by: Paolo Abeni <pabeni@redhat.com>
> > Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
> > ---
> >  include/linux/netdevice.h | 19 ++++++++++
> >  net/core/dev.c            | 79 +++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 98 insertions(+)
> > 
> > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > index f3a3b761abfb..42742a47f2c6 100644
> > --- a/include/linux/netdevice.h
> > +++ b/include/linux/netdevice.h
> > @@ -5279,6 +5279,25 @@ int __netdev_update_features(struct net_device *dev);
> >  void netdev_update_features(struct net_device *dev);
> >  void netdev_change_features(struct net_device *dev);
> >  
> > +/* netdevice features */
> > +#define VIRTUAL_DEV_VLAN_FEATURES	(NETIF_F_HW_CSUM | NETIF_F_SG | \
> > +					 NETIF_F_FRAGLIST | NETIF_F_GSO_SOFTWARE | \
> > +					 NETIF_F_GSO_ENCAP_ALL | \
> > +					 NETIF_F_HIGHDMA | NETIF_F_LRO)
> > +
> > +#define VIRTUAL_DEV_ENC_FEATURES	(NETIF_F_HW_CSUM | NETIF_F_SG | \
> > +					 NETIF_F_RXCSUM | NETIF_F_GSO_SOFTWARE | \
> > +					 NETIF_F_GSO_PARTIAL)
> > +
> > +#define VIRTUAL_DEV_MPLS_FEATURES	(NETIF_F_HW_CSUM | NETIF_F_SG | \
> > +					 NETIF_F_GSO_SOFTWARE)
> > +
> > +#define VIRTUAL_DEV_XFRM_FEATURES	(NETIF_F_HW_ESP | NETIF_F_HW_ESP_TX_CSUM | \
> > +					 NETIF_F_GSO_ESP)
> > +
> > +#define VIRTUAL_DEV_GSO_PARTIAL_FEATURES (NETIF_F_GSO_ESP)
> > +void netdev_compute_features_from_lowers(struct net_device *dev);
> > +
> >  void netif_stacked_transfer_operstate(const struct net_device *rootdev,
> >  					struct net_device *dev);
> >  
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 1d1650d9ecff..fcad2a9f6b65 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -12577,6 +12577,85 @@ netdev_features_t netdev_increment_features(netdev_features_t all,
> >  }
> >  EXPORT_SYMBOL(netdev_increment_features);
> >  
> > +/**
> > + *	netdev_compute_features_from_lowers - compute feature from lowers
> > + *	@dev: the upper device
> > + *
> > + *	Recompute the upper device's feature based on all lower devices.
> > + */
> > +void netdev_compute_features_from_lowers(struct net_device *dev)
> > +{
> > +	unsigned int dst_release_flag = IFF_XMIT_DST_RELEASE | IFF_XMIT_DST_RELEASE_PERM;
> > +	netdev_features_t gso_partial_features = VIRTUAL_DEV_GSO_PARTIAL_FEATURES;
> > +#ifdef CONFIG_XFRM_OFFLOAD
> > +	netdev_features_t xfrm_features  = VIRTUAL_DEV_XFRM_FEATURES;
>                                        ^ double space (in other places as well)
> 
> > +#endif
> > +	netdev_features_t mpls_features  = VIRTUAL_DEV_MPLS_FEATURES;
> > +	netdev_features_t vlan_features = VIRTUAL_DEV_VLAN_FEATURES;
> > +	netdev_features_t enc_features  = VIRTUAL_DEV_ENC_FEATURES;
> > +	unsigned short max_hard_header_len = ETH_HLEN;

Going back to this discussion about hard_header_len:

> hard_header_len is not really a feature, so does not sound like it
> belongs here. I'm pretty sure it's not needed at all.
> 
> It was added to the bond driver in 2006 by commit 54ef31371407 ("[PATCH]
> bonding: Handle large hard_header_len") citing panics with gianfar on
> xmit. In 2009 commit 93c1285c5d92 ("gianfar: reallocate skb when
> headroom is not enough for fcb") fixed the gianfar driver to stop
> assuming that it has enough room to push its custom header. Further,
> commit bee9e58c9e98 ("gianfar:don't add FCB length to hard_header_len")
> from 2012 fixed this driver to use needed_headroom instead of
> hard_header_len.
> 
> The team driver is also adjusting hard_header_len according to the lower
> devices, but it most likely copied it from the bond driver. On the other
> hand, the bridge driver does not mess with hard_header_len and no
> problems were reported there (that I know of).
> 
> Might be a good idea to remove this hard_header_len logic from bond and
> team and instead set their needed_headroom according to the lower device
> with the highest needed_headroom. Paolo added similar logic in bridge
> and ovs but the use case is a bit different there.

I'm not convinced removing adapting hard_header_len on bond/team is
correct, even with old and broken drivers getting fixed years
ago. hard_header_len will be used on the TX path (for some devices
like bridge/macvlan via dev_forward_skb() and similar helpers, for IP
tunnels setting their MTU, and via LL_RESERVED_SPACE).

So I think we should keep setting hard_header_len to the largest of
all lowers.

-- 
Sabrina