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

[Jason Wang]

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.

Thanks