From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lindbergh.monkeyblade.net (lindbergh.monkeyblade.net [23.128.96.19]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5078916421; Tue, 20 Jun 2023 14:15:41 +0000 (UTC) Received: from out30-132.freemail.mail.aliyun.com (out30-132.freemail.mail.aliyun.com [115.124.30.132]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5A6EFE6E; Tue, 20 Jun 2023 07:15:38 -0700 (PDT) X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R661e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018046049;MF=hengqi@linux.alibaba.com;NM=1;PH=DS;RN=13;SR=0;TI=SMTPD_---0VlcDhnY_1687270532; Received: from localhost(mailfrom:hengqi@linux.alibaba.com fp:SMTPD_---0VlcDhnY_1687270532) by smtp.aliyun-inc.com; Tue, 20 Jun 2023 22:15:33 +0800 Date: Tue, 20 Jun 2023 22:15:32 +0800 From: Heng Qi To: "Michael S. Tsirkin" Cc: netdev@vger.kernel.org, bpf@vger.kernel.org, Jason Wang , Xuan Zhuo , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend Subject: Re: [PATCH net-next 3/4] virtio-net: support coexistence of XDP and _F_GUEST_CSUM Message-ID: <20230620141532.GH74977@h68b04307.sqa.eu95> References: <20230619105738.117733-1-hengqi@linux.alibaba.com> <20230619105738.117733-4-hengqi@linux.alibaba.com> <20230619072320-mutt-send-email-mst@kernel.org> <20230620032430.GE74977@h68b04307.sqa.eu95> <20230620064711-mutt-send-email-mst@kernel.org> <20230620110148.GF74977@h68b04307.sqa.eu95> <20230620080926-mutt-send-email-mst@kernel.org> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230620080926-mutt-send-email-mst@kernel.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-Spam-Status: No, score=-9.9 required=5.0 tests=BAYES_00, ENV_AND_HDR_SPF_MATCH,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS, T_SCC_BODY_TEXT_LINE,UNPARSEABLE_RELAY,USER_IN_DEF_SPF_WL autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net On Tue, Jun 20, 2023 at 08:10:38AM -0400, Michael S. Tsirkin wrote: > On Tue, Jun 20, 2023 at 07:01:48PM +0800, Heng Qi wrote: > > On Tue, Jun 20, 2023 at 06:50:34AM -0400, Michael S. Tsirkin wrote: > > > On Tue, Jun 20, 2023 at 11:24:30AM +0800, Heng Qi wrote: > > > > On Mon, Jun 19, 2023 at 07:26:44AM -0400, Michael S. Tsirkin wrote: > > > > > On Mon, Jun 19, 2023 at 06:57:37PM +0800, Heng Qi wrote: > > > > > > We are now re-probing the csum related fields and trying > > > > > > to have XDP and RX hw checksum capabilities coexist on the > > > > > > XDP path. For the benefit of: > > > > > > 1. RX hw checksum capability can be used if XDP is loaded. > > > > > > 2. Avoid packet loss when loading XDP in the vm-vm scenario. > > > > > > > > > > > > Signed-off-by: Heng Qi > > > > > > Reviewed-by: Xuan Zhuo > > > > > > --- > > > > > > drivers/net/virtio_net.c | 36 ++++++++++++++++++++++++------------ > > > > > > 1 file changed, 24 insertions(+), 12 deletions(-) > > > > > > > > > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c > > > > > > index 07b4801d689c..25b486ab74db 100644 > > > > > > --- a/drivers/net/virtio_net.c > > > > > > +++ b/drivers/net/virtio_net.c > > > > > > @@ -1709,6 +1709,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq, > > > > > > struct net_device *dev = vi->dev; > > > > > > struct sk_buff *skb; > > > > > > struct virtio_net_hdr_mrg_rxbuf *hdr; > > > > > > + __u8 flags; > > > > > > > > > > > > if (unlikely(len < vi->hdr_len + ETH_HLEN)) { > > > > > > pr_debug("%s: short packet %i\n", dev->name, len); > > > > > > @@ -1717,6 +1718,8 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq, > > > > > > return; > > > > > > } > > > > > > > > > > > > + flags = ((struct virtio_net_hdr_mrg_rxbuf *)buf)->hdr.flags; > > > > > > + > > > > > > if (vi->mergeable_rx_bufs) > > > > > > skb = receive_mergeable(dev, vi, rq, buf, ctx, len, xdp_xmit, > > > > > > stats); > > > > > > > > > > what's going on here? > > > > > > > > Hi, Michael. > > > > > > > > Is your question about the function of this code? > > > > 1. If yes, > > > > this sentence saves the flags value in virtio-net-hdr in advance > > > > before entering the XDP processing logic, so that it can be used to > > > > judge further logic after XDP processing. > > > > > > > > If _NEEDS_CSUM is included in flags before XDP processing, then after > > > > XDP processing we need to re-probe the csum fields and calculate the > > > > pseudo-header checksum. > > > > > > Yes but we previously used this: > > > - hdr = skb_vnet_hdr(skb); > > > which pokes at the copy in skb cb. > > > > > > Is anything wrong with this? > > > > > > > This is where we save the hdr when there is no XDP loaded (note that > > this is the complete hdr, including flags, and also including GSO and > > other information). When XDP is loaded, because hdr is invalid, we will > > not save it into skb->cb. > > > > But the above situation is not what we want. Now our purpose is to save > > the hdr information before XDP processing, that is, when the driver has > > just received the packet and has not built the skb (in fact, we only > > need flags). Therefore, only flags are saved here. > > > > Thanks. > > I don't get it. this seems to be the only use of flags: > > > + if (unlikely(vi->xdp_enabled)) { > + if (virtnet_set_csum_after_xdp(vi, skb, flags) < 0) { > + pr_debug("%s: errors occurred in flow dissector setting csum", > + dev->name); > + goto frame_err; > + } > > looks like skb has already been created here. I explain more: First, this patchset only focuses on XDP loaded scenes. Then in the same-host vm-vm scenario, when the receiver loads XDP, this is the packet receiving process: 1. The driver receives a packet, represented by *buf. 2. The XDP program builds xdp_buff based on *buf. virtio_net_hdr_mrg_rxbuf is located in the headroom of xdp_buff. 3. The XDP program returns XDP_PASS and modifies the headroom, that is, the information in virtio_net_hdr_mrg_rxbuf becomes invalid (including flags). So information like csum_{start, offset} is no longer correct. Therefore, the skb converted from xdp_buff does not save information in skb cb. And, skb->ip_summed = CHECKSUM_NONE, there is a high probability of packet loss at this time because the incorrect check value. So, in order to solve this problem (not only), we save the flags in virtio_net_hdr_mrg_rxbuf before the #2 step above, which is the original information, just to know whether the packet is _NEEDS_CSUM or _DATA_VALID. If the saved flags contains _NEEDS_CSUM, then we use virtnet_set_csum_after_xdp() to recalculate csum_{start, offset}, pseudo-header checksum for the skb converted from xdp_buff. That is, the saved flags are only used to identify whether we want to re-probe for the packet to avoiding the packet dropping. Thanks. > > is there another use of flags that I missed? > > > > > > It seems preferable not to poke at the header an extra time. > > > > > > -- > > > MST