From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <stephen@networkplumber.org>
Subject: Re: [PATCH v3 1/4] vmxnet3: restore tx data ring support
Date: Tue, 12 Jan 2016 20:50:22 -0800
Message-ID: <20160112205022.118c3423@xeon-e3>
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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 >=3D datalen as in:
> >  m -> mb_segs=3D3, pktlen=3D1200, datalen=3D200 =20
> >    -> datalen=3D900
> >    -> datalen=3D100 =20
> >
> >There are two ways to fix this. You could test for nb_segs =3D=3D 1
> >or better yet. Optimize each segment it might be that the first
> >segment (or tail segment) would fit in the available data area. =20
>=20
> 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.
>=20
> m -> nb_segs=3D3, pktlen=3D128, datalen=3D64
>     -> datalen=3D32
>     -> datalen=3D32 =20
>=20
>=20
> It=E2=80=99s 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 =3D=3D 1).  I=E2=80=99ll 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.