From mboxrd@z Thu Jan 1 00:00:00 1970 From: Maxime Coquelin Subject: Re: [PATCH v3 7/9] net/virtio: Add MTU feature support Date: Fri, 7 Apr 2017 18:40:39 +0200 Message-ID: <1e0af6da-cb01-fb91-c0b0-efa08ebd9f21@redhat.com> References: <20170213142820.8964-1-maxime.coquelin@redhat.com> <20170312163406.17714-1-maxime.coquelin@redhat.com> <20170312163406.17714-8-maxime.coquelin@redhat.com> <0e5df10f-c61f-b1cd-a604-148379485ef2@intel.com> <12327337-70fd-168a-d836-026d619089f8@redhat.com> <5f8ae5f8-22a5-ef95-058d-6865b154bef0@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit To: "Tan, Jianfeng" , aconole@redhat.com, sodey@sonusnet.com, yuanhan.liu@linux.intel.com, thomas.monjalon@6wind.com, dev@dpdk.org Return-path: Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by dpdk.org (Postfix) with ESMTP id 5274C5689 for ; Fri, 7 Apr 2017 18:40:44 +0200 (CEST) In-Reply-To: <5f8ae5f8-22a5-ef95-058d-6865b154bef0@intel.com> List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" Hi Jianfeng, On 04/05/2017 04:50 PM, Tan, Jianfeng wrote: > > > On 4/5/2017 9:54 PM, Maxime Coquelin wrote: >> >> >> On 04/05/2017 11:42 AM, Tan, Jianfeng wrote: >>> Hi Maxime, >>> >>> Thank you for replying. >>> >>> On 4/5/2017 3:11 PM, Maxime Coquelin wrote: >>>> Hi Jianfeng, >>>> >>>> On 04/05/2017 06:52 AM, Tan, Jianfeng wrote: >>>>> Hi Maxime, >>>>> >>>>> Have some confusion about this feature. Please help confirm. >>>>> >>>>> (1) With this feature, we only support to advertise MTU value which is >>>>> defined by QEMU to frontend and backend driver separately. (2) But it >>>>> does not allow frontend driver to set a different MTU to QEMU and then >>>>> to vhost backend. >>>>> >>>>> Correct? >>>>> If it's correct, why not MTU works like (2)? >>>> >>>> Because idea is that the hosts advertises the maximum MTU value it >>>> supports. The frontend driver is free to use a smaller value. The goal >>>> of this change is to make possible to set a uniform MTU value across >>>> the infrastructure, the management tools giving a hint to the guests on >>>> the MTU value it should use. >>> >>> Based on that MTU is the maximum packet size that can be sent instead of >>> that can be received: >>> (1) Why vhost (as a device) does not drop any packets which are longer >>> than MTU when dequeue()? >> That's a good point. >> As when MTU value is negotiated, the guest agrees not to send larger >> packets. But we cannot trust the guest, so vhost needs to check the >> packet length. >> >>> (2) See some NICs also use MTU to calculate maximum packet size that can >>> be received, like ixgbe, i40e, shall we also do that? >> Can you give me some pointers to the code? > > Please refer ixgbe_dev_mtu_set(), and i40e_dev_mtu_set(). Thanks. Yes, we could also do that. I can send a patch for this and another one to check the size of the packet respects negotiated MTU value. Or maybe you want to do this? Regards, Maxime