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

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Date:   Wed, 15 May 2019 10:53:30 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Haiyang Zhang <haiyangz@...rosoft.com>
Cc:     KY Srinivasan <kys@...rosoft.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <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 17:50:25 +0000
Haiyang Zhang <haiyangz@...rosoft.com> wrote:

> > -----Original Message-----
> > From: Stephen Hemminger <stephen@...workplumber.org>
> > Sent: Wednesday, May 15, 2019 4:03 AM
> > To: KY Srinivasan <kys@...rosoft.com>; Haiyang Zhang
> > <haiyangz@...rosoft.com>; davem@...emloft.net
> > Cc: netdev@...r.kernel.org; Stephen Hemminger <sthemmin@...rosoft.com>
> > Subject: [RFC 1/2] netvsc: invoke xdp_generic from VF frame handler
> > 
> > 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).
> > 
> > Fixes: 0c195567a8f6 ("netvsc: transparent VF management")
> > Signed-off-by: Stephen Hemminger <sthemmin@...rosoft.com>
> > ---
> >  drivers/net/hyperv/netvsc_drv.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/net/hyperv/netvsc_drv.c b/drivers/net/hyperv/netvsc_drv.c
> > index 06393b215102..bb0fc1869bde 100644
> > --- a/drivers/net/hyperv/netvsc_drv.c
> > +++ b/drivers/net/hyperv/netvsc_drv.c
> > @@ -1999,9 +1999,15 @@ static rx_handler_result_t
> > netvsc_vf_handle_frame(struct sk_buff **pskb)
> >  	struct net_device_context *ndev_ctx = netdev_priv(ndev);
> >  	struct netvsc_vf_pcpu_stats *pcpu_stats
> >  		 = this_cpu_ptr(ndev_ctx->vf_stats);
> > +	struct bpf_prog *xdp_prog;
> > 
> >  	skb->dev = ndev;
> > 
> > +	xdp_prog = rcu_dereference(ndev->xdp_prog);
> > +	if (xdp_prog &&
> > +	    do_xdp_generic(xdp_prog, skb) != XDP_PASS)
> > +		return RX_HANDLER_CONSUMED;  
> 
> Looks fine overall.
> 
> The function do_xdp_generic() already checks NULL on xdp_prog,
> so we don't need to check it in our code. 
> 
> int do_xdp_generic(struct bpf_prog *xdp_prog, struct sk_buff *skb)
> {
>         if (xdp_prog) {
> 

The null check in the netvsc code was just an minor optimization
to avoid unnecessary function call in fast path.