Date:	Mon, 29 Sep 2014 22:59:39 +0300
From:	Or Gerlitz <gerlitz.or@...il.com>
To:	Tom Herbert <therbert@...gle.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	David Miller <davem@...emloft.net>
Cc:	Linux Netdev List <netdev@...r.kernel.org>,
	Thomas Graf <tgraf@...g.ch>,
	Pravin Shelar <pshelar@...ira.com>,
	John Fastabend <john.r.fastabend@...el.com>
Subject: Re: [PATCH] net: Add ndo_gso_check

On Mon, Sep 29, 2014 at 6:50 AM, Tom Herbert <therbert@...gle.com> wrote:
> Add ndo_gso_check which a device can define to indicate whether is
> is capable of doing GSO on a packet. This funciton would be called from
> the stack to determine whether software GSO is needed to be done.

please, no...

We should strive to have a model/architecture under which the driver
can clearly advertize up something (bits, collection of bits,
whatever) which the stack can run through some dec-sion making code
and decide if to GSO yes/no, do it ala SW or ala HW. As life, this
model need not be perfect and can be biased towards being
simple/robust/conservative and developed incrementally.

We certainly don't want each driver that support some sort of HW
offloads for tunnels (today we have 4-5, tomorrow maybe 40...) to
implement their super sophisticated/efficient "dig in a packet and
tell myself if I can GSO/CSUM it or not" code, while repeating (expect
many copy/paste bugs...) the other drivers' code.

Saying that, while looking quickly on the fm10k driver during it's
super fast upstream review cycle, I saw something which looked like
yellow light, I drove one, but now I see it was very much a red one:
the chain of calls:

fm10k_xmit_frame_ring --> fm10k_tso -->
fm10k_tx_encap_offload --> {fm10k_port_is_vxlan,  fm10k_port_is_nvgre}

is pretty much live example to what you are describing here, right?

L2 Drivers are not supposed to do such heavy duty lookups in packets
with tons of code that can be generic.

I would suggest as band aid to the current situation with non-VXLAN
UDP offloads enabled upstream and few drivers that don't really
support GSO/CSUM for udp tunnels which are not vxlan -- to have a
VXLAN bit which tells to the stack "I can do HW GSO to VXLAN" and have
the stack to enable HW GSO over these devices only to VXLAN. This
would bring the upstream code into consistent/well-defined state, and
we can take it from there.

Or.

SB another comment

> A driver should populate this function if it advertises GSO types for
> which there are combinations that it wouldn't be able to handle. For
> instance a device that performs UDP tunneling might only implement
> support for transparent Ethernet bridging type of inner packets
> or might have limitations on lengths of inner headers.
>
> Signed-off-by: Tom Herbert <therbert@...gle.com>
> ---
>  include/linux/netdevice.h | 12 +++++++++++-
>  net/core/dev.c            |  2 +-
>  2 files changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 9f5d293..f8c2027 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -997,6 +997,12 @@ typedef u16 (*select_queue_fallback_t)(struct net_device *dev,
>   *     Callback to use for xmit over the accelerated station. This
>   *     is used in place of ndo_start_xmit on accelerated net
>   *     devices.
> + * bool        (*ndo_gso_check) (struct sk_buff *skb,
> + *                       struct net_device *dev);
> + *     Called by core transmit path to determine if device is capable of
> + *     performing GSO on a packet. The device returns true if it is
> + *     able to GSO the packet, false otherwise. If the return value is
> + *     false the stack will do software GSO.
>   */
>  struct net_device_ops {
>         int                     (*ndo_init)(struct net_device *dev);
> @@ -1146,6 +1152,8 @@ struct net_device_ops {
>                                                         struct net_device *dev,
>                                                         void *priv);
>         int                     (*ndo_get_lock_subclass)(struct net_device *dev);
> +       bool                    (*ndo_gso_check) (struct sk_buff *skb,
> +                                                 struct net_device *dev);
>  };
>
>  /**
> @@ -3536,10 +3544,12 @@ static inline bool skb_gso_ok(struct sk_buff *skb, netdev_features_t features)
>                (!skb_has_frag_list(skb) || (features & NETIF_F_FRAGLIST));
>  }
>
> -static inline bool netif_needs_gso(struct sk_buff *skb,
> +static inline bool netif_needs_gso(struct net_device *dev, struct sk_buff *skb,
>                                    netdev_features_t features)
>  {
>         return skb_is_gso(skb) && (!skb_gso_ok(skb, features) ||
> +               (dev->netdev_ops->ndo_gso_check &&
> +                !dev->netdev_ops->ndo_gso_check(skb, dev)) ||
>                 unlikely((skb->ip_summed != CHECKSUM_PARTIAL) &&
>                          (skb->ip_summed != CHECKSUM_UNNECESSARY)));
>  }
> diff --git a/net/core/dev.c b/net/core/dev.c
> index e2ced01..8c2b9bb 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2680,7 +2680,7 @@ struct sk_buff *validate_xmit_skb(struct sk_buff *skb, struct net_device *dev)
>         if (skb->encapsulation)
>                 features &= dev->hw_enc_features;
>
> -       if (netif_needs_gso(skb, features)) {
> +       if (netif_needs_gso(dev, skb, features)) {
>                 struct sk_buff *segs;
>
>                 segs = skb_gso_segment(skb, features);

unrelated fix? best to put in a different patch.
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