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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2013-12-05 at 11:45 +0000, Richard Mortier wrote:
> On 5 Dec 2013, at 11:39, Ian Campbell wrote:
> 
> > 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.
> 
> i think balraj's question arises because the status of checksum
> offload can change mid-tcp-flow. how does he know whether it's on or
> off for a given packet?

It's a property of the NIC configuration.

In Linux for example a received skb gets a field set which indicates the
offload state at the time it was received.

Ian.