netdev - Re: [PATCH 2/6] xen-netfront: reduce gso_max_size to account for ethernet header

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Message-ID: <1364232271.29473.27.camel@edumazet-glaptop>
Date:	Mon, 25 Mar 2013 10:24:31 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	wei.liu2@...rix.com, xen-devel@...ts.xen.org,
	netdev@...r.kernel.org, ian.campbell@...rix.com,
	annie.li@...cle.com, konrad.wilk@...cle.com,
	david.vrabel@...rix.com
Subject: Re: [PATCH 2/6] xen-netfront: reduce gso_max_size to account for
 ethernet header

On Mon, 2013-03-25 at 12:59 -0400, David Miller wrote:

> If this GRO packet ends up being transmitted, the gso limit should be
> applied, otherwise we would be violating the device's advertised GSO
> limit value.
> 
> Assume that this kind of check is performed (it must), then I don't
> see how GRO can cause any problems for Xen.

It seems nobody cared to perform this generic check.

netif_skb_features() only deals with max_segs :

if (skb_shinfo(skb)->gso_segs > skb->dev->gso_max_segs) 
	features &= ~NETIF_F_GSO_MASK; 


dev->gso_max_size is currently only used to populate sk->sk_gso_max_size

For regular 1500 MTU and at most 17 frags per skb, its hardly a problem,
but it could happen with jumbo frames, or using loopback and splice()


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