netdev - Re: [PATCH v6 04/12] net/mlx4_en: add support for fast rx drop bpf program

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Message-ID: <20160710160540.GB6657@gmail.com>
Date:	Sun, 10 Jul 2016 09:05:42 -0700
From:	Brenden Blanco <bblanco@...mgrid.com>
To:	Tariq Toukan <ttoukan.linux@...il.com>
Cc:	Saeed Mahameed <saeedm@....mellanox.co.il>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Martin KaFai Lau <kafai@...com>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Ari Saha <as754m@....com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Or Gerlitz <gerlitz.or@...il.com>,
	john fastabend <john.fastabend@...il.com>,
	hannes@...essinduktion.org, Thomas Graf <tgraf@...g.ch>,
	Tom Herbert <tom@...bertland.com>,
	Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH v6 04/12] net/mlx4_en: add support for fast rx drop bpf
 program

On Sun, Jul 10, 2016 at 06:25:40PM +0300, Tariq Toukan wrote:
> 
> On 09/07/2016 10:58 PM, Saeed Mahameed wrote:
> >On Fri, Jul 8, 2016 at 5:15 AM, Brenden Blanco <bblanco@...mgrid.com> wrote:
> >>+               /* A bpf program gets first chance to drop the packet. It may
> >>+                * read bytes but not past the end of the frag.
> >>+                */
> >>+               if (prog) {
> >>+                       struct xdp_buff xdp;
> >>+                       dma_addr_t dma;
> >>+                       u32 act;
> >>+
> >>+                       dma = be64_to_cpu(rx_desc->data[0].addr);
> >>+                       dma_sync_single_for_cpu(priv->ddev, dma,
> >>+                                               priv->frag_info[0].frag_size,
> >>+                                               DMA_FROM_DEVICE);
> >In case of XDP_PASS we will dma_sync again in the normal path, this
> >can be improved by doing the dma_sync as soon as we can and once and
> >for all, regardless of the path the packet is going to take
> >(XDP_DROP/mlx4_en_complete_rx_desc/mlx4_en_rx_skb).
> I agree with Saeed, dma_sync is a heavy operation that is now done
> twice for all packets with XDP_PASS.
> We should try our best to avoid performance degradation in the flow
> of unfiltered packets.
Makes sense, do folks here see a way to do this cleanly?
> 
> Regards,
> Tariq