From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1031331AbeCAOQI convert rfc822-to-8bit (ORCPT ); Thu, 1 Mar 2018 09:16:08 -0500 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:42054 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1031264AbeCAOQG (ORCPT ); Thu, 1 Mar 2018 09:16:06 -0500 Date: Thu, 1 Mar 2018 15:16:01 +0100 From: Jesper Dangaard Brouer To: Jason Wang Cc: mst@redhat.com, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, john.fastabend@gmail.com, Alexei Starovoitov , brouer@redhat.com Subject: Re: [PATCH net-next 0/2] virtio-net: re enable XDP_REDIRECT for mergeable buffer Message-ID: <20180301151601.611d6b83@redhat.com> In-Reply-To: <872bf3ea-7631-4e5f-9a6a-d632213cd15c@redhat.com> References: <1519874345-10235-1-git-send-email-jasowang@redhat.com> <20180301101038.4253424d@redhat.com> <20180301113531.7b25e2df@redhat.com> <872bf3ea-7631-4e5f-9a6a-d632213cd15c@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 1 Mar 2018 21:15:36 +0800 Jason Wang wrote: > On 2018年03月01日 18:35, Jesper Dangaard Brouer wrote: > > On Thu, 1 Mar 2018 17:23:37 +0800 > > Jason Wang wrote: > > > >> On 2018年03月01日 17:10, Jesper Dangaard Brouer wrote: > >>> On Thu, 1 Mar 2018 11:19:03 +0800 > >>> Jason Wang wrote: > >>> > >>>> This series tries to re-enable XDP_REDIRECT for mergeable buffer which > >>>> was removed since commit 7324f5399b06 ("virtio_net: disable > >>>> XDP_REDIRECT in receive_mergeable() case"). Main concerns are: > >>>> > >>>> - not enough tailroom was reserved which breaks cpumap > >>> To address this at a more fundamental level, I would suggest that we/you > >>> instead extend XDP to know it's buffers "frame" size/end. (The > >>> assumption use to be, xdp_buff->data_hard_start + PAGE_SIZE, but > >>> ixgbe+virtio_net broke that assumption). > >>> > >>> It should actually be fairly easy to implement: > >>> * Simply extend xdp_buff with a "data_hard_end" pointer. > >> Right, and then cpumap can warn and drop packets with insufficient > >> tailroom. > >> > >> But it should be a patch on top of this I think. > > Hmmm, not really. If we/you instead fix the issue of XDP doesn't know > > the end/size of the frame, then we don't need this mixed XDP > > generic/native code path mixing. > > I know this but I'm still a little bit confused. According to the commit > log of 7324f5399b06 ("virtio_net: disable XDP_REDIRECT in > receive_mergeable() case"), you said: > > """ >     The longer explaination is that receive_mergeable() tries to >     work-around and satisfy these XDP requiresments e.g. by having a >     function xdp_linearize_page() that allocates and memcpy RX buffers >     around (in case packet is scattered across multiple rx buffers).  This >     does currently satisfy XDP_PASS, XDP_DROP and XDP_TX (but only because >     we have not implemented bpf_xdp_adjust_tail yet). > """ > > So I consider the tailroom is a must for the (future) tail adjustment. That is true, BUT implementing the "data_hard_end" extension is a pre-requisite. It will also be to catch the issue of too little tail-room if/when implementing bpf_xdp_adjust_tail(). It is of-cause a "nice-to-have", to fix this virtio_net driver's receive_mergeable() call to have enough tail-room, but I don't see it as a solution to the fundamental problem. > > You could re-enable native redirect, and push the responsibility to > > cpumap for detecting this too-small frame "missing tailroom" (and avoid > > crashing...). (If we really want to support this, cpumap could fallback > > to dev_alloc_skb, and handle it gracefully). > > > > Right but it will be slower than build_skb(). True, but bad argument in this context, as you are already using a similar function call napi_alloc_skb(). And it will be even slower to call generic-XDP code path. -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer