From mboxrd@z Thu Jan 1 00:00:00 1970 From: Maxime Coquelin Subject: Re: [PATCH v2] add mtu set in virtio Date: Wed, 7 Sep 2016 11:16:47 +0200 Message-ID: References: <20160829230240.20164-1-sodey@sonusnet.com> <20160907032547.GG23158@yliu-dev.sh.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Cc: souvikdey33 , stephen@networkplumber.org, huawei.xie@intel.com, dev@dpdk.org To: Yuanhan Liu Return-path: Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by dpdk.org (Postfix) with ESMTP id 7ADD97E24 for ; Wed, 7 Sep 2016 11:16:51 +0200 (CEST) In-Reply-To: <20160907032547.GG23158@yliu-dev.sh.intel.com> List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" On 09/07/2016 05:25 AM, Yuanhan Liu wrote: > On Tue, Aug 30, 2016 at 09:57:39AM +0200, Maxime Coquelin wrote: >> Hi Souvik, >> >> On 08/30/2016 01:02 AM, souvikdey33 wrote: >>> Signed-off-by: Souvik Dey >>> >>> Fixes: 1fb8e8896ca8 ("Signed-off-by: Souvik Dey ") >>> Reviewed-by: Stephen Hemminger >>> >>> Virtio interfaces should also support setting of mtu, as in case of cloud >>> it is expected to have the consistent mtu across the infrastructure that >>> the dhcp server sends and not hardcoded to 1500(default). >>> --- >>> drivers/net/virtio/virtio_ethdev.c | 12 ++++++++++++ >>> 1 file changed, 12 insertions(+) >> >> FYI, there are some on-going changes in the VIRTIO specification >> so that the VHOST interface exposes its MTU to its VIRTIO peer. >> It may also be used as an alternative of what you patch achieves. >> >> I am working on its implementation in Qemu/DPDK, our goal being to >> reduce performance drops for small packets with Rx mergeable buffers >> feature enabled. > > Mind to educate me a bit on how that works? Of course. Basically, this is a way to advise the MTU we want in the guest. In the guest, if GRO is not enabled: - In case of Kernel virtio-net, it could be used to size the SKBs at the expected MTU. If possible, we could disable Rx mergeable buffers. - In case of virtio PMD, if the MTU advised by host is lower than the pre-allocated mbuf size for the receive queue, then we should not need mergeable buffers. Does that sound reasonnable? Do I miss something? Thanks, Maxime