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: Tue, 10 Jan 2017 10:29:39 +0800 Message-ID: <9102bb4b-223a-d441-7546-8b4144d970fb@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> <068f0116-b37e-eb44-8c60-1781a9d5255e@redhat.com> <20170110010531-mutt-send-email-mst@kernel.org> <5874190B.9050505@gmail.com> <20170110012044-mutt-send-email-mst@kernel.org> <58742187.8050505@gmail.com> <20170110015759-mutt-send-email-mst@kernel.org> 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: "Michael S. Tsirkin" , John Fastabend Return-path: Received: from mx1.redhat.com ([209.132.183.28]:57416 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1764696AbdAJC3s (ORCPT ); Mon, 9 Jan 2017 21:29:48 -0500 In-Reply-To: <20170110015759-mutt-send-email-mst@kernel.org> Sender: netdev-owner@vger.kernel.org List-ID: On 2017年01月10日 07:58, Michael S. Tsirkin wrote: > On Mon, Jan 09, 2017 at 03:49:27PM -0800, John Fastabend wrote: >> On 17-01-09 03:24 PM, Michael S. Tsirkin wrote: >>> 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? >>> >> hmm in many of the drivers it is convention to use the mtu to set the rx >> buffer sizes and a receive side max length filter. For example in the Intel >> drivers if a packet with length greater than MTU + some headroom is received we >> drop it. I guess in the networking stack RX path though nothing forces this and >> virtio doesn't have any code to drop packets on rx size. >> >> In virtio I don't see any existing case currently. In the XDP case though we >> need to ensure packets fit in a page for the time being which is why I was >> looking at this code and generated this patch. > I'd say just look at the hardware max mtu. Ignore the configured mtu. > > Does this work for small buffers consider it always allocate skb with size of GOOD_PACKET_LEN? I think in any case, we should limit max_mtu to GOOD_PACKET_LEN for small buffers. Thanks