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

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Date:	Tue, 2 Dec 2014 17:41:58 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	"Michael S. Tsirkin" <mst@...hat.com>
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,
	jesse@...ira.com, pshelar@...ira.com
Subject: Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU

On 12/02/14 at 07:34pm, Michael S. Tsirkin wrote:
> On Tue, Dec 02, 2014 at 05:09:27PM +0000, Thomas Graf wrote:
> > On 12/02/14 at 01:48pm, Flavio Leitner wrote:
> > > What about containers or any other virtualization environment that
> > > doesn't use Virtio?
> > 
> > The host can dictate the MTU in that case for both veth or OVS
> > internal which would be primary container plumbing techniques.
> 
> It typically can't do this easily for VMs with emulated devices:
> real ethernet uses a fixed MTU.
> 
> IMHO it's confusing to suggest MTU as a fix for this bug, it's
> an unrelated optimization.
> ICMP_DEST_UNREACH/ICMP_FRAG_NEEDED is the right fix here.

PMTU discovery only resolves the issue if an actual IP stack is
running inside the VM. This may not be the case at all.

I agree that exposing an MTU towards the guest is not applicable
in all situations, in particular because it is difficult to decide
what MTU to expose. It is a relatively elegant solution in a lot
of virtualization host cases hooked up to an orchestration system
though.
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