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: Wed, 5 Apr 2017 15:54:00 +0200 Message-ID: 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: "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 22BC22A6C for ; Wed, 5 Apr 2017 15:54:05 +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 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? Thanks, Maxime