Subject: Re: [PATCHv12 bpf-next 1/6] bpf: run devmap xdp_prog on flush instead of bulk enqueue

From: Hangbin Liu <[email-masked]>
Date: Mon, 21 Dec 2020 16:38:41 +0800

Hi David,

I just aware that,
On Thu, Dec 17, 2020 at 09:07:03AM -0700, David Ahern wrote:
> > +static int dev_map_bpf_prog_run(struct bpf_prog *xdp_prog,
> > +				struct xdp_frame **frames, int n,
> > +				struct net_device *dev)
> > +{
> > +	struct xdp_txq_info txq = { .dev = dev };
> > +	struct xdp_buff xdp;
> > +	int i, nframes = 0;
> > +
> > +	for (i = 0; i < n; i++) {
> > +		struct xdp_frame *xdpf = frames[i];
> > +		u32 act;
> > +		int err;
> > +
> > +		xdp_convert_frame_to_buff(xdpf, &xdp);
> > +		xdp.txq = &txq;
> > +
> > +		act = bpf_prog_run_xdp(xdp_prog, &xdp);
> > +		switch (act) {
> > +		case XDP_PASS:
> > +			err = xdp_update_frame_from_buff(&xdp, xdpf);
> > +			if (unlikely(err < 0))
> > +				xdp_return_frame_rx_napi(xdpf);
> > +			else
> > +				frames[nframes++] = xdpf;
> > +			break;
> > +		default:
> > +			bpf_warn_invalid_xdp_action(act);
> > +			fallthrough;
> > +		case XDP_ABORTED:
> > +			trace_xdp_exception(dev, xdp_prog, act);
> > +			fallthrough;
> > +		case XDP_DROP:
> > +			xdp_return_frame_rx_napi(xdpf);
> > +			break;
> > +		}
> > +	}
> > +	return n - nframes; /* dropped frames count */
> 
> just return nframes here, since ...

If we return nframes here,
> 
> > +}
> > +
> >  static void bq_xmit_all(struct xdp_dev_bulk_queue *bq, u32 flags)
> >  {
> >  	struct net_device *dev = bq->dev;
> >  	int sent = 0, drops = 0, err = 0;
> > +	unsigned int cnt = bq->count;
> > +	unsigned int xdp_drop;
> >  	int i;
> >  
> > -	if (unlikely(!bq->count))
> > +	if (unlikely(!cnt))
> >  		return;
> >  
> > -	for (i = 0; i < bq->count; i++) {
> > +	for (i = 0; i < cnt; i++) {
> >  		struct xdp_frame *xdpf = bq->q[i];
> >  
> >  		prefetch(xdpf);
> >  	}
> >  
> > -	sent = dev->netdev_ops->ndo_xdp_xmit(dev, bq->count, bq->q, flags);
> > +	if (unlikely(bq->xdp_prog)) {
> > +		xdp_drop = dev_map_bpf_prog_run(bq->xdp_prog, bq->q, cnt, dev);
> > +		cnt -= xdp_drop;
> 
> ... that is apparently what you really want.

then this will be 

		cnt = dev_map_bpf_prog_run(bq->xdp_prog, bq->q, cnt, dev);
		xdp_drop = bq->count - cnt;

So there is no much difference whether we return passed frames or dropped
frames.
> > +		if (!cnt) {
> > +			sent = 0;
> > +			drops = xdp_drop;
> > +			goto out;
> > +		}
> > +	}

Thanks
Hangbin