From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Tan, Jianfeng" Subject: Re: [PATCH v3 7/9] net/virtio: Add MTU feature support Date: Wed, 5 Apr 2017 22:50:54 +0800 Message-ID: <5f8ae5f8-22a5-ef95-058d-6865b154bef0@intel.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> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit To: Maxime Coquelin , aconole@redhat.com, sodey@sonusnet.com, yuanhan.liu@linux.intel.com, thomas.monjalon@6wind.com, dev@dpdk.org Return-path: Received: from mga05.intel.com (mga05.intel.com [192.55.52.43]) by dpdk.org (Postfix) with ESMTP id D637C2B84 for ; Wed, 5 Apr 2017 16:50:57 +0200 (CEST) In-Reply-To: List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" 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, Jianfeng