netdev - Re: [RFC 1/2] netvsc: invoke xdp_generic from VF frame handler

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Date:   Thu, 16 May 2019 15:36:52 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     kys@...rosoft.com, haiyangz@...rosoft.com, davem@...emloft.net,
        netdev@...r.kernel.org, Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: [RFC 1/2] netvsc: invoke xdp_generic from VF frame handler


On 2019/5/15 下午11:23, Stephen Hemminger wrote:
> On Wed, 15 May 2019 16:12:42 +0800
> Jason Wang <jasowang@...hat.com> wrote:
>
>> On 2019/5/15 下午4:03, Stephen Hemminger wrote:
>>> XDP generic does not work correctly with the Hyper-V/Azure netvsc
>>> device because of packet processing order. Only packets on the
>>> synthetic path get seen by the XDP program. The VF device packets
>>> are not seen.
>>>
>>> By the time the packets that arrive on the VF are handled by
>>> netvsc after the first pass of XDP generic (on the VF) has already
>>> been done.
>>>
>>> A fix for the netvsc device is to do this in the VF packet handler.
>>> by directly calling do_xdp_generic() if XDP program is present
>>> on the parent device.
>>>
>>> A riskier but maybe better alternative would be to do this netdev core
>>> code after the receive handler is invoked (if RX_HANDLER_ANOTHER
>>> is returned).
>>
>> Something like what I propose at
>> https://lore.kernel.org/patchwork/patch/973819/ ?
>>
>> It belongs to a series that try to make XDP (both native and generic)
>> work for stacked device. But for some reason (probably performance), the
>> maintainer seems not like the idea.
>>
>> Maybe it's time to reconsider that?
>>
>> Thanks
>
> I like your generic solution but it introduces a change in semantics.
> Netvsc always changes device when returning a ANOTHER but do all devices?
> If some other stacked device did this then there a chance that using
> XDP on that device would see same packet twice.


Good point.  Can we simply add a check and call XDP only if dev is 
changed in this case?

Thanks