[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] 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. Ian. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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