From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [net PATCH] net: virtio: cap mtu when XDP programs are running Date: Tue, 10 Jan 2017 01:24:45 +0200 Message-ID: <20170110012044-mutt-send-email-mst@kernel.org> References: <20170102223031.11541.28717.stgit@john-Precision-Tower-5810> <402027b4-58c7-aa1b-5079-74e31448f544@redhat.com> <586BD5D5.6020100@gmail.com> <1caf1ffc-0f46-067e-0f0d-a93b408b4ffd@redhat.com> <586D458F.5050705@gmail.com> <068f0116-b37e-eb44-8c60-1781a9d5255e@redhat.com> <20170110010531-mutt-send-email-mst@kernel.org> <5874190B.9050505@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: Jason Wang , john.r.fastabend@intel.com, netdev@vger.kernel.org, alexei.starovoitov@gmail.com, daniel@iogearbox.net To: John Fastabend Return-path: Received: from mx1.redhat.com ([209.132.183.28]:35318 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753839AbdAIXYr (ORCPT ); Mon, 9 Jan 2017 18:24:47 -0500 Content-Disposition: inline In-Reply-To: <5874190B.9050505@gmail.com> Sender: netdev-owner@vger.kernel.org List-ID: On Mon, Jan 09, 2017 at 03:13:15PM -0800, John Fastabend wrote: > On 17-01-09 03:05 PM, Michael S. Tsirkin wrote: > > On Thu, Jan 05, 2017 at 11:09:14AM +0800, Jason Wang wrote: > >> > >> > >> On 2017年01月05日 02:57, John Fastabend wrote: > >>> [...] > >>> > >>>> On 2017年01月04日 00:48, John Fastabend wrote: > >>>>> On 17-01-02 10:14 PM, Jason Wang wrote: > >>>>>> On 2017年01月03日 06:30, John Fastabend wrote: > >>>>>>> XDP programs can not consume multiple pages so we cap the MTU to > >>>>>>> avoid this case. Virtio-net however only checks the MTU at XDP > >>>>>>> program load and does not block MTU changes after the program > >>>>>>> has loaded. > >>>>>>> > >>>>>>> This patch sets/clears the max_mtu value at XDP load/unload time. > >>>>>>> > >>>>>>> Signed-off-by: John Fastabend > >>>>>>> --- > >>> [...] > >>> > >>>>> OK so this logic is a bit too simply. When it resets the max_mtu I guess it > >>>>> needs to read the mtu via > >>>>> > >>>>> virtio_cread16(vdev, ...) > >>>>> > >>>>> or we may break the negotiated mtu. > >>>> Yes, this is a problem (even use ETH_MAX_MTU). We may need a method to notify > >>>> the device about the mtu in this case which is not supported by virtio now. > >>> Note this is not really a XDP specific problem. The guest can change the MTU > >>> after init time even without XDP which I assume should ideally result in a > >>> notification if the MTU is negotiated. > >> > >> Yes, Michael, do you think we need add some mechanism to notify host about > >> MTU change in this case? > >> > >> Thanks > > > > Why does host care? > > > > Well the guest will drop packets after mtu has been reduced. I didn't know. What place in code does this? > Although the guest > by reducing its MTU in some sense must expect this. Likewise if the host were > to change MTU after virtio_net probe time the guest would not learn about it. The spec explicitly disallows this last one. > I think at best negotiating the mtu is just a hint? If system _really_ cares > we could use lldp or some other out of band mechanism to learn/set/adjust MTU > on both systems and it would be more robust. I'm not actually convinced this > is a problem in bare metal systems we have the same issue with physical > switches and solve it out of band via configuration, protocols, etc. > > .John ATM we don't have negotiation in virtio, just a max mtu limit. This doesn't free guest from configuring mtu correctly, just helps it avoid doing something clearly bogus. -- MST