From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Fastabend Subject: Re: Questions on XDP Date: Thu, 16 Feb 2017 14:36:41 -0800 Message-ID: <58A62979.1050600@gmail.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: Tom Herbert , Alexei Starovoitov , John Fastabend , Jesper Dangaard Brouer , Daniel Borkmann To: Alexander Duyck , Netdev Return-path: Received: from mail-pf0-f170.google.com ([209.85.192.170]:36656 "EHLO mail-pf0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933507AbdBPWgy (ORCPT ); Thu, 16 Feb 2017 17:36:54 -0500 Received: by mail-pf0-f170.google.com with SMTP id 189so8459602pfu.3 for ; Thu, 16 Feb 2017 14:36:54 -0800 (PST) In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: On 17-02-16 12:41 PM, Alexander Duyck wrote: > So I'm in the process of working on enabling XDP for the Intel NICs > and I had a few questions so I just thought I would put them out here > to try and get everything sorted before I paint myself into a corner. > Added Daniel. > So my first question is why does the documentation mention 1 frame per > page for XDP? Is this with the intention at some point to try and > support page flipping into user space, or is it supposed to have been > for the use with an API such as the AF_PACKET mmap stuff? If I am not > mistaken the page flipping has been tried in the past and failed, and > as far as the AF_PACKET stuff my understanding is that the pages had > to be mapped beforehand so it doesn't gain us anything without a > hardware offload to a pre-mapped queue. +1 here. The implementation for virtio does not use page per packet and works fine. And agreed AF_PACKET does not require it. If anyone has page-flipping code I would be happy to benchmark it. > > Second I was wondering about supporting jumbo frames and scatter > gather. Specifically if I let XDP handle the first 2-3K of a frame, > and then processed the remaining portion of the frame following the > directive set forth based on the first frame would that be good enough > to satisfy XDP or do I actually have to support 1 linear buffer > always. For now yes. But, I need a solution to support 64k TSO packets or else VM to VM traffic is severely degraded in my vswitch use case. > > Finally I was looking at xdp_adjust_head. From what I can tell all > that is technically required to support it is allowing the head to be > adjusted either in or out. I'm assuming there is some amount of > padding that is preferred. With the setup I have currently I am > guaranteeing at least NET_SKB_PAD + NET_IP_ALIGN, however I have found > that there should be enough room for 192 bytes on an x86 system if I > am using a 2K buffer. I'm just wondering if that is enough padding or > if we need more for XDP. > Not surprisingly I'm also in agreement here it would help the ixgbe implementation out. > Anyway sorry for the stupid questions but I haven't been paying close > of attention to this and was mostly focused on the DMA bits needed to > support this so now I am playing catch-up. None of the above are stupid IMO. Let me send out the ixgbe implementation later this afternoon so you can have a look at my interpretation of the rules. > > - Alex >