> > - as you defer ack until xennetback_tx_free and the mbuf can live ~forever,
> >   you can capture all buffers.
> >   (i think that linux netback has the same problem.)
> 
> I wondered about this. Can the kernel really grab a mbuf forever ? For
> which reason ?

eg. when an application never read from the socket.

> An easy fix would be to switch back to copy when we are low on buffers.

yes, it can be a workaround.

YAMAMOTO Takashi