Re: [dpdk-dev] [PATCH 0/2] virtio fix false offload claims

[Olivier MATZ <olivier.matz@6wind.com>]

On Wed, Aug 23, 2017 at 08:31:35AM -0700, Stephen Hemminger wrote:
> On Wed, 23 Aug 2017 11:30:26 +0200
> Olivier MATZ <olivier.matz@6wind.com> wrote:
> 
> > Hello,
> > 
> > On Sat, Jul 08, 2017 at 11:12:22AM +0800, Yuanhan Liu wrote:
> > > On Fri, Jul 07, 2017 at 12:52:48PM -0700, Stephen Hemminger wrote:  
> > > > While doing code for Hyper-V, noticed that the virtio driver was
> > > > confused about receive versus transmit offloads.  The virtio
> > > > checksum offload is L4 (TCP/UDP) only, not IPv4. Also, TSO
> > > > and LRO are not the same.
> > > > 
> > > > This may break some program that was assuming it was getting offloads
> > > > that it wasn't.  
> > > 
> > > Applied to dpdk-next-virtio.
> > > 
> > > And I think they should be backported to stable releases, thus,
> > > 
> > >     Cc: stable@dpdk.org
> > > 
> > > Thanks.
> > > 
> > > 	--yliu  
> > > > 
> > > > Stephen Hemminger (2):
> > > >   virtio: don't falsely claim to do IP checksum
> > > >   virtio: don't claim to support LRO
> > > > 
> > > >  drivers/net/virtio/virtio_ethdev.c | 30 +++++-------------------------
> > > >  1 file changed, 5 insertions(+), 25 deletions(-)
> > > > 
> > > > -- 
> > > > 2.11.0  
> > 
> > I think these 2 commits break the virtio offload, which can be tested as
> > described in this test plan:
> > http://dpdk.org/ml/archives/dev/2016-October/048092.html
> > 
> > First, about checksum: the description of rxmode->hw_ip_checksum is:
> > 
> >      hw_ip_checksum   : 1, /**< IP/UDP/TCP checksum offload enable. */
> > 
> > So, while I agree the name is not well chosen, it is valid to set it
> > for virtio to enable L4 checksum.
> > 
> > Then about LRO: setting rxmode->enable_lro is a way to tell the host that the
> > guest is ok to receive tso packets. From the guest point of view, it is like
> > enabling lro on a physical driver. Again, it is valid and useful to do this.
> > 
> > Before removing these features, it would have been nice to have a quick look at
> > the commits that introduced them.
> 
> I am ok with keeping LRO as long as the documentation changed.  And virtio
> driver did some enforcement. 
> 
> For checksums, the hw_ip_checksum flag either needs to be more fine grain (IP, UDP, TCP)
> which would be best, or virtio would have to check IP checksum in software.
> 

For checksum, yes, the rxconf should be more fine-grained and renamed.

But apart from the name which is confusing, it was not wrong.
Setting hw_ip_checksum=1 means: "allow the driver to return packets with
checksums flags != unknown". These flags are good,bad,unknown,none for
both l3 and l4. So virtio driver always return unknown for l3, and
none|unknown|good|bad for l4, depending on what the host passed.