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 17:16:42 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     vyasevic@...hat.com, 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 2017/05/19 16:09, Vlad Yasevich wrote:
> 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.

Right for stacked vlan device.
But I think the check was there for packets from guests forwarded by
bridge to vlan device so it was a problem before 8cb65d000.

>> 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.

At least HW_CSUM in the check was introduced intentionally.
https://www.spinics.net/lists/netdev/msg152016.html

And I think HW_CSUM should work with any packets.
You know, include/linux/skbuff.h says
>  *	NETIF_F_HW_CSUM	- The driver (or its device) is able to compute one
>  *			  IP (one's complement) checksum for any combination
>  *			  of protocols or protocol layering.

We should be able to safely enable it.

...But you are so worried about layer2 protocol handling of HW_CSUM
devices, I'm ok with disabling it for now.

-- 
Toshiaki Makita