From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [net PATCH] net: virtio: cap mtu when XDP programs are running Date: Thu, 5 Jan 2017 11:09:14 +0800 Message-ID: <068f0116-b37e-eb44-8c60-1781a9d5255e@redhat.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Cc: john.r.fastabend@intel.com, netdev@vger.kernel.org, alexei.starovoitov@gmail.com, daniel@iogearbox.net To: John Fastabend , mst@redhat.com Return-path: Received: from mx1.redhat.com ([209.132.183.28]:53526 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751946AbdAEDJW (ORCPT ); Wed, 4 Jan 2017 22:09:22 -0500 In-Reply-To: <586D458F.5050705@gmail.com> Sender: netdev-owner@vger.kernel.org List-ID: 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