From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from dpdk.org (dpdk.org [92.243.14.124]) by inbox.dpdk.org (Postfix) with ESMTP id 1AE0CA054F; Tue, 18 Feb 2020 15:20:30 +0100 (CET) Received: from [92.243.14.124] (localhost [127.0.0.1]) by dpdk.org (Postfix) with ESMTP id E60861C119; Tue, 18 Feb 2020 15:20:29 +0100 (CET) Received: from mga12.intel.com (mga12.intel.com [192.55.52.136]) by dpdk.org (Postfix) with ESMTP id B97EE1C11A for ; Tue, 18 Feb 2020 15:20:28 +0100 (CET) X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga004.fm.intel.com ([10.253.24.48]) by fmsmga106.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 18 Feb 2020 06:20:27 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,456,1574150400"; d="scan'208";a="258568386" Received: from silpixa00399839.ir.intel.com (HELO silpixa00399839.ger.corp.intel.com) ([10.237.222.8]) by fmsmga004.fm.intel.com with ESMTP; 18 Feb 2020 06:20:26 -0800 From: Ciara Loftus To: dev@dpdk.org Cc: xiaolong.ye@intel.com, yinan.wang@intel.com, Ciara Loftus Date: Tue, 18 Feb 2020 14:03:59 +0000 Message-Id: <20200218140359.7533-1-ciara.loftus@intel.com> X-Mailer: git-send-email 2.25.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Subject: [dpdk-dev] [PATCH] doc: update af_xdp documentation on MTU limitations X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" Explain how kernel driver RX buffer sizes affect the maximum MTU size in practice. Signed-off-by: Ciara Loftus --- doc/guides/nics/af_xdp.rst | 26 +++++++++++++++++++++++--- 1 file changed, 23 insertions(+), 3 deletions(-) diff --git a/doc/guides/nics/af_xdp.rst b/doc/guides/nics/af_xdp.rst index b434b25df..07bdd29e2 100644 --- a/doc/guides/nics/af_xdp.rst +++ b/doc/guides/nics/af_xdp.rst @@ -18,9 +18,6 @@ packets through the socket which would bypass the kernel network stack. Current implementation only supports single queue, multi-queues feature will be added later. -Note that MTU of AF_XDP PMD is limited due to XDP lacks support for -fragmentation. - AF_XDP PMD enables need_wakeup flag by default if it is supported. This need_wakeup feature is used to support executing application and driver on the same core efficiently. This feature not only has a large positive performance @@ -57,3 +54,26 @@ The following example will set up an af_xdp interface in DPDK: .. code-block:: console --vdev net_af_xdp,iface=ens786f1 + +Limitations +----------- + +- **MTU** + + The MTU of the AF_XDP PMD is limited due to the XDP requirement of one packet + per page. In the PMD we report the maximum MTU for zero copy to be equal + to the page size less the frame overhead introduced by AF_XDP (XDP HR = 256) + and DPDK (frame headroom = 320). With a 4K page size this works out at 3520. + However in practice this value may be even smaller, due to differences between + the supported RX buffer sizes of the underlying kernel netdev driver. + + For example, the largest RX buffer size supported by the underlying kernel driver + which is less than the page size (4096B) may be 3072B. In this case, the maximum + MTU value will be at most 3072, but likely even smaller than this, once relevant + headers are accounted for eg. Ethernet and VLAN. + + To determine the actual maximum MTU value of the interface you are using with the + AF_XDP PMD, consult the documentation for the kernel driver. + + Note: The AF_XDP PMD will fail to initialise if an MTU which violates the driver's + conditions as above is set prior to launching the application. -- 2.25.0