netdev - Re: xdp_redirect ifindex vs port. Was: best API for returning/setting egress port?

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Message-ID: <b15f490d-5490-e309-9626-d35b8e932483@fb.com>
Date:   Thu, 27 Apr 2017 22:30:46 -0700
From:   Alexei Starovoitov <ast@...com>
To:     John Fastabend <john.fastabend@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Andy Gospodarek <andy@...yhouse.net>
CC:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Daniel Borkmann <borkmann@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xdp-newbies@...r.kernel.org" <xdp-newbies@...r.kernel.org>
Subject: Re: xdp_redirect ifindex vs port. Was: best API for returning/setting
 egress port?

On 4/27/17 10:06 PM, John Fastabend wrote:
> That is more or less what I was thinking as well. The other question
> I have though is should we have a bpf_redirect() call for the simple
> case where I use the ifindex directly. This will be helpful for taking
> existing programs from tc_cls into xdp. I think it makes sense to have
> both bpf_tx_allports(), bpf_tx_port(), and bpf_redirect().

I think so too.
Once netdevice is stored into netdev_array map the netdevice is pinned
and we need to figure out what to do if somebody tries to delete it.
Should we add a new netlink notifier that this netdev's refcnt is
almost zero and it's only in netdev_array(s) ?
or should it be deleted from the array(s) automatically and
then user space will be notified post-deletion?
Both approaches have their pros and cons.

Whereas raw ifindex approach (via bpf_redirect) doesn't have these
caveats. It's clear to both bpf prog and user space that ifindex
can be stale and user space needs to monitor netdevs and update
programs/maps.