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Date: Sun, 30 Nov 2014 11:26:40 +0100 From: Florian Westphal <fw@...len.de> To: Fan Du <fan.du@...el.com> Cc: netdev@...r.kernel.org, davem@...emloft.net, fw@...len.de Subject: Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU Fan Du <fan.du@...el.com> wrote: > Test scenario: two KVM guests sitting in different > hosts communicate to each other with a vxlan tunnel. > > All interface MTU is default 1500 Bytes, from guest point > of view, its skb gso_size could be as bigger as 1448Bytes, > however after guest skb goes through vxlan encapuslation, > individual segments length of a gso packet could exceed > physical NIC MTU 1500, which will be lost at recevier side. > > So it's possible in virtualized environment, locally created > skb len after encapslation could be bigger than underlayer > MTU. In such case, it's reasonable to do GSO first, > then fragment any packet bigger than MTU as possible. > > +---------------+ TX RX +---------------+ > | KVM Guest | -> ... -> | KVM Guest | > +-+-----------+-+ +-+-----------+-+ > |Qemu/VirtIO| |Qemu/VirtIO| > +-----------+ +-----------+ > | | > v tap0 tap0 v > +-----------+ +-----------+ > | ovs bridge| | ovs bridge| > +-----------+ +-----------+ > | vxlan vxlan | > v v > +-----------+ +-----------+ > | NIC | <------> | NIC | > +-----------+ +-----------+ > > Steps to reproduce: > 1. Using kernel builtin openvswitch module to setup ovs bridge. > 2. Runing iperf without -M, communication will stuck. Hmm, do we really want to suport bridges containing interfaces with different MTUs? It seems to me to only clean solution is to set tap0 MTU so that it accounts for the bridge encap overhead. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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