From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vlad Yasevich Subject: Re: [PATCH 1/2] mactap: Fix checksum errors for non-gso packets in bridge mode Date: Wed, 23 Apr 2014 16:30:22 -0400 Message-ID: <535822DE.5020704@redhat.com> References: <1398271901-32534-1-git-send-email-vyasevic@redhat.com> <1398271901-32534-2-git-send-email-vyasevic@redhat.com> <20140423192022.GA28446@redhat.com> <53581700.9010205@redhat.com> <20140423201050.GC28446@redhat.com> Reply-To: vyasevic@redhat.com Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, daniel.lezcano@free.fr, nightnord@gmail.com, kaber@trash.net, eric.dumazet@gmail.com, jasowang@redhat.com To: "Michael S. Tsirkin" Return-path: Received: from mx1.redhat.com ([209.132.183.28]:43166 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752748AbaDWUa6 (ORCPT ); Wed, 23 Apr 2014 16:30:58 -0400 In-Reply-To: <20140423201050.GC28446@redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: On 04/23/2014 04:10 PM, Michael S. Tsirkin wrote: > On Wed, Apr 23, 2014 at 03:39:44PM -0400, Vlad Yasevich wrote: >> On 04/23/2014 03:20 PM, Michael S. Tsirkin wrote: >>> On Wed, Apr 23, 2014 at 12:51:40PM -0400, Vlad Yasevich wrote: >>>> The following is a problematic configuration: >>>> >>>> VM1: virtio-net device connected to macvtap0@eth0 >>>> VM2: e1000 device connect to macvtap1@eth0 >>>> >>>> The problem is is that virtio-net supports checksum offloading >>>> and thus sends the packets to the host with CHECKSUM_PARTIAL set. >>>> On the other hand, e1000 does not support any acceleration. >>>> >>>> For small TCP packets (and this includes the 3-way handshake), >>>> e1000 ends up receiving packets that only have a partial checksum >>>> set. This causes TCP to fail checksum validation and to drop >>>> packets. As a result tcp connections can not be established. >>>> >>>> Commit 3e4f8b787370978733ca6cae452720a4f0c296b8 >>>> macvtap: Perform GSO on forwarding path. >>>> fixes this issue for large packets wthat will end up undergoing GSO. >>>> This commit adds a check for the non-GSO case and attempts to >>>> compute the checksum for partially checksummed packets in the >>>> non-GSO case. >>>> >>>> CC: Daniel Lezcano >>>> CC: Patrick McHardy >>>> CC: Andrian Nord >>>> CC: Eric Dumazet >>>> CC: Michael S. Tsirkin >>>> CC: Jason Wang >>>> Signed-off-by: Vlad Yasevich >>>> --- >>>> drivers/net/macvtap.c | 7 +++++++ >>>> 1 file changed, 7 insertions(+) >>>> >>>> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c >>>> index ff111a8..ba91084 100644 >>>> --- a/drivers/net/macvtap.c >>>> +++ b/drivers/net/macvtap.c >>>> @@ -322,6 +322,13 @@ static rx_handler_result_t macvtap_handle_frame(struct sk_buff **pskb) >>>> segs = nskb; >>>> } >>>> } else { >>>> + /* If we receive a partial checksum and the tap side >>>> + * doesn't support checksum offload, compute the checksum. >>>> + */ >>>> + if (skb->ip_summed == CHECKSUM_PARTIAL && >>>> + !(features & NETIF_F_ALL_CSUM) && >>>> + skb_checksum_help(skb)) >>>> + goto drop; >>> >>> Hmm confused by NETIF_F_ALL_CSUM here. >>> >>> features come from here: >>> feature_mask = NETIF_F_HW_CSUM; >>> >>> if (arg & (TUN_F_TSO4 | TUN_F_TSO6)) { >>> if (arg & TUN_F_TSO_ECN) >>> feature_mask |= NETIF_F_TSO_ECN; >>> if (arg & TUN_F_TSO4) >>> feature_mask |= NETIF_F_TSO; >>> if (arg & TUN_F_TSO6) >>> feature_mask |= NETIF_F_TSO6; >>> } >>> >>> if (arg & TUN_F_UFO) >>> feature_mask |= NETIF_F_UFO; >>> >>> >>> okay so why not just check that NETIF_F_HW_CSUM is set? >> >> We can do that, but it doesn't make much difference. > > Seems cleaner to test a single bit otherwise one is left > wondering what happens if only one bit matches. I can certainly do a single test, but if we ever change it, this will be another palace that would have to change. The above is also what dev_start_hard_xmit() does. > >>> >>> Also does it matter whether specific offloads are enabled? >>> >> >> No it doesn't matter at all. The packet is not a GSO packet >> so no other acceleration is used. > > Hmm how do we know it's not a gso packet? > All I see is need_gso test which means it needs segmentation. Part of netif_needs_gso() is a test for skb_is_gso(). So it it's gso and doesn't need segmentation (meaning the guest can receive large packets), then partial checksum is OK. > > >> Also, other offloads are dependent on checksum. >> >> -vlad > > Right so what if checksum is on, but segmentation is off? > Not the case with e1000 today but can be with other userspace. > In this case, the skb will be in need to segmentation and will take a different branch. -vlad > >>> >>>> skb_queue_tail(&q->sk.sk_receive_queue, skb); >>>> } >>>> >>>> -- >>>> 1.9.0