Re: [Patch net v3] mlx5: force CHECKSUM_NONE for short ethernet frames

From: Eric Dumazet <eric.dumazet@gmail.com>
To: Cong Wang <xiyou.wangcong@gmail.com>, saeedm@dev.mellanox.co.il
Cc: Eric Dumazet <edumazet@google.com>,
	Linux Kernel Network Developers <netdev@vger.kernel.org>,
	Saeed Mahameed <saeedm@mellanox.com>,
	Tariq Toukan <tariqt@mellanox.com>
Subject: Re: [Patch net v3] mlx5: force CHECKSUM_NONE for short ethernet frames
Date: Tue, 4 Dec 2018 13:16:35 -0800	[thread overview]
Message-ID: <325adc28-b9b7-947c-a3a8-ae848c224957@gmail.com> (raw)
In-Reply-To: <CAM_iQpUqhaJ0_hJZfZoJtu50K5bae0oR5f1jAUVR1F7id6YLzQ@mail.gmail.com>

On 12/04/2018 12:35 PM, Cong Wang wrote:
> On Tue, Dec 4, 2018 at 11:17 AM Saeed Mahameed
> <saeedm@dev.mellanox.co.il> wrote:
>>
>> On Mon, Dec 3, 2018 at 11:52 PM Eric Dumazet <edumazet@google.com> wrote:
>>>
>>> On Mon, Dec 3, 2018 at 11:30 PM Cong Wang <xiyou.wangcong@gmail.com> wrote:
>>>>
>>>> On Mon, Dec 3, 2018 at 11:08 PM Eric Dumazet <edumazet@google.com> wrote:
>>>>>
>>>>> The hardware has probably validated the L3 & L4 checksum just fine.
>>>>>
>>>>> Note that if ip_summed is CHECKSUM_UNNECESSARY, the padding bytes (if any)
>>>>> have no impact on the csum that has been verified by the NIC.
>>>>
>>>>
>>>> Why? Why does the hardware validates L3/L4 checksum when it
>>>> supplies a full-packet checksum? What's its point here?
>>>
>>> The point is that the driver author can decide what is best.
>>>
>>> For native IP+TCP or IP+UDP, the NIC has the ability to fully
>>> understand the packet and fully validate the checksum.
>>
>> Also for Native IP4 and IP6 plain L3 packets.
>> The Hardware validates the csum when it can, and always provides
>> checksum complete for all packets.
>> One of the reason to validate is that sometimes we want to skip
>> checksum complete, but still leverage the hw validation,
>> like in your patch :), or LRO case, or many other cases in other
>> kernels/OSes/drivers.
> 
> This sounds wrong to me too.
> 
> If the HW already validates it, the software doesn't need to do it,
> therefore must skip hw csum for performance gain.,
> 
> 
>>
>> So i agree with Eric, let's jump to checksum_unnecessary for short packets.
> 
> This is odd, if Eric is right, then we should completely get rid of
> CHECKSUM_COMPLETE. Short packets are not exceptions.
> 
> I still don't understand why people including Eric kept fixing this
> thing which could be just removed from the very beginning.
> Sounds like nobody even looked into it until my patch.
> 

Erm I never suggested to get rid of CHECKSUM_COMPLETE...
My suggestion was to reorder the mlx5 logic to match mlx4 one.

CHECKSUM_COMPLETE is very nice _when_/_if_ the NIC is unable to
fully dissect a packet and validate L4, as a fallback.

I am pretty sure for example that IP reassembly can benefit from CHECKSUM_COMPLETE.
(Although for some reason mlx4 code does not handle IPv6 fragments in its CHECKSUM_COMPLETE path)