RE: [Xen-devel] Question about TCP checksum offload in Xen

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxx] On Behalf Of Ian Campbell
> Sent: 05 December 2013 11:39
> To: Anil Madhavapeddy
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Balraj Singh; Mirage List
> Subject: Re: [Xen-devel] Question about TCP checksum offload in Xen
> 
> On Thu, 2013-12-05 at 11:29 +0000, Anil Madhavapeddy wrote:
> > On Tue, Dec 03, 2013 at 01:00:23PM +0000, Balraj Singh wrote:
> > > Hi,
> > >
> > > I'm working on verifying TCP checksums on incoming packets in Mirage,
> but
> > > I've run into a bit of a problem.
> > >
> > > If TCP checksum offload is turned on on a virtual interface (this is the
> > > default), and if the TCP connection is local to the machine, it looks like
> > > Xen does not calculate the checksum at all.  This may be valid because
> Xen
> > > may be providing a stronger guarantee, but it means that incoming
> packets
> > > don't have a valid checksum in the header.  This then means that in
> Mirage
> > > we can't just have checksum verification turned on all the time.  This
> > > would have been the safe fall back option and detecting that checksum
> > > offload is on, and then not duplicating the verification in Mirage would
> > > have been an optimisation.  But it looks like this is not an option.  Now 
> > > I
> > > need to know for every incoming packet whether checksum verification
> should
> > > be done or not.  It should ideally be for every packet since chksum
> offload
> > > can be turned off and on on the VIF and existing tcp connections should
> > > continue.  If not every packet, I need to get a notification or 
> > > efficiently
> > > detect right away that the setting is changed on the VIF.
> >
> > This is a question that seems to keep coming up even for Linux and
> > Windows, as the combination of local<->local VMs vs local<->off-host and
> > the checksum offload is quite confusing.
> >
> > CCing xen-devel: is the appropriate behaviour for a guest VM that wants to
> > use checksum offloading in all situations documented anywhere?
> 
> I don't understand the question/concern. If you have enabled checksum
> offload then of course you don't recalculate the checksum, that's the
> whole point of offloading it.
> 

If your frontend doesn't advertise checksum offload at ring connect time then 
the backend should not advertise checksum offload to the network stack and 
hence all packets passing to the VM should have valid checksum. If your 
frontend *does* advertise offload then you need to look at the NETRXF flags in 
the head descriptor of any packet you handle to determine the presence/validity 
of any checksum.

  Paul