netdev - Re: [PATCH net-next 4/4] act_mirred: use ACT

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Date:   Mon, 16 Jul 2018 16:39:55 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: Re: [PATCH net-next 4/4] act_mirred: use ACT_REDIRECT when possible

On Fri, Jul 13, 2018 at 2:55 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> When mirred is invoked from the ingress path, and it wants to redirect
> the processed packet, it can now use the ACT_REDIRECT action,
> filling the tcf_result accordingly.
>
> This avoids a skb_clone() in the TC S/W data path giving a ~10%
> improvement in forwarding performances. Overall TC S/W performances
> are now comparable to the kernel openswitch datapath.

Avoiding skb_clone() for redirection is cool, but why need to use
skb_do_redirect() here?

There is a subtle difference here:

skb_do_redirect() calls __bpf_rx_skb() which calls
dev_forward_skb().

while the current mirred action doesn't scrub packets when
redirecting to ingress (from egress). Although I forget if it is
intentionally.

Also, skb->skb_iif is unset in skb_do_redirect() when
redirecting to ingress, I recall we have to set it correctly
for input routing. Probably yet another reason why we
can't scrub it, unless my memory goes wrong. :)

Thanks!