Re: [PATCH net-next 4/4] act_mirred: use ACT_REDIRECT when possible

[Cong Wang <xiyou.wangcong@gmail.com>]

On Fri, Jul 13, 2018 at 2:55 AM Paolo Abeni <pabeni@redhat.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!