From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Graf Subject: Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU Date: Tue, 2 Dec 2014 17:41:58 +0000 Message-ID: <20141202174158.GB9457@casper.infradead.org> References: <1417156385-18276-1-git-send-email-fan.du@intel.com> <1417158128.3268.2@smtp.corp.redhat.com> <5A90DA2E42F8AE43BC4A093BF0678848DED92B@SHSMSX104.ccr.corp.intel.com> <20141201135225.GA16814@casper.infradead.org> <20141202154839.GB5344@t520.home> <20141202170927.GA9457@casper.infradead.org> <20141202173401.GB4126@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "Du, Fan" , 'Jason Wang' , "netdev@vger.kernel.org" , "davem@davemloft.net" , "fw@strlen.de" , dev@openvswitch.org, jesse@nicira.com, pshelar@nicira.com To: "Michael S. Tsirkin" Return-path: Received: from casper.infradead.org ([85.118.1.10]:37892 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751124AbaLBRmB (ORCPT ); Tue, 2 Dec 2014 12:42:01 -0500 Content-Disposition: inline In-Reply-To: <20141202173401.GB4126@redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: 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.