netdev - Re: [RFC 00/12] io_uring zerocopy send

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Date:   Wed, 1 Dec 2021 20:15:28 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     David Ahern <dsahern@...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 19:20, David Ahern wrote:
> 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

I see, thanks. I still don't use the skb_orphan_frags_rx() hack
and it doesn't go through the loopback (for my dummy tests), just
dummy_xmit() and no mention of loopback in perf data, see the
flamegraph. Don't know what is the catch.

I'm illiterate of the routing paths. Can it be related to
the "ip route add"? How do you get an ipv4 address for the device?

> 
> (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
> 

-- 
Pavel Begunkov