linux-kernel - Re: [PATCH net-next 0/2] net: two updates related to UDP GSO

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Date:   Mon, 7 Sep 2020 21:32:22 +0800
From:   tanhuazhong <tanhuazhong@...wei.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Jakub Kicinski <kuba@...nel.org>
CC:     David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        <salil.mehta@...wei.com>, <yisen.zhuang@...wei.com>,
        <linuxarm@...wei.com>
Subject: Re: [PATCH net-next 0/2] net: two updates related to UDP GSO



On 2020/9/7 17:22, Willem de Bruijn wrote:
> On Sun, Sep 6, 2020 at 8:42 PM Jakub Kicinski <kuba@...nel.org> wrote:
>>
>> On Sat, 5 Sep 2020 14:11:11 +0800 Huazhong Tan wrote:
>>> There are two updates relates to UDP GSO.
>>> #1 adds a new GSO type for UDPv6
>>> #2 adds check for UDP GSO when csum is disable in netdev_fix_features().
>>>
>>> Changes since RFC V2:
>>> - modifies the timing of setting UDP GSO type when doing UDP GRO in #1.
>>>
>>> Changes since RFC V1:
>>> - updates NETIF_F_GSO_LAST suggested by Willem de Bruijn.
>>>    and add NETIF_F_GSO_UDPV6_L4 feature for each driver who support UDP GSO in #1.
>>>    - add #2 who needs #1.
>>
>> Please CC people who gave you feedback (Willem).
>>
>> I don't feel good about this series. IPv6 is not optional any more.
>> AFAIU you have some issues with csum support in your device? Can you
>> use .ndo_features_check() to handle this?
>>
>> The change in semantics of NETIF_F_GSO_UDP_L4 from "v4 and v6" to
>> "just v4" can trip people over; this is not a new feature people
>> may be depending on the current semantics.
>>
>> Willem, what are your thoughts on this?
> 
> If that is the only reason, +1 on fixing it up in the driver's
> ndo_features_check.
> 

Hi, Willem & Jakub.

This series mainly fixes the feature dependency between hardware 
checksum and UDP GSO.
When turn off hardware checksum offload, run 'ethtool -k [devname]'
we can see TSO is off as well, but udp gso still is on.

[root@...alhost ~]# ethtool -K eth0 tx off
Actual changes:
tx-checksumming: off
	tx-checksum-ipv4: off
	tx-checksum-ipv6: off
	tx-checksum-sctp: off
tcp-segmentation-offload: off
	tx-tcp-segmentation: off [requested on]
	tx-tcp-ecn-segmentation: off [requested on]
	tx-tcp6-segmentation: off [requested on]
[root@...alhost ~]# ethtool -k eth0
Features for eth0:
rx-checksumming: on
tx-checksumming: off
	tx-checksum-ipv4: off
	tx-checksum-ip-generic: off [fixed]
	tx-checksum-ipv6: off
	tx-checksum-fcoe-crc: off [fixed]
	tx-checksum-sctp: off
...
tcp-segmentation-offload: off
	tx-tcp-segmentation: off [requested on]
	tx-tcp-ecn-segmentation: off [requested on]
	tx-tcp-mangleid-segmentation: off
	tx-tcp6-segmentation: off [requested on]
udp-fragmentation-offload: off
generic-segmentation-offload: on
generic-receive-offload: on
...
tx-udp-segmentation: on
...

.ndo_feature_check seems unnecessary.
Because the stack has already do this check.
in __ip_append_data() below if branch will not run if hardware checksum 
offload is off.
         if (transhdrlen &&
             length + fragheaderlen <= mtu &&
             rt->dst.dev->features & (NETIF_F_HW_CSUM | NETIF_F_IP_CSUM) &&
             (!(flags & MSG_MORE) || cork->gso_size) &&
             (!exthdrlen || (rt->dst.dev->features & 
NETIF_F_HW_ESP_TX_CSUM)))
                 csummode = CHECKSUM_PARTIAL;
...
so skb->ip_summed set as CHECKSUM_NONE,
then in udp_send_skb()
         if (cork->gso_size) {
		...
                 if (skb->ip_summed != CHECKSUM_PARTIAL || is_udplite ||
                     dst_xfrm(skb_dst(skb))) {
                         kfree_skb(skb); 
 

                         return -EIO;
                 }
the packet who needs udp gso will return ERROR.

For this kind of problem how could we fix it?

Thanks.
Huazhong.

> .
>