linux-kernel - Re: [RFC PATCH net-next 11/11] net: enetc: add TX support for zero-copy XDP sockets

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Date:   Wed, 8 Feb 2023 19:08:15 +0200
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH net-next 11/11] net: enetc: add TX support for
 zero-copy XDP sockets

Hi Maciej,

On Wed, Feb 08, 2023 at 05:37:35PM +0100, Maciej Fijalkowski wrote:
> On Mon, Feb 06, 2023 at 12:08:37PM +0200, Vladimir Oltean wrote:
> 
> Hey Vladimir,
> 
> > Schedule NAPI by hand from enetc_xsk_wakeup(), and send frames from the
> > XSK TX queue from NAPI context. Add them to the completion queue from
> > the enetc_clean_tx_ring() procedure which is common for all kinds of
> > traffic.
> > 
> > We reuse one of the TX rings for XDP (XDP_TX/XDP_REDIRECT) for XSK as
> > well. They are already cropped from the TX rings that the network stack
> > can use when XDP is enabled (with or without AF_XDP).
> > 
> > As for XDP_REDIRECT calling enetc's ndo_xdp_xmit, I'm not sure if that
> > can run simultaneously with enetc_poll() (on different CPUs, but towards
> > the same TXQ). I guess it probably can, but idk what to do about it.
> > The problem is that enetc_xdp_xmit() sends to
> > priv->xdp_tx_ring[smp_processor_id()], while enetc_xsk_xmit() and XDP_TX
> > send to priv->xdp_tx_ring[NAPI instance]. So when the NAPI instance runs
> 
> Why not use cpu id on the latter then?

Hmm, because I want the sendto() syscall to trigger wakeup of the NAPI
that sends traffic to the proper queue_id, rather than to the queue_id
affine to the CPU that the sendto() syscall was made?