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

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Date:   Wed, 15 May 2019 08:23:56 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Jason Wang <jasowang@...hat.com>
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 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.