On Wed, 13 Jan 2016 02:20:01 +0000
Yong Wang [off-list ref] wrote:

quoted

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.