netdev - Re: [PATCH v3] net: Make skb_segment not to compute checksum if network controller supports checksumming

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Message-Id: <20200321.200634.534398039682553516.davem@davemloft.net>
Date:   Sat, 21 Mar 2020 20:06:34 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     kyk.segfault@...il.com
Cc:     David.Laight@...lab.com, eric.dumazet@...il.com,
        netdev@...r.kernel.org, willemdebruijn.kernel@...il.com
Subject: Re: [PATCH v3] net: Make skb_segment not to compute checksum if
 network controller supports checksumming

From: Yadu Kishore <kyk.segfault@...il.com>
Date: Tue, 17 Mar 2020 14:08:38 +0530

> 
>> I think you can rebase and submit against net-next.
> 
>> If your patch isn't active in the networking development patchwork instance,
>> it is not pending to be applied and you must resend it.
> 
> Rebasing the patch on net-next and resending it.
> 
> Problem:
> TCP checksum in the output path is not being offloaded during GSO
> in the following case:
> The network driver does not support scatter-gather but supports
> checksum offload with NETIF_F_HW_CSUM.
> 
> Cause:
> skb_segment calls skb_copy_and_csum_bits if the network driver
> does not announce NETIF_F_SG. It does not check if the driver
> supports NETIF_F_HW_CSUM.
> So for devices which might want to offload checksum but do not support SG
> there is currently no way to do so if GSO is enabled.
> 
> Solution:
> In skb_segment check if the network controller does checksum and if so
> call skb_copy_bits instead of skb_copy_and_csum_bits.
> 
> Testing:
> Without the patch, ran iperf TCP traffic with NETIF_F_HW_CSUM enabled
> in the network driver. Observed the TCP checksum offload is not happening
> since the skbs received by the driver in the output path have
> skb->ip_summed set to CHECKSUM_NONE.
> 
> With the patch ran iperf TCP traffic and observed that TCP checksum
> is being offloaded with skb->ip_summed set to CHECKSUM_PARTIAL.
> Also tested with the patch by disabling NETIF_F_HW_CSUM in the driver
> to cover the newly introduced if-else code path in skb_segment.
> 
> Link: https://lore.kernel.org/netdev/CA+FuTSeYGYr3Umij+Mezk9CUcaxYwqEe5sPSuXF8jPE2yMFJAw@mail.gmail.com
> Signed-off-by: Yadu Kishore <kyk.segfault@...il.com>

Willem, please review and ACK/NACK.

Thank you.