Re: [Qemu-devel] [PATCH V3 05/10] net/net.c: Add vnet header length to SocketReadState

[Jason Wang <jasowang@redhat.com>]

On 2017年05月09日 12:03, Zhang Chen wrote:
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> On 05/09/2017 10:40 AM, Jason Wang wrote:
>>
>>
>> On 2017年05月08日 11:47, Zhang Chen wrote:
>>>
>>>
>>> On 05/03/2017 06:42 PM, Jason Wang wrote:
>>>>
>>>>
>>>> On 2017年05月03日 11:43, Zhang Chen wrote:
>>>>>
>>>>>
>>>>> On 05/02/2017 12:53 PM, Jason Wang wrote:
>>>>>>
>>>>>>
>>>>>> On 2017年04月28日 17:47, Zhang Chen wrote:
>>>>>>> Address Jason Wang's comments add vnet header length to 
>>>>>>> SocketReadState.
>>>>>>
>>>>>> Instead of saying this, you can add "Suggested-by: Jason Wang 
>>>>>> <jasowang@redhat.com>" above your sign-off.
>>>>>
>>>>> OK.
>>>>>
>>>>>>
>>>>>>> So we change net_fill_rstate() to read
>>>>>>> struct  {int size; int vnet_hdr_len; const uint8_t buf[];}.
>>>>>>
>>>>>> This makes me thinking about the backward compatibility. I think 
>>>>>> we'd better try to keep it as much as possible. E.g how about 
>>>>>> pack vnet_hdr_len into higher bits of size?
>>>>>>
>>>>>
>>>>> Do you means split uint32_t size to uint16_t size and uint16_t 
>>>>> vnet_hdr_len ?
>>>>> If yes, we also can't keep compatibility with old version.
>>>>> Old code send a uint32_t size, we read it as uint16_t size is 
>>>>> always wrong.
>>>>>
>>>>> Thanks
>>>>> Zhang Chen
>>>>
>>>> Consider it's unlikely to send or receive packet >= 64K, we can 
>>>> reuse higher bits (e.g highest 8 bits). Then we can still read 
>>>> uint32_t and then check its highest 8 bits. If it was zero, we know 
>>>> vnet header is zero, if not it could be read as vnet header length.
>>>
>>> I got your point, but in this way, packet size must < 64K, if size 
>>> >=64K we still can't maintain the backward compatibility,
>>> For the packet sender that didn't know the potential packet size 
>>> limit, I think we should make code explicitly declared like
>>> currently code. Otherwise we will make other people confused and 
>>> make code difficult to maintain.
>>>
>>> Thanks
>>> Zhang Chen
>>>
>>
>> Yes, this is an possible issue in the future. Rethink about this, 
>> what if we just compare vnet header? Is there any issue of doing 
>> this? (Sorry, I remember this is a reason, but forget now).
>
> Yes, we can compare all packet with vnet header, the compare 
> performance is very low. but we can't parse raw packet to tcp/udp/icmp 
> packet, because we didn't know the vnet_hdr_len as the offset.
>
> Thanks
> Zhang Chen

Aha, so I think it's time to use the new format but:

- probably need a new option to enable this support for filter
- let's don't touch socket backend, and make it still can work with 
filters whose vnet_hder is off

Thanks

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>> Thanks
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