netdev - Re: [PATCH v2 RFC] 8139cp: Fix GSO MSS handling

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Message-ID: <1443096325.29850.151.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 24 Sep 2015 05:05:25 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Francois Romieu <romieu@...zoreil.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 RFC] 8139cp: Fix GSO MSS handling

On Thu, 2015-09-24 at 11:38 +0100, David Woodhouse wrote:
> When fixing the TSO support I noticed we just mask ->gso_size with the
> MSSMask value and don't care about the consequences.
> 
> Provide a .ndo_features_check() method which drops the NETIF_F_TSO
> feature for any skb which would exceed the maximum, and thus forces it
> to be segmented by software.
> 
> Then we can stop the masking in cp_start_xmit(), and just WARN if the
> maximum is exceeded, which should now never happen.
> 
> Finally, Francois Romieu noticed that we didn't even have the right
> value for MSSMask anyway; it should be 0x7ff (11 bits) not 0xfff.
> 
> Signed-off-by: David Woodhouse <David.Woodhouse@...el.com>
> ---
> OK, netif_set_gso_max_size() was *not*, as I had naïvely assumed, a way
> to set the maximum value of ->gso_size. That's an unfortunate set of
> naming decisions.

Right, netif_skb_features() only has the following checks :

if (gso_segs > dev->gso_max_segs || gso_segs < dev->gso_min_segs)
    features &= ~NETIF_F_GSO_MASK;

But now we have .ndo_features_check() we could remove this generic check
from fast path.


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