linux-kernel - Re: [PATCH net] virtio-net: unbreak cusmed packet for small buffer XDP

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Date:   Wed, 28 Jun 2017 20:05:06 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] virtio-net: unbreak cusmed packet for small buffer
 XDP



On 2017年06月28日 12:01, Michael S. Tsirkin wrote:
> On Wed, Jun 28, 2017 at 11:40:30AM +0800, Jason Wang wrote:
>>
>> On 2017年06月28日 11:31, Michael S. Tsirkin wrote:
>>> On Wed, Jun 28, 2017 at 10:45:18AM +0800, Jason Wang wrote:
>>>> On 2017年06月28日 10:17, Michael S. Tsirkin wrote:
>>>>> On Wed, Jun 28, 2017 at 10:14:34AM +0800, Jason Wang wrote:
>>>>>> On 2017年06月28日 10:02, Michael S. Tsirkin wrote:
>>>>>>> On Wed, Jun 28, 2017 at 09:54:03AM +0800, Jason Wang wrote:
>>>>>>>> We should allow csumed packet for small buffer, otherwise XDP_PASS
>>>>>>>> won't work correctly.
>>>>>>>>
>>>>>>>> Fixes commit bb91accf2733 ("virtio-net: XDP support for small buffers")
>>>>>>>> Signed-off-by: Jason Wang<jasowang@...hat.com>
>>>>>>> The issue would be VIRTIO_NET_HDR_F_DATA_VALID might be set.
>>>>>>> What do you think?
>>>>>> I think it's safe. For XDP_PASS, it work like in the past.
>>>>> That's the part I don't get. With DATA_VALID csum in packet is wrong, XDP
>>>>> tools assume it's value.
>>>> DATA_VALID is CHECKSUM_UNCESSARY on the host, and according to the comment
>>>> in skbuff.h
>>>>
>>>>
>>>> "
>>>>    *   The hardware you're dealing with doesn't calculate the full checksum
>>>>    *   (as in CHECKSUM_COMPLETE), but it does parse headers and verify
>>>> checksums
>>>>    *   for specific protocols. For such packets it will set
>>>> CHECKSUM_UNNECESSARY
>>>>    *   if their checksums are okay. skb->csum is still undefined in this case
>>>>    *   though. A driver or device must never modify the checksum field in the
>>>>    *   packet even if checksum is verified.
>>>> "
>>>>
>>>> The csum is correct I believe?
>>>>
>>>> Thanks
>>> That's on input. But I think for tun it's output, where that is equivalent
>>> to CHECKSUM_NONE
>>>
>>>
>> Yes, but the comment said:
>>
>> "
>> CKSUM_NONE:
>>   *
>>   *   The skb was already checksummed by the protocol, or a checksum is not
>>   *   required.
>>   *
>>   * CHECKSUM_UNNECESSARY:
>>   *
>>   *   This has the same meaning on as CHECKSUM_NONE for checksum offload on
>>   *   output.
>>   *
>> "
>>
>> So still correct I think?
>>
>> Thanks
> Hmm maybe I mean NEEDS_CHECKSUM actually.
>
> I'll need to re-read the spec.
>

Not sure this is an issue. But if it is, we can probably checksum the 
packet before passing it to XDP. But it would be a little slow.

Thanks