netdev - Re: [PATCH net 1/3] vlan: Fix tcp checksums offloads for Q-in-Q vlan.

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Date:   Fri, 19 May 2017 03:09:46 -0400
From:   Vlad Yasevich <vyasevic@...hat.com>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        Vladislav Yasevich <vyasevich@...il.com>,
        netdev@...r.kernel.org
Cc:     mkubecek@...e.cz
Subject: Re: [PATCH net 1/3] vlan: Fix tcp checksums offloads for Q-in-Q vlan.

On 05/18/2017 10:13 PM, Toshiaki Makita wrote:
> On 2017/05/18 22:31, Vladislav Yasevich wrote:
>> It appears that since commit 8cb65d000, Q-in-Q vlans have been
>> broken.  The series that commit is part of enabled TSO and checksum
>> offloading on Q-in-Q vlans.  However, most HW we support can't handle
>> it.  To work around the issue, the above commit added a function that
>> turns off offloads on Q-in-Q devices, but it left the checksum offload.
>> That will cause issues with most older devices that supprort very basic
>> checksum offload capabilities as well as some newer devices (we've
>> reproduced te problem with both be2net and bnx).
>>
>> To solve this for everyone, turn off checksum offloading feature
>> by default when sending Q-in-Q traffic.  Devices that are proven to
>> work can provided a corrected ndo_features_check implemetation.
>>
>> Fixes: 8cb65d000 ("net: Move check for multiple vlans to drivers")
>> CC: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
>> Signed-off-by: Vladislav Yasevich <vyasevic@...hat.com>
>> ---
>>  include/linux/if_vlan.h | 1 -
>>  1 file changed, 1 deletion(-)
>>
>> diff --git a/include/linux/if_vlan.h b/include/linux/if_vlan.h
>> index 8d5fcd6..ae537f0 100644
>> --- a/include/linux/if_vlan.h
>> +++ b/include/linux/if_vlan.h
>> @@ -619,7 +619,6 @@ static inline netdev_features_t vlan_features_check(const struct sk_buff *skb,
>>  						     NETIF_F_SG |
>>  						     NETIF_F_HIGHDMA |
>>  						     NETIF_F_FRAGLIST |
>> -						     NETIF_F_HW_CSUM |
>>  						     NETIF_F_HW_VLAN_CTAG_TX |
>>  						     NETIF_F_HW_VLAN_STAG_TX);
>>  
> 
> I guess HW_CSUM theoretically can handle Q-in-Q packets and the problem
> is IP_CSUM and IPV6_CSUM.
> So wouldn't it be better to leave HW_CSUM and drop IP_CSUM/IPV6_CSUM,
> i.e. change intersection into bitwise AND?
> 

It wasn't really a problem before accelerations got enabled on q-in-q
vlans.

> The intersection was introduced in db115037bb57 ("net: fix checksum
> features handling in netif_skb_features()"), but I guess for this
> particular check the intersection was not needed.
> 

So, to put it another way, leave the intersection with HW_CSUM in the mask,
and then do:

  return features & ~(NETIF_F_IP_CSUM|NETIF_F_IPV6_CSUM);

This might work, but it assumes that everyone who announce HW_CSUM can
do q-in-q vlans.  It's been a bit of a pain tracking this down and I'd rather
fix it for everyone and let individual driver authors verify that Q-in-Q works
correctly with HW checksum.  However, I am willing to do the above if
that's what people want.

-vlad