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

[Zhang Chen]

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 
> > > <address@hidden>" 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


> Thanks
>
>
>
> .

--
Thanks
Zhang Chen