linux-kernel - Re: [RFC 00/12] io

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Date:   Wed, 1 Dec 2021 12:20:56 -0700
From:   David Ahern <dsahern@...il.com>
To:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [RFC 00/12] io_uring zerocopy send

On 12/1/21 12:11 PM, Pavel Begunkov wrote:
> btw, why a dummy device would ever go through loopback? It doesn't
> seem to make sense, though may be missing something.

You are sending to a local ip address, so the fib_lookup returns
RTN_LOCAL. The code makes dev_out the loopback:

https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/net/ipv4/route.c#n2773

(you are not using vrf so ignore the l3mdev reference). loopback device
has the logic to put the skb back in the stack for Rx processing:

https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/drivers/net/loopback.c#n68