netdev - Re: [RFC PATCH 00/14] Introducing AF

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Message-ID: <CAF=yD-+esffK3CscpLGNnL9PdcvqP=04EELC_g--NXuv5W=_og@mail.gmail.com>
Date:   Fri, 3 Nov 2017 13:34:36 +0900
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <ast@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        michael.lundkvist@...csson.com, ravineet.singh@...csson.com,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        jesse.brandeburg@...el.com, anjali.singhai@...el.com,
        rami.rosen@...el.com, jeffrey.b.shaw@...el.com,
        ferruh.yigit@...el.com, qi.z.zhang@...el.com
Subject: Re: [RFC PATCH 00/14] Introducing AF_PACKET V4 support

On Tue, Oct 31, 2017 at 9:41 PM, Björn Töpel <bjorn.topel@...il.com> wrote:
> From: Björn Töpel <bjorn.topel@...el.com>
>
> This RFC introduces AF_PACKET_V4 and PACKET_ZEROCOPY that are
> optimized for high performance packet processing and zero-copy
> semantics. Throughput improvements can be up to 40x compared to V2 and
> V3 for the micro benchmarks included. Would be great to get your
> feedback on it.
>
> The main difference between V4 and V2/V3 is that TX and RX descriptors
> are separated from packet buffers.

Cool feature. I'm looking forward to the netdev talk. Aside from the
inline comments in the patches, a few architecture questions.

Is TX support needed? Existing PACKET_TX_RING already sends out
packets without copying directly from the tx_ring. Indirection through a
descriptor ring is not helpful on TX if all packets still have to come from
a pre-registered packet pool. The patch set adds a lot of tx-only code
and is complex enough without it.

Can you use the existing PACKET_V2 format for the packet pool? The
v4 format is nearly the same as V2. Using the same version might avoid
some code duplication and simplify upgrading existing legacy code.
Instead of continuing to add new versions whose behavior is implicit,
perhaps we can add explicit mode PACKET_INDIRECT to PACKET_V2.

Finally, is it necessary to define a new descriptor ring format? Same for the
packet array and frame set. The kernel already has a few, such as virtio for
the first, skb_array/ptr_ring, even linux list for the second. These containers
add a lot of new boilerplate code. If new formats are absolutely necessary,
at least we should consider making them generic (like skb_array and
ptr_ring). But I'd like to understand first why, e.g., virtio cannot be used.