netdev - Re: [RFC PATCH 7/9] GSO: Support partial segmentation offload

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Date:	Sun, 27 Mar 2016 22:36:12 -0700
From:	Jesse Gross <jesse@...nel.org>
To:	Alexander Duyck <aduyck@...antis.com>
Cc:	Edward Cree <ecree@...arflare.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Alexander Duyck <alexander.duyck@...il.com>,
	Tom Herbert <tom@...bertland.com>
Subject: Re: [RFC PATCH 7/9] GSO: Support partial segmentation offload

On Fri, Mar 18, 2016 at 4:25 PM, Alexander Duyck <aduyck@...antis.com> wrote:
> diff --git a/net/core/dev.c b/net/core/dev.c
> index edb7179bc051..666cf427898b 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2711,6 +2711,19 @@ struct sk_buff *__skb_gso_segment(struct sk_buff *skb,
[...]
> +       /* Only report GSO partial support if it will enable us to
> +        * support segmentation on this frame without needing additional
> +        * work.
> +        */
> +       if (features & NETIF_F_GSO_PARTIAL) {
> +               netdev_features_t partial_features;
> +               struct net_device *dev = skb->dev;
> +
> +               partial_features = dev->features & dev->gso_partial_features;
> +               if (!skb_gso_ok(skb, features | partial_features))
> +                       features &= ~NETIF_F_GSO_PARTIAL;

I think we need to add NETIF_F_GSO_ROBUST into the skb_gso_ok() check
- otherwise packets coming from VMs fail this test and we lose GSO
partial. It's totally safe to expose this feature, since we'll compute
gso_segs anyways.

> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index f044f970f1a6..bdcba77e164c 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -3281,6 +3291,15 @@ perform_csum_check:
>          */
>         segs->prev = tail;
>
> +       /* Update GSO info on first skb in partial sequence. */
> +       if (partial_segs) {
> +               skb_shinfo(segs)->gso_size = mss / partial_segs;

One small thing: this gso_size is the same as the original MSS, right?
It seems like we could trivially stick it in a local variable and
avoid the extra division.

> +               skb_shinfo(segs)->gso_segs = partial_segs;
> +               skb_shinfo(segs)->gso_type = skb_shinfo(head_skb)->gso_type |
> +                                            SKB_GSO_PARTIAL;

Since we're computing the gso_segs ourselves, it might be nice to
strip out SKB_GSO_DODGY when we set the type.

I just wanted to say that this is really nice work - I was expecting
it to turn out to be really messy and unmaintainable but this is very
clean. Thanks!