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Message-ID: <53591A20.6080009@redhat.com>
Date: Thu, 24 Apr 2014 10:05:20 -0400
From: Vlad Yasevich <vyasevic@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
CC: netdev@...r.kernel.org, daniel.lezcano@...e.fr,
nightnord@...il.com, kaber@...sh.net, eric.dumazet@...il.com,
jasowang@...hat.com
Subject: Re: [PATCH 1/2] mactap: Fix checksum errors for non-gso packets in
bridge mode
On 04/24/2014 03:26 AM, Michael S. Tsirkin wrote:
> On Wed, Apr 23, 2014 at 04:30:22PM -0400, Vlad Yasevich wrote:
>> 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@...0
>>>>>> VM2: e1000 device connect to macvtap1@...0
>>>>>>
>>>>>> 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 <daniel.lezcano@...e.fr>
>>>>>> CC: Patrick McHardy <kaber@...sh.net>
>>>>>> CC: Andrian Nord <nightnord@...il.com>
>>>>>> CC: Eric Dumazet <eric.dumazet@...il.com>
>>>>>> CC: Michael S. Tsirkin <mst@...hat.com>
>>>>>> CC: Jason Wang <jasowang@...hat.com>
>>>>>> Signed-off-by: Vlad Yasevich <vyasevic@...hat.com>
>>>>>> ---
>>>>>> 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.
>
> Hmm change what exactly? Add support for selectively
> disabling checksum for specific protocols?
Yes. Right now, we kind-of lump them all together, but there
are some protocols that are not accounted for in HW_CSUM.
For instance, I am looking to add SCTP checksum offload. It
would be very useful if the host has SCTP-capable nic.
>
>> The above is also what dev_start_hard_xmit() does.
>
> Yes and I was wondering about that too, but check it out: that one
> calls: netif_skb_dev_features which in turn calls harmonize_features.
> And there we have:
> if (skb->ip_summed != CHECKSUM_NONE &&
> !can_checksum_protocol(features, skb_network_protocol(skb, &tmp))) {
> features &= ~NETIF_F_ALL_CSUM;
> } else if (illegal_highdma(dev, skb)) {
> features &= ~NETIF_F_SG;
> }
>
> So NETIF_F_HW_CSUM is tested because it's cleared by a per-protocol
> handling here which is not there in your patch.
>
> Your patch is still correct - the reason harmonize_features is not
> necessary is because tap either sets HW_CSUM or nothing,
> can_checksum_protocol is always true or always false. But since we rely
> on this anyway, isn't it better to make this explicit?
>
> Alternatively let's clarify the comment here:
>
>>>>>> + /* If we receive a partial checksum and the tap side
>>>>>> + * doesn't support checksum offload, compute the checksum.
>
> Add:
>
> + * Note: it doesn't matter which checksum feature to
> + * check, we either support them all or none.
>
>>>>>> + */
>
> Fine?
Looks good to me. I'll update and re-submit.
Thanks
-vlad
>
>>>
>>>>>
>>>>> 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.
>
> That is correct- thanks for the clarification.
>
>
>>>
>>>
>>>> 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
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