netdev - Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU

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Message-ID: <20141202154839.GB5344@t520.home>
Date:	Tue, 2 Dec 2014 13:48:39 -0200
From:	Flavio Leitner <fbl@...hat.com>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	"Du, Fan" <fan.du@...el.com>, "'Jason Wang'" <jasowang@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"fw@...len.de" <fw@...len.de>, dev@...nvswitch.org, mst@...hat.com,
	jesse@...ira.com, pshelar@...ira.com
Subject: Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU

On Mon, Dec 01, 2014 at 01:52:25PM +0000, Thomas Graf wrote:
> On 11/30/14 at 10:08am, Du, Fan wrote:
> > >-----Original Message-----
> > >From: Jason Wang [mailto:jasowang@...hat.com]
> > >Sent: Friday, November 28, 2014 3:02 PM
> > >To: Du, Fan
> > >Cc: netdev@...r.kernel.org; davem@...emloft.net; fw@...len.de; Du, Fan
> > >Subject: Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU
> > >On Fri, Nov 28, 2014 at 2:33 PM, 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.
> > >
> > >Is this issue specific to ovs or ipv4? Path MTU discovery should help in this case I
> > >believe.
> > 
> > Problem here is host stack push local over-sized gso skb down to NIC, and perform GSO there
> > without any further ip segmentation.
> > 
> > Reasonable behavior is do gso first at ip level, if gso-ed skb is bigger than MTU && df is set, 
> > Then push ICMP_DEST_UNREACH/ICMP_FRAG_NEEDED message back to sender to adjust mtu.
> 
> Aside from this. I think Virtio should provide a MTU hint to the guest
> to adjust MTU in the vNIC to account for both overhead or support for
> jumbo frames in the underlay transparently without relying on PMTU or
> MSS hints. I remember we talked about this a while ago with at least
> Michael but haven't done actual code work on it yet.

What about containers or any other virtualization environment that
doesn't use Virtio?

fbl
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