netdev - Re: [RFC net-next 2/2] net: disable UDP GSO feature when CSUM is disabled

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Date:   Wed, 8 Jul 2020 19:47:09 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     tanhuazhong <tanhuazhong@...wei.com>,
        Eric Dumazet <eric.dumazet@...il.com>, davem@...emloft.net,
        willemb@...gle.com
Cc:     netdev@...r.kernel.org, linuxarm@...wei.com, kuba@...nel.org
Subject: Re: [RFC net-next 2/2] net: disable UDP GSO feature when CSUM is
 disabled



On 7/8/20 7:30 PM, tanhuazhong wrote:
> 
> 
> On 2020/7/8 13:36, Eric Dumazet wrote:
>>
>>
>> On 7/7/20 8:48 PM, Huazhong Tan wrote:
>>> Since UDP GSO feature is depended on checksum offload, so disable
>>> UDP GSO feature when CSUM is disabled, then from user-space also
>>> can see UDP GSO feature is disabled.
>>>
>>> Signed-off-by: Huazhong Tan <tanhuazhong@...wei.com>
>>> ---
>>>   net/core/dev.c | 6 ++++++
>>>   1 file changed, 6 insertions(+)
>>>
>>> diff --git a/net/core/dev.c b/net/core/dev.c
>>> index c02bae9..dcb6b35 100644
>>> --- a/net/core/dev.c
>>> +++ b/net/core/dev.c
>>> @@ -9095,6 +9095,12 @@ static netdev_features_t netdev_fix_features(struct net_device *dev,
>>>           features &= ~NETIF_F_TSO6;
>>>       }
>>>   +    if ((features & NETIF_F_GSO_UDP_L4) && !(features & NETIF_F_HW_CSUM) &&
>>> +        (!(features & NETIF_F_IP_CSUM) || !(features & NETIF_F_IPV6_CSUM))) {
>>
>> This would prevent a device providing IPv4 checksum only (no IPv6 csum support) from sending IPv4 UDP GSO packets ?
>>
> 
> Yes, not like TCP (who uses NETIF_F_TSO for IPv4 and NETIF_F_TSO6 for IPv6),
> UDP only has a NETIF_F_GSO_UDP_L4 for both IPv4 and IPv6.
> I cannot find a better way to do it with combined IPv4 and IPv6 csum together.
> For this issue, is there any good idea to fix it?

This could be done in an ndo_fix_features(), or ndo_features_check()

Or maybe we do not care, but this should probably be documented.