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

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Date:	Sun, 10 Jul 2016 18:25:40 +0300
From:	Tariq Toukan <ttoukan.linux@...il.com>
To:	Saeed Mahameed <saeedm@....mellanox.co.il>,
	Brenden Blanco <bblanco@...mgrid.com>
Cc:	"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 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.

Regards,
Tariq