netdev - Re: [PATCH net-next] af_packet: Provide a TPACKET_V2 compatible Tx path for TPACKET

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Date:   Fri, 30 Dec 2016 16:33:47 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Willem de Bruijn <willemb@...gle.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next] af_packet: Provide a TPACKET_V2 compatible Tx
 path for TPACKET_V3

On Fri, Dec 30, 2016 at 11:54 AM, Sowmini Varadhan
<sowmini.varadhan@...cle.com> wrote:
> Although TPACKET_V3 Rx has some benefits over TPACKET_V2 Rx,
> *_v3 does not currently have Tx support. As a result an application
> that wants the best perf for Tx and Rx (e.g. to handle
> request/response transacations) ends up needing 2 sockets,
> one with *_v2 for Tx and another with *_v3 for Rx.
>
> This patch enables TPACKET_V2 compatible Tx features in TPACKET_V3
> so that _v3 supports at least as many features as _v2.

Once we define the interface as equivalent to v2, we cannot redefine it to
support v3-only features later.

> Signed-off-by: Sowmini Varadhan <sowmini.varadhan@...cle.com>
> ---
>  Documentation/networking/packet_mmap.txt |    6 ++++--
>  net/packet/af_packet.c                   |   23 +++++++++++++++--------
>  2 files changed, 19 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/networking/packet_mmap.txt b/Documentation/networking/packet_mmap.txt
> index daa015a..6425062 100644
> --- a/Documentation/networking/packet_mmap.txt
> +++ b/Documentation/networking/packet_mmap.txt
> @@ -565,7 +565,7 @@ where 'tpacket_version' can be TPACKET_V1 (default), TPACKET_V2, TPACKET_V3.
>                    (void *)hdr + TPACKET_ALIGN(sizeof(struct tpacket_hdr))
>
>  TPACKET_V2 --> TPACKET_V3:
> -       - Flexible buffer implementation:
> +       - Flexible buffer implementation for RX_RING:
>                 1. Blocks can be configured with non-static frame-size

This is one of the main advantages of the v3 interface, and also
relevant to Tx. The current implementation does not consult
tpacket3_hdr->tp_next_offset and would preclude adding that
later.