Message-ID: <0b92f046-5ac3-7138-2775-59fadee6e17a@gmail.com>
Date:   Wed, 1 Dec 2021 12:20:56 -0700
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Subject: Re: [RFC 00/12] io_uring zerocopy send
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To:     Pavel Begunkov <asml.silence@gmail.com>, io-uring@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Jakub Kicinski <kuba@kernel.org>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        "David S . Miller" <davem@davemloft.net>,
        Willem de Bruijn <willemb@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>, Jens Axboe <axboe@kernel.dk>
References: <cover.1638282789.git.asml.silence@gmail.com>
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 <889c0306-afed-62cd-d95b-a20b8e798979@gmail.com>
From:   David Ahern <dsahern@gmail.com>
In-Reply-To: <889c0306-afed-62cd-d95b-a20b8e798979@gmail.com>
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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