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Re: [Qemu-discuss] GRO not happening in VM with VxLAN

From: Santosh R
Subject: Re: [Qemu-discuss] GRO not happening in VM with VxLAN
Date: Thu, 2 Jul 2015 00:20:22 +0530

Since the vxlan UDP header checksum is 0, udp_tunnel_gro_complete (called via vxlan_gro_complete) is setting SKB_GSO_UDP_TUNNEL in skb_shinfo(skb)->gso_type.
Later when bridge interface tries to forward this packet to tap interface (br_dev_queue_push_xmit -> __dev_queue_xmit -> validate_xmit_skb) netif_needs_gso will succeed (skb_gso_ok returns 0)
resulting in packet segmentation (which was earlier aggregated) as tap interface doesn't have this feature. Is there a way to enable tx-udp_tnl-segmentation for tap interface?

# ethtool -K tap0 tx-udp_tnl-segmentation on
Could not change any device features

# ethtool -k tap0 | grep -i tnl
tx-udp_tnl-segmentation: off [fixed]

On Tue, Jun 30, 2015 at 12:24 PM, Santosh R <address@hidden> wrote:

On Mon, Jun 29, 2015 at 9:24 PM, Santosh R <address@hidden> wrote:

On Mon, Jun 29, 2015 at 9:04 PM, Vlad Yasevich <address@hidden> wrote:
On 06/29/2015 01:46 AM, Santosh R wrote:
> All,
>    I am testing VxLAN performance in VM. For this I am using below command
> to bring up the VM.
> # qemu-system-x86_64 -m 4096 -smp 4 -boot c -device
> virtio-net-pci,netdev=tap0,mac=00:11:22:33:44:55 -netdev
> tap,id=tap0,script=no,vhost=on  -drive file=/root/vdisk_rhel65.img &
> This is resulting in lower throughput in VM. On looking further I found
> that GRO is not happing on the tap interface.
> Using tcpdump I could see GRO happeing in physcial nic, vxlan and bridge
> interface. but _not_ on tap interface.

I don't think this is GRO.  GRO  only happens at the lowest possible layer.
For packets coming from external sources, that happens just above the nic
when the packet enters the host for the first time.  From then on, the features
of other devices determine if the large aggregated packet will be segmented
again or not.

> So, I thought to use veth pair instead of the tap interface in VM. What is
> the command to do this?

veth pair will not help with this.  you need a tap or a macvtap to have
VM  talk to host.

> Also,
> 1. Why is rx-checksumming off for tap interface?

because tap doesn't do checksum validation.  it allows lower level devices or
VM to do so.

> 2. Why is GRO not happening with tap interface?

Again.  I don't think it's GRO.  Try capturing traffic at tap and see if you get
large packets (more the 1 mtu long) on the tap.  If not, then someone else is fragmenting
packets and you need to find who.
[Santosh]:  Thanks for the reply.
With VxLAN, aggregated packets are seen on physical, VxLAN and bridge interface but _not_ on the tap interface.
Without VxLAN, I do see aggregated (GRO) packets all the way upto VM (i.e. physical, bridge and tap interface).
As you rightly pointed, looks like VxLAN packets are getting fragmented. Any pointers to look for?
By the way, I was referring below link for VxLAN testing and ran into the GRO issue.
This mentions "The veth0 is suppose to be connected to the VM, while the veth1 is connected to the Linux bridge".

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