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SubjectRe: [PATCH net] virtio-net: unbreak cusmed packet for small buffer XDP
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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@redhat.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

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