netdev - Re: [Patch net] mlx4: set csum_complete_sw bit when fixing complete csum

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Date:	Mon, 27 Jun 2016 14:49:58 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Tom Herbert <tom@...bertland.com>
Cc:	Or Gerlitz <gerlitz.or@...il.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Shani Michaeli <shanim@...lanox.com>,
	Tariq Toukan <tariqt@...lanox.com>,
	Yishai Hadas <yishaih@...lanox.com>,
	Carol L Soto <clsoto@...ibm.com>
Subject: Re: [Patch net] mlx4: set csum_complete_sw bit when fixing complete csum

On Mon, Jun 27, 2016 at 2:47 PM, Tom Herbert <tom@...bertland.com> wrote:
> On Mon, Jun 27, 2016 at 2:44 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>> On Mon, Jun 27, 2016 at 2:08 PM, Or Gerlitz <gerlitz.or@...il.com> wrote:
>>> On Mon, Jun 27, 2016 at 9:22 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>>>> The stack doesn't trust the complete csum by hardware
>>>> even when it is correct.
>>>
>>> Can you explain that a little further?
>>
>> Sure, here is the code in __skb_checksum_complete():
>>
>>         /* skb->csum holds pseudo checksum */
>>         sum = csum_fold(csum_add(skb->csum, csum));
>>         if (likely(!sum)) {
>>                 if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE) &&
>>                     !skb->csum_complete_sw)
>>                         netdev_rx_csum_fault(skb->dev);
>>         }
>>
>> So when sum == 0, it means the checksum is correct. And
>> we already set ->ip_summed to CHECKSUM_COMPLETE
>> after check_csum(), and ->csum_complete_sw is initialized
>> to 0 when we allocate the skb. This is why we trigger
>> netdev_rx_csum_fault().
>>
> Yes, but this also means that the driver gave the stack a checksum
> complete value that was incorrect. That's an error.

That is the whole purpose of commit f8c6455bb04b944edb69e,
isn't it?