Re: [PATCH v3 1/4] vmxnet3: restore tx data ring support

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 13 Jan 2016 02:20:01 +0000
Yong Wang <yongwang@vmware.com> wrote:

> >Good idea to use a local region which optmizes the copy in the host,
> >but this implementation needs to be more general.
> >
> >As written it is broken for multi-segment packets. A multi-segment
> >packet will have a pktlen >= datalen as in:
> >  m -> mb_segs=3, pktlen=1200, datalen=200  
> >    -> datalen=900
> >    -> datalen=100  
> >
> >There are two ways to fix this. You could test for nb_segs == 1
> >or better yet. Optimize each segment it might be that the first
> >segment (or tail segment) would fit in the available data area.  
> 
> Currently the vmxnet3 backend has a limitation of 128B data area so
> it should work even for the multi-segmented pkt shown above. But
> I agree it does not work for all multi-segmented packets.  The
> following packet will be such an example.
> 
> m -> nb_segs=3, pktlen=128, datalen=64
>     -> datalen=32
>     -> datalen=32  
> 
> 
> It’s unclear if/how we might get into such a multi-segmented pkt
> but I agree we should handle this case.  Patch updated taking the
> simple approach (checking for nb_segs == 1).  I’ll leave the
> optimization as a future patch.

Such a packet can happen when adding a tunnel header such as VXLAN
and the underlying packet is shared (refcnt > 1) or does not have
enough headroom for the tunnel header.